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llvm-project/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
Jeremy Morse 34b139594a
[NFC][DebugInfo] Switch more call-sites to using iterator-insertion (#124283) 
To finalise the "RemoveDIs" work removing debug intrinsics, we're
updating call sites that insert instructions to use iterators instead.
This set of changes are those where it's not immediately obvious that
just calling getIterator to fetch an iterator is correct, and one or two
places where more than one line needs to change.

Overall the same rule holds though: iterators generated for the start of
a block such as getFirstNonPHIIt need to be passed into insert/move
methods without being unwrapped/rewrapped, everything else can use
getIterator.
2025-01-27 16:44:14 +00:00

3728 lines
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///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/InstructionCost.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <numeric>
#include <optional>
#include <utility>
#define DEBUG_TYPE "simple-loop-unswitch"
using namespace llvm;
using namespace llvm::PatternMatch;
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumSelects, "Number of selects turned into branches for unswitching");
STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
STATISTIC(NumTrivial, "Number of unswitches that are trivial");
STATISTIC(
NumCostMultiplierSkipped,
"Number of unswitch candidates that had their cost multiplier skipped");
STATISTIC(NumInvariantConditionsInjected,
"Number of invariant conditions injected and unswitched");
static cl::opt<bool> EnableNonTrivialUnswitch(
"enable-nontrivial-unswitch", cl::init(false), cl::Hidden,
cl::desc("Forcibly enables non-trivial loop unswitching rather than "
"following the configuration passed into the pass."));
static cl::opt<int>
UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden,
cl::desc("The cost threshold for unswitching a loop."));
static cl::opt<bool> EnableUnswitchCostMultiplier(
"enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden,
cl::desc("Enable unswitch cost multiplier that prohibits exponential "
"explosion in nontrivial unswitch."));
static cl::opt<int> UnswitchSiblingsToplevelDiv(
"unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden,
cl::desc("Toplevel siblings divisor for cost multiplier."));
static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
"unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
cl::desc("Number of unswitch candidates that are ignored when calculating "
"cost multiplier."));
static cl::opt<bool> UnswitchGuards(
"simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
cl::desc("If enabled, simple loop unswitching will also consider "
"llvm.experimental.guard intrinsics as unswitch candidates."));
static cl::opt<bool> DropNonTrivialImplicitNullChecks(
"simple-loop-unswitch-drop-non-trivial-implicit-null-checks",
cl::init(false), cl::Hidden,
cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
"null checks to save time analyzing if we can keep it."));
static cl::opt<unsigned>
MSSAThreshold("simple-loop-unswitch-memoryssa-threshold",
cl::desc("Max number of memory uses to explore during "
"partial unswitching analysis"),
cl::init(100), cl::Hidden);
static cl::opt<bool> FreezeLoopUnswitchCond(
"freeze-loop-unswitch-cond", cl::init(true), cl::Hidden,
cl::desc("If enabled, the freeze instruction will be added to condition "
"of loop unswitch to prevent miscompilation."));
static cl::opt<bool> InjectInvariantConditions(
"simple-loop-unswitch-inject-invariant-conditions", cl::Hidden,
cl::desc("Whether we should inject new invariants and unswitch them to "
"eliminate some existing (non-invariant) conditions."),
cl::init(true));
static cl::opt<unsigned> InjectInvariantConditionHotnesThreshold(
"simple-loop-unswitch-inject-invariant-condition-hotness-threshold",
cl::Hidden, cl::desc("Only try to inject loop invariant conditions and "
"unswitch on them to eliminate branches that are "
"not-taken 1/<this option> times or less."),
cl::init(16));
AnalysisKey ShouldRunExtraSimpleLoopUnswitch::Key;
namespace {
struct CompareDesc {
BranchInst *Term;
Value *Invariant;
BasicBlock *InLoopSucc;
CompareDesc(BranchInst *Term, Value *Invariant, BasicBlock *InLoopSucc)
: Term(Term), Invariant(Invariant), InLoopSucc(InLoopSucc) {}
};
struct InjectedInvariant {
ICmpInst::Predicate Pred;
Value *LHS;
Value *RHS;
BasicBlock *InLoopSucc;
InjectedInvariant(ICmpInst::Predicate Pred, Value *LHS, Value *RHS,
BasicBlock *InLoopSucc)
: Pred(Pred), LHS(LHS), RHS(RHS), InLoopSucc(InLoopSucc) {}
};
struct NonTrivialUnswitchCandidate {
Instruction *TI = nullptr;
TinyPtrVector<Value *> Invariants;
std::optional<InstructionCost> Cost;
std::optional<InjectedInvariant> PendingInjection;
NonTrivialUnswitchCandidate(
Instruction *TI, ArrayRef<Value *> Invariants,
std::optional<InstructionCost> Cost = std::nullopt,
std::optional<InjectedInvariant> PendingInjection = std::nullopt)
: TI(TI), Invariants(Invariants), Cost(Cost),
PendingInjection(PendingInjection) {};
bool hasPendingInjection() const { return PendingInjection.has_value(); }
};
} // end anonymous namespace.
// Helper to skip (select x, true, false), which matches both a logical AND and
// OR and can confuse code that tries to determine if \p Cond is either a
// logical AND or OR but not both.
static Value *skipTrivialSelect(Value *Cond) {
Value *CondNext;
while (match(Cond, m_Select(m_Value(CondNext), m_One(), m_Zero())))
Cond = CondNext;
return Cond;
}
/// Collect all of the loop invariant input values transitively used by the
/// homogeneous instruction graph from a given root.
///
/// This essentially walks from a root recursively through loop variant operands
/// which have perform the same logical operation (AND or OR) and finds all
/// inputs which are loop invariant. For some operations these can be
/// re-associated and unswitched out of the loop entirely.
static TinyPtrVector<Value *>
collectHomogenousInstGraphLoopInvariants(const Loop &L, Instruction &Root,
const LoopInfo &LI) {
assert(!L.isLoopInvariant(&Root) &&
"Only need to walk the graph if root itself is not invariant.");
TinyPtrVector<Value *> Invariants;
bool IsRootAnd = match(&Root, m_LogicalAnd());
bool IsRootOr = match(&Root, m_LogicalOr());
// Build a worklist and recurse through operators collecting invariants.
SmallVector<Instruction *, 4> Worklist;
SmallPtrSet<Instruction *, 8> Visited;
Worklist.push_back(&Root);
Visited.insert(&Root);
do {
Instruction &I = *Worklist.pop_back_val();
for (Value *OpV : I.operand_values()) {
// Skip constants as unswitching isn't interesting for them.
if (isa<Constant>(OpV))
continue;
// Add it to our result if loop invariant.
if (L.isLoopInvariant(OpV)) {
Invariants.push_back(OpV);
continue;
}
// If not an instruction with the same opcode, nothing we can do.
Instruction *OpI = dyn_cast<Instruction>(skipTrivialSelect(OpV));
if (OpI && ((IsRootAnd && match(OpI, m_LogicalAnd())) ||
(IsRootOr && match(OpI, m_LogicalOr())))) {
// Visit this operand.
if (Visited.insert(OpI).second)
Worklist.push_back(OpI);
}
}
} while (!Worklist.empty());
return Invariants;
}
static void replaceLoopInvariantUses(const Loop &L, Value *Invariant,
Constant &Replacement) {
assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
// Replace uses of LIC in the loop with the given constant.
// We use make_early_inc_range as set invalidates the iterator.
for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
Instruction *UserI = dyn_cast<Instruction>(U.getUser());
// Replace this use within the loop body.
if (UserI && L.contains(UserI))
U.set(&Replacement);
}
}
/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
/// incoming values along this edge.
static bool areLoopExitPHIsLoopInvariant(const Loop &L,
const BasicBlock &ExitingBB,
const BasicBlock &ExitBB) {
for (const Instruction &I : ExitBB) {
auto *PN = dyn_cast<PHINode>(&I);
if (!PN)
// No more PHIs to check.
return true;
// If the incoming value for this edge isn't loop invariant the unswitch
// won't be trivial.
if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
return false;
}
llvm_unreachable("Basic blocks should never be empty!");
}
/// Copy a set of loop invariant values \p ToDuplicate and insert them at the
/// end of \p BB and conditionally branch on the copied condition. We only
/// branch on a single value.
static void buildPartialUnswitchConditionalBranch(
BasicBlock &BB, ArrayRef<Value *> Invariants, bool Direction,
BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, bool InsertFreeze,
const Instruction *I, AssumptionCache *AC, const DominatorTree &DT) {
IRBuilder<> IRB(&BB);
SmallVector<Value *> FrozenInvariants;
for (Value *Inv : Invariants) {
if (InsertFreeze && !isGuaranteedNotToBeUndefOrPoison(Inv, AC, I, &DT))
Inv = IRB.CreateFreeze(Inv, Inv->getName() + ".fr");
FrozenInvariants.push_back(Inv);
}
Value *Cond = Direction ? IRB.CreateOr(FrozenInvariants)
: IRB.CreateAnd(FrozenInvariants);
IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
Direction ? &NormalSucc : &UnswitchedSucc);
}
/// Copy a set of loop invariant values, and conditionally branch on them.
static void buildPartialInvariantUnswitchConditionalBranch(
BasicBlock &BB, ArrayRef<Value *> ToDuplicate, bool Direction,
BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, Loop &L,
MemorySSAUpdater *MSSAU) {
ValueToValueMapTy VMap;
for (auto *Val : reverse(ToDuplicate)) {
Instruction *Inst = cast<Instruction>(Val);
Instruction *NewInst = Inst->clone();
NewInst->insertInto(&BB, BB.end());
RemapInstruction(NewInst, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
VMap[Val] = NewInst;
if (!MSSAU)
continue;
MemorySSA *MSSA = MSSAU->getMemorySSA();
if (auto *MemUse =
dyn_cast_or_null<MemoryUse>(MSSA->getMemoryAccess(Inst))) {
auto *DefiningAccess = MemUse->getDefiningAccess();
// Get the first defining access before the loop.
while (L.contains(DefiningAccess->getBlock())) {
// If the defining access is a MemoryPhi, get the incoming
// value for the pre-header as defining access.
if (auto *MemPhi = dyn_cast<MemoryPhi>(DefiningAccess))
DefiningAccess =
MemPhi->getIncomingValueForBlock(L.getLoopPreheader());
else
DefiningAccess = cast<MemoryDef>(DefiningAccess)->getDefiningAccess();
}
MSSAU->createMemoryAccessInBB(NewInst, DefiningAccess,
NewInst->getParent(),
MemorySSA::BeforeTerminator);
}
}
IRBuilder<> IRB(&BB);
Value *Cond = VMap[ToDuplicate[0]];
IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
Direction ? &NormalSucc : &UnswitchedSucc);
}
/// Rewrite the PHI nodes in an unswitched loop exit basic block.
///
/// Requires that the loop exit and unswitched basic block are the same, and
/// that the exiting block was a unique predecessor of that block. Rewrites the
/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
/// PHI nodes from the old preheader that now contains the unswitched
/// terminator.
static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH) {
for (PHINode &PN : UnswitchedBB.phis()) {
// When the loop exit is directly unswitched we just need to update the
// incoming basic block. We loop to handle weird cases with repeated
// incoming blocks, but expect to typically only have one operand here.
for (auto i : seq<int>(0, PN.getNumOperands())) {
assert(PN.getIncomingBlock(i) == &OldExitingBB &&
"Found incoming block different from unique predecessor!");
PN.setIncomingBlock(i, &OldPH);
}
}
}
/// Rewrite the PHI nodes in the loop exit basic block and the split off
/// unswitched block.
///
/// Because the exit block remains an exit from the loop, this rewrites the
/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
/// nodes into the unswitched basic block to select between the value in the
/// old preheader and the loop exit.
static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH,
bool FullUnswitch) {
assert(&ExitBB != &UnswitchedBB &&
"Must have different loop exit and unswitched blocks!");
BasicBlock::iterator InsertPt = UnswitchedBB.begin();
for (PHINode &PN : ExitBB.phis()) {
auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
PN.getName() + ".split");
NewPN->insertBefore(InsertPt);
// Walk backwards over the old PHI node's inputs to minimize the cost of
// removing each one. We have to do this weird loop manually so that we
// create the same number of new incoming edges in the new PHI as we expect
// each case-based edge to be included in the unswitched switch in some
// cases.
// FIXME: This is really, really gross. It would be much cleaner if LLVM
// allowed us to create a single entry for a predecessor block without
// having separate entries for each "edge" even though these edges are
// required to produce identical results.
for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != &OldExitingBB)
continue;
Value *Incoming = PN.getIncomingValue(i);
if (FullUnswitch)
// No more edge from the old exiting block to the exit block.
PN.removeIncomingValue(i);
NewPN->addIncoming(Incoming, &OldPH);
}
// Now replace the old PHI with the new one and wire the old one in as an
// input to the new one.
PN.replaceAllUsesWith(NewPN);
NewPN->addIncoming(&PN, &ExitBB);
}
}
/// Hoist the current loop up to the innermost loop containing a remaining exit.
///
/// Because we've removed an exit from the loop, we may have changed the set of
/// loops reachable and need to move the current loop up the loop nest or even
/// to an entirely separate nest.
static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU, ScalarEvolution *SE) {
// If the loop is already at the top level, we can't hoist it anywhere.
Loop *OldParentL = L.getParentLoop();
if (!OldParentL)
return;
SmallVector<BasicBlock *, 4> Exits;
L.getExitBlocks(Exits);
Loop *NewParentL = nullptr;
for (auto *ExitBB : Exits)
if (Loop *ExitL = LI.getLoopFor(ExitBB))
if (!NewParentL || NewParentL->contains(ExitL))
NewParentL = ExitL;
if (NewParentL == OldParentL)
return;
// The new parent loop (if different) should always contain the old one.
if (NewParentL)
assert(NewParentL->contains(OldParentL) &&
"Can only hoist this loop up the nest!");
// The preheader will need to move with the body of this loop. However,
// because it isn't in this loop we also need to update the primary loop map.
assert(OldParentL == LI.getLoopFor(&Preheader) &&
"Parent loop of this loop should contain this loop's preheader!");
LI.changeLoopFor(&Preheader, NewParentL);
// Remove this loop from its old parent.
OldParentL->removeChildLoop(&L);
// Add the loop either to the new parent or as a top-level loop.
if (NewParentL)
NewParentL->addChildLoop(&L);
else
LI.addTopLevelLoop(&L);
// Remove this loops blocks from the old parent and every other loop up the
// nest until reaching the new parent. Also update all of these
// no-longer-containing loops to reflect the nesting change.
for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
OldContainingL = OldContainingL->getParentLoop()) {
llvm::erase_if(OldContainingL->getBlocksVector(),
[&](const BasicBlock *BB) {
return BB == &Preheader || L.contains(BB);
});
OldContainingL->getBlocksSet().erase(&Preheader);
for (BasicBlock *BB : L.blocks())
OldContainingL->getBlocksSet().erase(BB);
// Because we just hoisted a loop out of this one, we have essentially
// created new exit paths from it. That means we need to form LCSSA PHI
// nodes for values used in the no-longer-nested loop.
formLCSSA(*OldContainingL, DT, &LI, SE);
// We shouldn't need to form dedicated exits because the exit introduced
// here is the (just split by unswitching) preheader. However, after trivial
// unswitching it is possible to get new non-dedicated exits out of parent
// loop so let's conservatively form dedicated exit blocks and figure out
// if we can optimize later.
formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU,
/*PreserveLCSSA*/ true);
}
}
// Return the top-most loop containing ExitBB and having ExitBB as exiting block
// or the loop containing ExitBB, if there is no parent loop containing ExitBB
// as exiting block.
static Loop *getTopMostExitingLoop(const BasicBlock *ExitBB,
const LoopInfo &LI) {
Loop *TopMost = LI.getLoopFor(ExitBB);
Loop *Current = TopMost;
while (Current) {
if (Current->isLoopExiting(ExitBB))
TopMost = Current;
Current = Current->getParentLoop();
}
return TopMost;
}
/// Unswitch a trivial branch if the condition is loop invariant.
///
/// This routine should only be called when loop code leading to the branch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and one of the successors is a loop exit. This
/// allows us to unswitch without duplicating the loop, making it trivial.
///
/// If this routine fails to unswitch the branch it returns false.
///
/// If the branch can be unswitched, this routine splits the preheader and
/// hoists the branch above that split. Preserves loop simplified form
/// (splitting the exit block as necessary). It simplifies the branch within
/// the loop to an unconditional branch but doesn't remove it entirely. Further
/// cleanup can be done with some simplifycfg like pass.
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
assert(BI.isConditional() && "Can only unswitch a conditional branch!");
LLVM_DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n");
// The loop invariant values that we want to unswitch.
TinyPtrVector<Value *> Invariants;
// When true, we're fully unswitching the branch rather than just unswitching
// some input conditions to the branch.
bool FullUnswitch = false;
Value *Cond = skipTrivialSelect(BI.getCondition());
if (L.isLoopInvariant(Cond)) {
Invariants.push_back(Cond);
FullUnswitch = true;
} else {
if (auto *CondInst = dyn_cast<Instruction>(Cond))
Invariants = collectHomogenousInstGraphLoopInvariants(L, *CondInst, LI);
if (Invariants.empty()) {
LLVM_DEBUG(dbgs() << " Couldn't find invariant inputs!\n");
return false;
}
}
// Check that one of the branch's successors exits, and which one.
bool ExitDirection = true;
int LoopExitSuccIdx = 0;
auto *LoopExitBB = BI.getSuccessor(0);
if (L.contains(LoopExitBB)) {
ExitDirection = false;
LoopExitSuccIdx = 1;
LoopExitBB = BI.getSuccessor(1);
if (L.contains(LoopExitBB)) {
LLVM_DEBUG(dbgs() << " Branch doesn't exit the loop!\n");
return false;
}
}
auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx);
auto *ParentBB = BI.getParent();
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB)) {
LLVM_DEBUG(dbgs() << " Loop exit PHI's aren't loop-invariant!\n");
return false;
}
// When unswitching only part of the branch's condition, we need the exit
// block to be reached directly from the partially unswitched input. This can
// be done when the exit block is along the true edge and the branch condition
// is a graph of `or` operations, or the exit block is along the false edge
// and the condition is a graph of `and` operations.
if (!FullUnswitch) {
if (ExitDirection ? !match(Cond, m_LogicalOr())
: !match(Cond, m_LogicalAnd())) {
LLVM_DEBUG(dbgs() << " Branch condition is in improper form for "
"non-full unswitch!\n");
return false;
}
}
LLVM_DEBUG({
dbgs() << " unswitching trivial invariant conditions for: " << BI
<< "\n";
for (Value *Invariant : Invariants) {
dbgs() << " " << *Invariant << " == true";
if (Invariant != Invariants.back())
dbgs() << " ||";
dbgs() << "\n";
}
});
// If we have scalar evolutions, we need to invalidate them including this
// loop, the loop containing the exit block and the topmost parent loop
// exiting via LoopExitBB.
if (SE) {
if (const Loop *ExitL = getTopMostExitingLoop(LoopExitBB, LI))
SE->forgetLoop(ExitL);
else
// Forget the entire nest as this exits the entire nest.
SE->forgetTopmostLoop(&L);
SE->forgetBlockAndLoopDispositions();
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we are
// unswitching. We need to split this if there are other loop predecessors.
// Because the loop is in simplified form, *any* other predecessor is enough.
BasicBlock *UnswitchedBB;
if (FullUnswitch && LoopExitBB->getUniquePredecessor()) {
assert(LoopExitBB->getUniquePredecessor() == BI.getParent() &&
"A branch's parent isn't a predecessor!");
UnswitchedBB = LoopExitBB;
} else {
UnswitchedBB =
SplitBlock(LoopExitBB, LoopExitBB->begin(), &DT, &LI, MSSAU, "", false);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Actually move the invariant uses into the unswitched position. If possible,
// we do this by moving the instructions, but when doing partial unswitching
// we do it by building a new merge of the values in the unswitched position.
OldPH->getTerminator()->eraseFromParent();
if (FullUnswitch) {
// If fully unswitching, we can use the existing branch instruction.
// Splice it into the old PH to gate reaching the new preheader and re-point
// its successors.
BI.moveBefore(*OldPH, OldPH->end());
BI.setCondition(Cond);
if (MSSAU) {
// Temporarily clone the terminator, to make MSSA update cheaper by
// separating "insert edge" updates from "remove edge" ones.
BI.clone()->insertInto(ParentBB, ParentBB->end());
} else {
// Create a new unconditional branch that will continue the loop as a new
// terminator.
Instruction *NewBI = BranchInst::Create(ContinueBB, ParentBB);
NewBI->setDebugLoc(BI.getDebugLoc());
}
BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB);
BI.setSuccessor(1 - LoopExitSuccIdx, NewPH);
} else {
// Only unswitching a subset of inputs to the condition, so we will need to
// build a new branch that merges the invariant inputs.
if (ExitDirection)
assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalOr()) &&
"Must have an `or` of `i1`s or `select i1 X, true, Y`s for the "
"condition!");
else
assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalAnd()) &&
"Must have an `and` of `i1`s or `select i1 X, Y, false`s for the"
" condition!");
buildPartialUnswitchConditionalBranch(
*OldPH, Invariants, ExitDirection, *UnswitchedBB, *NewPH,
FreezeLoopUnswitchCond, OldPH->getTerminator(), nullptr, DT);
}
// Update the dominator tree with the added edge.
DT.insertEdge(OldPH, UnswitchedBB);
// After the dominator tree was updated with the added edge, update MemorySSA
// if available.
if (MSSAU) {
SmallVector<CFGUpdate, 1> Updates;
Updates.push_back({cfg::UpdateKind::Insert, OldPH, UnswitchedBB});
MSSAU->applyInsertUpdates(Updates, DT);
}
// Finish updating dominator tree and memory ssa for full unswitch.
if (FullUnswitch) {
if (MSSAU) {
Instruction *Term = ParentBB->getTerminator();
// Remove the cloned branch instruction and create unconditional branch
// now.
Instruction *NewBI = BranchInst::Create(ContinueBB, ParentBB);
NewBI->setDebugLoc(Term->getDebugLoc());
Term->eraseFromParent();
MSSAU->removeEdge(ParentBB, LoopExitBB);
}
DT.deleteEdge(ParentBB, LoopExitBB);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Rewrite the relevant PHI nodes.
if (UnswitchedBB == LoopExitBB)
rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH);
else
rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
*ParentBB, *OldPH, FullUnswitch);
// The constant we can replace all of our invariants with inside the loop
// body. If any of the invariants have a value other than this the loop won't
// be entered.
ConstantInt *Replacement = ExitDirection
? ConstantInt::getFalse(BI.getContext())
: ConstantInt::getTrue(BI.getContext());
// Since this is an i1 condition we can also trivially replace uses of it
// within the loop with a constant.
for (Value *Invariant : Invariants)
replaceLoopInvariantUses(L, Invariant, *Replacement);
// If this was full unswitching, we may have changed the nesting relationship
// for this loop so hoist it to its correct parent if needed.
if (FullUnswitch)
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
LLVM_DEBUG(dbgs() << " done: unswitching trivial branch...\n");
++NumTrivial;
++NumBranches;
return true;
}
/// Unswitch a trivial switch if the condition is loop invariant.
///
/// This routine should only be called when loop code leading to the switch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and that at least one of the successors is a loop
/// exit. This allows us to unswitch without duplicating the loop, making it
/// trivial.
///
/// If this routine fails to unswitch the switch it returns false.
///
/// If the switch can be unswitched, this routine splits the preheader and
/// copies the switch above that split. If the default case is one of the
/// exiting cases, it copies the non-exiting cases and points them at the new
/// preheader. If the default case is not exiting, it copies the exiting cases
/// and points the default at the preheader. It preserves loop simplified form
/// (splitting the exit blocks as necessary). It simplifies the switch within
/// the loop by removing now-dead cases. If the default case is one of those
/// unswitched, it replaces its destination with a new basic block containing
/// only unreachable. Such basic blocks, while technically loop exits, are not
/// considered for unswitching so this is a stable transform and the same
/// switch will not be revisited. If after unswitching there is only a single
/// in-loop successor, the switch is further simplified to an unconditional
/// branch. Still more cleanup can be done with some simplifycfg like pass.
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
LLVM_DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n");
Value *LoopCond = SI.getCondition();
// If this isn't switching on an invariant condition, we can't unswitch it.
if (!L.isLoopInvariant(LoopCond))
return false;
auto *ParentBB = SI.getParent();
// The same check must be used both for the default and the exit cases. We
// should never leave edges from the switch instruction to a basic block that
// we are unswitching, hence the condition used to determine the default case
// needs to also be used to populate ExitCaseIndices, which is then used to
// remove cases from the switch.
auto IsTriviallyUnswitchableExitBlock = [&](BasicBlock &BBToCheck) {
// BBToCheck is not an exit block if it is inside loop L.
if (L.contains(&BBToCheck))
return false;
// BBToCheck is not trivial to unswitch if its phis aren't loop invariant.
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, BBToCheck))
return false;
// We do not unswitch a block that only has an unreachable statement, as
// it's possible this is a previously unswitched block. Only unswitch if
// either the terminator is not unreachable, or, if it is, it's not the only
// instruction in the block.
auto *TI = BBToCheck.getTerminator();
bool isUnreachable = isa<UnreachableInst>(TI);
return !isUnreachable || &*BBToCheck.getFirstNonPHIOrDbg() != TI;
};
SmallVector<int, 4> ExitCaseIndices;
for (auto Case : SI.cases())
if (IsTriviallyUnswitchableExitBlock(*Case.getCaseSuccessor()))
ExitCaseIndices.push_back(Case.getCaseIndex());
BasicBlock *DefaultExitBB = nullptr;
SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0);
if (IsTriviallyUnswitchableExitBlock(*SI.getDefaultDest())) {
DefaultExitBB = SI.getDefaultDest();
} else if (ExitCaseIndices.empty())
return false;
LLVM_DEBUG(dbgs() << " unswitching trivial switch...\n");
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// We may need to invalidate SCEVs for the outermost loop reached by any of
// the exits.
Loop *OuterL = &L;
if (DefaultExitBB) {
// Check the loop containing this exit.
Loop *ExitL = getTopMostExitingLoop(DefaultExitBB, LI);
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
}
for (unsigned Index : ExitCaseIndices) {
auto CaseI = SI.case_begin() + Index;
// Compute the outer loop from this exit.
Loop *ExitL = getTopMostExitingLoop(CaseI->getCaseSuccessor(), LI);
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
}
if (SE) {
if (OuterL)
SE->forgetLoop(OuterL);
else
SE->forgetTopmostLoop(&L);
}
if (DefaultExitBB) {
// Clear out the default destination temporarily to allow accurate
// predecessor lists to be examined below.
SI.setDefaultDest(nullptr);
}
// Store the exit cases into a separate data structure and remove them from
// the switch.
SmallVector<std::tuple<ConstantInt *, BasicBlock *,
SwitchInstProfUpdateWrapper::CaseWeightOpt>,
4> ExitCases;
ExitCases.reserve(ExitCaseIndices.size());
SwitchInstProfUpdateWrapper SIW(SI);
// We walk the case indices backwards so that we remove the last case first
// and don't disrupt the earlier indices.
for (unsigned Index : reverse(ExitCaseIndices)) {
auto CaseI = SI.case_begin() + Index;
// Save the value of this case.
auto W = SIW.getSuccessorWeight(CaseI->getSuccessorIndex());
ExitCases.emplace_back(CaseI->getCaseValue(), CaseI->getCaseSuccessor(), W);
// Delete the unswitched cases.
SIW.removeCase(CaseI);
}
// Check if after this all of the remaining cases point at the same
// successor.
BasicBlock *CommonSuccBB = nullptr;
if (SI.getNumCases() > 0 &&
all_of(drop_begin(SI.cases()), [&SI](const SwitchInst::CaseHandle &Case) {
return Case.getCaseSuccessor() == SI.case_begin()->getCaseSuccessor();
}))
CommonSuccBB = SI.case_begin()->getCaseSuccessor();
if (!DefaultExitBB) {
// If we're not unswitching the default, we need it to match any cases to
// have a common successor or if we have no cases it is the common
// successor.
if (SI.getNumCases() == 0)
CommonSuccBB = SI.getDefaultDest();
else if (SI.getDefaultDest() != CommonSuccBB)
CommonSuccBB = nullptr;
}
// Split the preheader, so that we know that there is a safe place to insert
// the switch.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
OldPH->getTerminator()->eraseFromParent();
// Now add the unswitched switch. This new switch instruction inherits the
// debug location of the old switch, because it semantically replace the old
// one.
auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH);
NewSI->setDebugLoc(SIW->getDebugLoc());
SwitchInstProfUpdateWrapper NewSIW(*NewSI);
// Rewrite the IR for the unswitched basic blocks. This requires two steps.
// First, we split any exit blocks with remaining in-loop predecessors. Then
// we update the PHIs in one of two ways depending on if there was a split.
// We walk in reverse so that we split in the same order as the cases
// appeared. This is purely for convenience of reading the resulting IR, but
// it doesn't cost anything really.
SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
// Handle the default exit if necessary.
// FIXME: It'd be great if we could merge this with the loop below but LLVM's
// ranges aren't quite powerful enough yet.
if (DefaultExitBB) {
if (pred_empty(DefaultExitBB)) {
UnswitchedExitBBs.insert(DefaultExitBB);
rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH);
} else {
auto *SplitBB =
SplitBlock(DefaultExitBB, DefaultExitBB->begin(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
}
}
// Note that we must use a reference in the for loop so that we update the
// container.
for (auto &ExitCase : reverse(ExitCases)) {
// Grab a reference to the exit block in the pair so that we can update it.
BasicBlock *ExitBB = std::get<1>(ExitCase);
// If this case is the last edge into the exit block, we can simply reuse it
// as it will no longer be a loop exit. No mapping necessary.
if (pred_empty(ExitBB)) {
// Only rewrite once.
if (UnswitchedExitBBs.insert(ExitBB).second)
rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH);
continue;
}
// Otherwise we need to split the exit block so that we retain an exit
// block from the loop and a target for the unswitched condition.
BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
if (!SplitExitBB) {
// If this is the first time we see this, do the split and remember it.
SplitExitBB = SplitBlock(ExitBB, ExitBB->begin(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
}
// Update the case pair to point to the split block.
std::get<1>(ExitCase) = SplitExitBB;
}
// Now add the unswitched cases. We do this in reverse order as we built them
// in reverse order.
for (auto &ExitCase : reverse(ExitCases)) {
ConstantInt *CaseVal = std::get<0>(ExitCase);
BasicBlock *UnswitchedBB = std::get<1>(ExitCase);
NewSIW.addCase(CaseVal, UnswitchedBB, std::get<2>(ExitCase));
}
// If the default was unswitched, re-point it and add explicit cases for
// entering the loop.
if (DefaultExitBB) {
NewSIW->setDefaultDest(DefaultExitBB);
NewSIW.setSuccessorWeight(0, DefaultCaseWeight);
// We removed all the exit cases, so we just copy the cases to the
// unswitched switch.
for (const auto &Case : SI.cases())
NewSIW.addCase(Case.getCaseValue(), NewPH,
SIW.getSuccessorWeight(Case.getSuccessorIndex()));
} else if (DefaultCaseWeight) {
// We have to set branch weight of the default case.
uint64_t SW = *DefaultCaseWeight;
for (const auto &Case : SI.cases()) {
auto W = SIW.getSuccessorWeight(Case.getSuccessorIndex());
assert(W &&
"case weight must be defined as default case weight is defined");
SW += *W;
}
NewSIW.setSuccessorWeight(0, SW);
}
// If we ended up with a common successor for every path through the switch
// after unswitching, rewrite it to an unconditional branch to make it easy
// to recognize. Otherwise we potentially have to recognize the default case
// pointing at unreachable and other complexity.
if (CommonSuccBB) {
BasicBlock *BB = SI.getParent();
// We may have had multiple edges to this common successor block, so remove
// them as predecessors. We skip the first one, either the default or the
// actual first case.
bool SkippedFirst = DefaultExitBB == nullptr;
for (auto Case : SI.cases()) {
assert(Case.getCaseSuccessor() == CommonSuccBB &&
"Non-common successor!");
(void)Case;
if (!SkippedFirst) {
SkippedFirst = true;
continue;
}
CommonSuccBB->removePredecessor(BB,
/*KeepOneInputPHIs*/ true);
}
// Now nuke the switch and replace it with a direct branch.
Instruction *NewBI = BranchInst::Create(CommonSuccBB, BB);
NewBI->setDebugLoc(SIW->getDebugLoc());
SIW.eraseFromParent();
} else if (DefaultExitBB) {
assert(SI.getNumCases() > 0 &&
"If we had no cases we'd have a common successor!");
// Move the last case to the default successor. This is valid as if the
// default got unswitched it cannot be reached. This has the advantage of
// being simple and keeping the number of edges from this switch to
// successors the same, and avoiding any PHI update complexity.
auto LastCaseI = std::prev(SI.case_end());
SI.setDefaultDest(LastCaseI->getCaseSuccessor());
SIW.setSuccessorWeight(
0, SIW.getSuccessorWeight(LastCaseI->getSuccessorIndex()));
SIW.removeCase(LastCaseI);
}
// Walk the unswitched exit blocks and the unswitched split blocks and update
// the dominator tree based on the CFG edits. While we are walking unordered
// containers here, the API for applyUpdates takes an unordered list of
// updates and requires them to not contain duplicates.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
for (auto *UnswitchedExitBB : UnswitchedExitBBs) {
DTUpdates.push_back({DT.Delete, ParentBB, UnswitchedExitBB});
DTUpdates.push_back({DT.Insert, OldPH, UnswitchedExitBB});
}
for (auto SplitUnswitchedPair : SplitExitBBMap) {
DTUpdates.push_back({DT.Delete, ParentBB, SplitUnswitchedPair.first});
DTUpdates.push_back({DT.Insert, OldPH, SplitUnswitchedPair.second});
}
if (MSSAU) {
MSSAU->applyUpdates(DTUpdates, DT, /*UpdateDT=*/true);
if (VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
} else {
DT.applyUpdates(DTUpdates);
}
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
// We may have changed the nesting relationship for this loop so hoist it to
// its correct parent if needed.
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU, SE);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
++NumTrivial;
++NumSwitches;
LLVM_DEBUG(dbgs() << " done: unswitching trivial switch...\n");
return true;
}
/// This routine scans the loop to find a branch or switch which occurs before
/// any side effects occur. These can potentially be unswitched without
/// duplicating the loop. If a branch or switch is successfully unswitched the
/// scanning continues to see if subsequent branches or switches have become
/// trivial. Once all trivial candidates have been unswitched, this routine
/// returns.
///
/// The return value indicates whether anything was unswitched (and therefore
/// changed).
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
bool Changed = false;
// If loop header has only one reachable successor we should keep looking for
// trivial condition candidates in the successor as well. An alternative is
// to constant fold conditions and merge successors into loop header (then we
// only need to check header's terminator). The reason for not doing this in
// LoopUnswitch pass is that it could potentially break LoopPassManager's
// invariants. Folding dead branches could either eliminate the current loop
// or make other loops unreachable. LCSSA form might also not be preserved
// after deleting branches. The following code keeps traversing loop header's
// successors until it finds the trivial condition candidate (condition that
// is not a constant). Since unswitching generates branches with constant
// conditions, this scenario could be very common in practice.
BasicBlock *CurrentBB = L.getHeader();
SmallPtrSet<BasicBlock *, 8> Visited;
Visited.insert(CurrentBB);
do {
// Check if there are any side-effecting instructions (e.g. stores, calls,
// volatile loads) in the part of the loop that the code *would* execute
// without unswitching.
if (MSSAU) // Possible early exit with MSSA
if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB))
if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end()))
return Changed;
if (llvm::any_of(*CurrentBB,
[](Instruction &I) { return I.mayHaveSideEffects(); }))
return Changed;
Instruction *CurrentTerm = CurrentBB->getTerminator();
if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// Don't bother trying to unswitch past a switch with a constant
// condition. This should be removed prior to running this pass by
// simplifycfg.
if (isa<Constant>(SI->getCondition()))
return Changed;
if (!unswitchTrivialSwitch(L, *SI, DT, LI, SE, MSSAU))
// Couldn't unswitch this one so we're done.
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If unswitching turned the terminator into an unconditional branch then
// we can continue. The unswitching logic specifically works to fold any
// cases it can into an unconditional branch to make it easier to
// recognize here.
auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator());
if (!BI || BI->isConditional())
return Changed;
CurrentBB = BI->getSuccessor(0);
continue;
}
auto *BI = dyn_cast<BranchInst>(CurrentTerm);
if (!BI)
// We do not understand other terminator instructions.
return Changed;
// Don't bother trying to unswitch past an unconditional branch or a branch
// with a constant value. These should be removed by simplifycfg prior to
// running this pass.
if (!BI->isConditional() ||
isa<Constant>(skipTrivialSelect(BI->getCondition())))
return Changed;
// Found a trivial condition candidate: non-foldable conditional branch. If
// we fail to unswitch this, we can't do anything else that is trivial.
if (!unswitchTrivialBranch(L, *BI, DT, LI, SE, MSSAU))
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If we only unswitched some of the conditions feeding the branch, we won't
// have collapsed it to a single successor.
BI = cast<BranchInst>(CurrentBB->getTerminator());
if (BI->isConditional())
return Changed;
// Follow the newly unconditional branch into its successor.
CurrentBB = BI->getSuccessor(0);
// When continuing, if we exit the loop or reach a previous visited block,
// then we can not reach any trivial condition candidates (unfoldable
// branch instructions or switch instructions) and no unswitch can happen.
} while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second);
return Changed;
}
/// Build the cloned blocks for an unswitched copy of the given loop.
///
/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
/// after the split block (`SplitBB`) that will be used to select between the
/// cloned and original loop.
///
/// This routine handles cloning all of the necessary loop blocks and exit
/// blocks including rewriting their instructions and the relevant PHI nodes.
/// Any loop blocks or exit blocks which are dominated by a different successor
/// than the one for this clone of the loop blocks can be trivially skipped. We
/// use the `DominatingSucc` map to determine whether a block satisfies that
/// property with a simple map lookup.
///
/// It also correctly creates the unconditional branch in the cloned
/// unswitched parent block to only point at the unswitched successor.
///
/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
/// block splitting is correctly reflected in `LoopInfo`, essentially all of
/// the cloned blocks (and their loops) are left without full `LoopInfo`
/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
/// blocks to them but doesn't create the cloned `DominatorTree` structure and
/// instead the caller must recompute an accurate DT. It *does* correctly
/// update the `AssumptionCache` provided in `AC`.
static BasicBlock *buildClonedLoopBlocks(
Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
ValueToValueMapTy &VMap,
SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU,
ScalarEvolution *SE) {
SmallVector<BasicBlock *, 4> NewBlocks;
NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
// We will need to clone a bunch of blocks, wrap up the clone operation in
// a helper.
auto CloneBlock = [&](BasicBlock *OldBB) {
// Clone the basic block and insert it before the new preheader.
BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
NewBB->moveBefore(LoopPH);
// Record this block and the mapping.
NewBlocks.push_back(NewBB);
VMap[OldBB] = NewBB;
return NewBB;
};
// We skip cloning blocks when they have a dominating succ that is not the
// succ we are cloning for.
auto SkipBlock = [&](BasicBlock *BB) {
auto It = DominatingSucc.find(BB);
return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
};
// First, clone the preheader.
auto *ClonedPH = CloneBlock(LoopPH);
// Then clone all the loop blocks, skipping the ones that aren't necessary.
for (auto *LoopBB : L.blocks())
if (!SkipBlock(LoopBB))
CloneBlock(LoopBB);
// Split all the loop exit edges so that when we clone the exit blocks, if
// any of the exit blocks are *also* a preheader for some other loop, we
// don't create multiple predecessors entering the loop header.
for (auto *ExitBB : ExitBlocks) {
if (SkipBlock(ExitBB))
continue;
// When we are going to clone an exit, we don't need to clone all the
// instructions in the exit block and we want to ensure we have an easy
// place to merge the CFG, so split the exit first. This is always safe to
// do because there cannot be any non-loop predecessors of a loop exit in
// loop simplified form.
auto *MergeBB = SplitBlock(ExitBB, ExitBB->begin(), &DT, &LI, MSSAU);
// Rearrange the names to make it easier to write test cases by having the
// exit block carry the suffix rather than the merge block carrying the
// suffix.
MergeBB->takeName(ExitBB);
ExitBB->setName(Twine(MergeBB->getName()) + ".split");
// Now clone the original exit block.
auto *ClonedExitBB = CloneBlock(ExitBB);
assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
"Cloned exit block has the wrong successor!");
// Remap any cloned instructions and create a merge phi node for them.
for (auto ZippedInsts : llvm::zip_first(
llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
llvm::make_range(ClonedExitBB->begin(),
std::prev(ClonedExitBB->end())))) {
Instruction &I = std::get<0>(ZippedInsts);
Instruction &ClonedI = std::get<1>(ZippedInsts);
// The only instructions in the exit block should be PHI nodes and
// potentially a landing pad.
assert(
(isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
"Bad instruction in exit block!");
// We should have a value map between the instruction and its clone.
assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
// Forget SCEVs based on exit phis in case SCEV looked through the phi.
if (SE)
if (auto *PN = dyn_cast<PHINode>(&I))
SE->forgetLcssaPhiWithNewPredecessor(&L, PN);
BasicBlock::iterator InsertPt = MergeBB->getFirstInsertionPt();
auto *MergePN =
PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi");
MergePN->insertBefore(InsertPt);
MergePN->setDebugLoc(InsertPt->getDebugLoc());
I.replaceAllUsesWith(MergePN);
MergePN->addIncoming(&I, ExitBB);
MergePN->addIncoming(&ClonedI, ClonedExitBB);
}
}
// Rewrite the instructions in the cloned blocks to refer to the instructions
// in the cloned blocks. We have to do this as a second pass so that we have
// everything available. Also, we have inserted new instructions which may
// include assume intrinsics, so we update the assumption cache while
// processing this.
Module *M = ClonedPH->getParent()->getParent();
for (auto *ClonedBB : NewBlocks)
for (Instruction &I : *ClonedBB) {
RemapDbgRecordRange(M, I.getDbgRecordRange(), VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
RemapInstruction(&I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
if (auto *II = dyn_cast<AssumeInst>(&I))
AC.registerAssumption(II);
}
// Update any PHI nodes in the cloned successors of the skipped blocks to not
// have spurious incoming values.
for (auto *LoopBB : L.blocks())
if (SkipBlock(LoopBB))
for (auto *SuccBB : successors(LoopBB))
if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
for (PHINode &PN : ClonedSuccBB->phis())
PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
// Remove the cloned parent as a predecessor of any successor we ended up
// cloning other than the unswitched one.
auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
for (auto *SuccBB : successors(ParentBB)) {
if (SuccBB == UnswitchedSuccBB)
continue;
auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB));
if (!ClonedSuccBB)
continue;
ClonedSuccBB->removePredecessor(ClonedParentBB,
/*KeepOneInputPHIs*/ true);
}
// Replace the cloned branch with an unconditional branch to the cloned
// unswitched successor.
auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
Instruction *ClonedTerminator = ClonedParentBB->getTerminator();
// Trivial Simplification. If Terminator is a conditional branch and
// condition becomes dead - erase it.
Value *ClonedConditionToErase = nullptr;
if (auto *BI = dyn_cast<BranchInst>(ClonedTerminator))
ClonedConditionToErase = BI->getCondition();
else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator))
ClonedConditionToErase = SI->getCondition();
Instruction *BI = BranchInst::Create(ClonedSuccBB, ClonedParentBB);
BI->setDebugLoc(ClonedTerminator->getDebugLoc());
ClonedTerminator->eraseFromParent();
if (ClonedConditionToErase)
RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr,
MSSAU);
// If there are duplicate entries in the PHI nodes because of multiple edges
// to the unswitched successor, we need to nuke all but one as we replaced it
// with a direct branch.
for (PHINode &PN : ClonedSuccBB->phis()) {
bool Found = false;
// Loop over the incoming operands backwards so we can easily delete as we
// go without invalidating the index.
for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != ClonedParentBB)
continue;
if (!Found) {
Found = true;
continue;
}
PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false);
}
}
// Record the domtree updates for the new blocks.
SmallPtrSet<BasicBlock *, 4> SuccSet;
for (auto *ClonedBB : NewBlocks) {
for (auto *SuccBB : successors(ClonedBB))
if (SuccSet.insert(SuccBB).second)
DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB});
SuccSet.clear();
}
return ClonedPH;
}
/// Recursively clone the specified loop and all of its children.
///
/// The target parent loop for the clone should be provided, or can be null if
/// the clone is a top-level loop. While cloning, all the blocks are mapped
/// with the provided value map. The entire original loop must be present in
/// the value map. The cloned loop is returned.
static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
const ValueToValueMapTy &VMap, LoopInfo &LI) {
auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
ClonedL.reserveBlocks(OrigL.getNumBlocks());
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
ClonedL.addBlockEntry(ClonedBB);
if (LI.getLoopFor(BB) == &OrigL)
LI.changeLoopFor(ClonedBB, &ClonedL);
}
};
// We specially handle the first loop because it may get cloned into
// a different parent and because we most commonly are cloning leaf loops.
Loop *ClonedRootL = LI.AllocateLoop();
if (RootParentL)
RootParentL->addChildLoop(ClonedRootL);
else
LI.addTopLevelLoop(ClonedRootL);
AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
if (OrigRootL.isInnermost())
return ClonedRootL;
// If we have a nest, we can quickly clone the entire loop nest using an
// iterative approach because it is a tree. We keep the cloned parent in the
// data structure to avoid repeatedly querying through a map to find it.
SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
// Build up the loops to clone in reverse order as we'll clone them from the
// back.
for (Loop *ChildL : llvm::reverse(OrigRootL))
LoopsToClone.push_back({ClonedRootL, ChildL});
do {
Loop *ClonedParentL, *L;
std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
Loop *ClonedL = LI.AllocateLoop();
ClonedParentL->addChildLoop(ClonedL);
AddClonedBlocksToLoop(*L, *ClonedL);
for (Loop *ChildL : llvm::reverse(*L))
LoopsToClone.push_back({ClonedL, ChildL});
} while (!LoopsToClone.empty());
return ClonedRootL;
}
/// Build the cloned loops of an original loop from unswitching.
///
/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
/// operation. We need to re-verify that there even is a loop (as the backedge
/// may not have been cloned), and even if there are remaining backedges the
/// backedge set may be different. However, we know that each child loop is
/// undisturbed, we only need to find where to place each child loop within
/// either any parent loop or within a cloned version of the original loop.
///
/// Because child loops may end up cloned outside of any cloned version of the
/// original loop, multiple cloned sibling loops may be created. All of them
/// are returned so that the newly introduced loop nest roots can be
/// identified.
static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
const ValueToValueMapTy &VMap, LoopInfo &LI,
SmallVectorImpl<Loop *> &NonChildClonedLoops) {
Loop *ClonedL = nullptr;
auto *OrigPH = OrigL.getLoopPreheader();
auto *OrigHeader = OrigL.getHeader();
auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
// We need to know the loops of the cloned exit blocks to even compute the
// accurate parent loop. If we only clone exits to some parent of the
// original parent, we want to clone into that outer loop. We also keep track
// of the loops that our cloned exit blocks participate in.
Loop *ParentL = nullptr;
SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
ClonedExitsInLoops.reserve(ExitBlocks.size());
for (auto *ExitBB : ExitBlocks)
if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoopMap[ClonedExitBB] = ExitL;
ClonedExitsInLoops.push_back(ClonedExitBB);
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
}
assert((!ParentL || ParentL == OrigL.getParentLoop() ||
ParentL->contains(OrigL.getParentLoop())) &&
"The computed parent loop should always contain (or be) the parent of "
"the original loop.");
// We build the set of blocks dominated by the cloned header from the set of
// cloned blocks out of the original loop. While not all of these will
// necessarily be in the cloned loop, it is enough to establish that they
// aren't in unreachable cycles, etc.
SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
for (auto *BB : OrigL.blocks())
if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
ClonedLoopBlocks.insert(ClonedBB);
// Rebuild the set of blocks that will end up in the cloned loop. We may have
// skipped cloning some region of this loop which can in turn skip some of
// the backedges so we have to rebuild the blocks in the loop based on the
// backedges that remain after cloning.
SmallVector<BasicBlock *, 16> Worklist;
SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
for (auto *Pred : predecessors(ClonedHeader)) {
// The only possible non-loop header predecessor is the preheader because
// we know we cloned the loop in simplified form.
if (Pred == ClonedPH)
continue;
// Because the loop was in simplified form, the only non-loop predecessor
// should be the preheader.
assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
"header other than the preheader "
"that is not part of the loop!");
// Insert this block into the loop set and on the first visit (and if it
// isn't the header we're currently walking) put it into the worklist to
// recurse through.
if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
Worklist.push_back(Pred);
}
// If we had any backedges then there *is* a cloned loop. Put the header into
// the loop set and then walk the worklist backwards to find all the blocks
// that remain within the loop after cloning.
if (!BlocksInClonedLoop.empty()) {
BlocksInClonedLoop.insert(ClonedHeader);
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(BlocksInClonedLoop.count(BB) &&
"Didn't put block into the loop set!");
// Insert any predecessors that are in the possible set into the cloned
// set, and if the insert is successful, add them to the worklist. Note
// that we filter on the blocks that are definitely reachable via the
// backedge to the loop header so we may prune out dead code within the
// cloned loop.
for (auto *Pred : predecessors(BB))
if (ClonedLoopBlocks.count(Pred) &&
BlocksInClonedLoop.insert(Pred).second)
Worklist.push_back(Pred);
}
ClonedL = LI.AllocateLoop();
if (ParentL) {
ParentL->addBasicBlockToLoop(ClonedPH, LI);
ParentL->addChildLoop(ClonedL);
} else {
LI.addTopLevelLoop(ClonedL);
}
NonChildClonedLoops.push_back(ClonedL);
ClonedL->reserveBlocks(BlocksInClonedLoop.size());
// We don't want to just add the cloned loop blocks based on how we
// discovered them. The original order of blocks was carefully built in
// a way that doesn't rely on predecessor ordering. Rather than re-invent
// that logic, we just re-walk the original blocks (and those of the child
// loops) and filter them as we add them into the cloned loop.
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
continue;
// Directly add the blocks that are only in this loop.
if (LI.getLoopFor(BB) == &OrigL) {
ClonedL->addBasicBlockToLoop(ClonedBB, LI);
continue;
}
// We want to manually add it to this loop and parents.
// Registering it with LoopInfo will happen when we clone the top
// loop for this block.
for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
PL->addBlockEntry(ClonedBB);
}
// Now add each child loop whose header remains within the cloned loop. All
// of the blocks within the loop must satisfy the same constraints as the
// header so once we pass the header checks we can just clone the entire
// child loop nest.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
continue;
#ifndef NDEBUG
// We should never have a cloned child loop header but fail to have
// all of the blocks for that child loop.
for (auto *ChildLoopBB : ChildL->blocks())
assert(BlocksInClonedLoop.count(
cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
"Child cloned loop has a header within the cloned outer "
"loop but not all of its blocks!");
#endif
cloneLoopNest(*ChildL, ClonedL, VMap, LI);
}
}
// Now that we've handled all the components of the original loop that were
// cloned into a new loop, we still need to handle anything from the original
// loop that wasn't in a cloned loop.
// Figure out what blocks are left to place within any loop nest containing
// the unswitched loop. If we never formed a loop, the cloned PH is one of
// them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
if (BlocksInClonedLoop.empty())
UnloopedBlockSet.insert(ClonedPH);
for (auto *ClonedBB : ClonedLoopBlocks)
if (!BlocksInClonedLoop.count(ClonedBB))
UnloopedBlockSet.insert(ClonedBB);
// Copy the cloned exits and sort them in ascending loop depth, we'll work
// backwards across these to process them inside out. The order shouldn't
// matter as we're just trying to build up the map from inside-out; we use
// the map in a more stably ordered way below.
auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return ExitLoopMap.lookup(LHS)->getLoopDepth() <
ExitLoopMap.lookup(RHS)->getLoopDepth();
});
// Populate the existing ExitLoopMap with everything reachable from each
// exit, starting from the inner most exit.
while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
Loop *ExitL = ExitLoopMap.lookup(ExitBB);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
Worklist.push_back(ExitBB);
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == ClonedPH)
continue;
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlockSet.erase(PredBB)) {
assert(
(BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
"Predecessor not mapped to a loop!");
continue;
}
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in an order that doesn't rely on
// predecessor order (which in turn relies on use list order).
bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
(void)Inserted;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
Worklist.push_back(PredBB);
}
} while (!Worklist.empty());
}
// Now that the ExitLoopMap gives as mapping for all the non-looping cloned
// blocks to their outer loops, walk the cloned blocks and the cloned exits
// in their original order adding them to the correct loop.
// We need a stable insertion order. We use the order of the original loop
// order and map into the correct parent loop.
for (auto *BB : llvm::concat<BasicBlock *const>(
ArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
if (Loop *OuterL = ExitLoopMap.lookup(BB))
OuterL->addBasicBlockToLoop(BB, LI);
#ifndef NDEBUG
for (auto &BBAndL : ExitLoopMap) {
auto *BB = BBAndL.first;
auto *OuterL = BBAndL.second;
assert(LI.getLoopFor(BB) == OuterL &&
"Failed to put all blocks into outer loops!");
}
#endif
// Now that all the blocks are placed into the correct containing loop in the
// absence of child loops, find all the potentially cloned child loops and
// clone them into whatever outer loop we placed their header into.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
continue;
#ifndef NDEBUG
for (auto *ChildLoopBB : ChildL->blocks())
assert(VMap.count(ChildLoopBB) &&
"Cloned a child loop header but not all of that loops blocks!");
#endif
NonChildClonedLoops.push_back(cloneLoopNest(
*ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
}
}
static void
deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
DominatorTree &DT, MemorySSAUpdater *MSSAU) {
// Find all the dead clones, and remove them from their successors.
SmallVector<BasicBlock *, 16> DeadBlocks;
for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks))
for (const auto &VMap : VMaps)
if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB)))
if (!DT.isReachableFromEntry(ClonedBB)) {
for (BasicBlock *SuccBB : successors(ClonedBB))
SuccBB->removePredecessor(ClonedBB);
DeadBlocks.push_back(ClonedBB);
}
// Remove all MemorySSA in the dead blocks
if (MSSAU) {
SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
DeadBlocks.end());
MSSAU->removeBlocks(DeadBlockSet);
}
// Drop any remaining references to break cycles.
for (BasicBlock *BB : DeadBlocks)
BB->dropAllReferences();
// Erase them from the IR.
for (BasicBlock *BB : DeadBlocks)
BB->eraseFromParent();
}
static void deleteDeadBlocksFromLoop(Loop &L,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU,
ScalarEvolution *SE,
LPMUpdater &LoopUpdater) {
// Find all the dead blocks tied to this loop, and remove them from their
// successors.
SmallSetVector<BasicBlock *, 8> DeadBlockSet;
// Start with loop/exit blocks and get a transitive closure of reachable dead
// blocks.
SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
ExitBlocks.end());
DeathCandidates.append(L.blocks().begin(), L.blocks().end());
while (!DeathCandidates.empty()) {
auto *BB = DeathCandidates.pop_back_val();
if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
for (BasicBlock *SuccBB : successors(BB)) {
SuccBB->removePredecessor(BB);
DeathCandidates.push_back(SuccBB);
}
DeadBlockSet.insert(BB);
}
}
// Remove all MemorySSA in the dead blocks
if (MSSAU)
MSSAU->removeBlocks(DeadBlockSet);
// Filter out the dead blocks from the exit blocks list so that it can be
// used in the caller.
llvm::erase_if(ExitBlocks,
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
// Walk from this loop up through its parents removing all of the dead blocks.
for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
for (auto *BB : DeadBlockSet)
ParentL->getBlocksSet().erase(BB);
llvm::erase_if(ParentL->getBlocksVector(),
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
}
// Now delete the dead child loops. This raw delete will clear them
// recursively.
llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
if (!DeadBlockSet.count(ChildL->getHeader()))
return false;
assert(llvm::all_of(ChildL->blocks(),
[&](BasicBlock *ChildBB) {
return DeadBlockSet.count(ChildBB);
}) &&
"If the child loop header is dead all blocks in the child loop must "
"be dead as well!");
LoopUpdater.markLoopAsDeleted(*ChildL, ChildL->getName());
if (SE)
SE->forgetBlockAndLoopDispositions();
LI.destroy(ChildL);
return true;
});
// Remove the loop mappings for the dead blocks and drop all the references
// from these blocks to others to handle cyclic references as we start
// deleting the blocks themselves.
for (auto *BB : DeadBlockSet) {
// Check that the dominator tree has already been updated.
assert(!DT.getNode(BB) && "Should already have cleared domtree!");
LI.changeLoopFor(BB, nullptr);
// Drop all uses of the instructions to make sure we won't have dangling
// uses in other blocks.
for (auto &I : *BB)
if (!I.use_empty())
I.replaceAllUsesWith(PoisonValue::get(I.getType()));
BB->dropAllReferences();
}
// Actually delete the blocks now that they've been fully unhooked from the
// IR.
for (auto *BB : DeadBlockSet)
BB->eraseFromParent();
}
/// Recompute the set of blocks in a loop after unswitching.
///
/// This walks from the original headers predecessors to rebuild the loop. We
/// take advantage of the fact that new blocks can't have been added, and so we
/// filter by the original loop's blocks. This also handles potentially
/// unreachable code that we don't want to explore but might be found examining
/// the predecessors of the header.
///
/// If the original loop is no longer a loop, this will return an empty set. If
/// it remains a loop, all the blocks within it will be added to the set
/// (including those blocks in inner loops).
static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
LoopInfo &LI) {
SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
auto *PH = L.getLoopPreheader();
auto *Header = L.getHeader();
// A worklist to use while walking backwards from the header.
SmallVector<BasicBlock *, 16> Worklist;
// First walk the predecessors of the header to find the backedges. This will
// form the basis of our walk.
for (auto *Pred : predecessors(Header)) {
// Skip the preheader.
if (Pred == PH)
continue;
// Because the loop was in simplified form, the only non-loop predecessor
// is the preheader.
assert(L.contains(Pred) && "Found a predecessor of the loop header other "
"than the preheader that is not part of the "
"loop!");
// Insert this block into the loop set and on the first visit and, if it
// isn't the header we're currently walking, put it into the worklist to
// recurse through.
if (LoopBlockSet.insert(Pred).second && Pred != Header)
Worklist.push_back(Pred);
}
// If no backedges were found, we're done.
if (LoopBlockSet.empty())
return LoopBlockSet;
// We found backedges, recurse through them to identify the loop blocks.
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
// No need to walk past the header.
if (BB == Header)
continue;
// Because we know the inner loop structure remains valid we can use the
// loop structure to jump immediately across the entire nested loop.
// Further, because it is in loop simplified form, we can directly jump
// to its preheader afterward.
if (Loop *InnerL = LI.getLoopFor(BB))
if (InnerL != &L) {
assert(L.contains(InnerL) &&
"Should not reach a loop *outside* this loop!");
// The preheader is the only possible predecessor of the loop so
// insert it into the set and check whether it was already handled.
auto *InnerPH = InnerL->getLoopPreheader();
assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
"but not contain the inner loop "
"preheader!");
if (!LoopBlockSet.insert(InnerPH).second)
// The only way to reach the preheader is through the loop body
// itself so if it has been visited the loop is already handled.
continue;
// Insert all of the blocks (other than those already present) into
// the loop set. We expect at least the block that led us to find the
// inner loop to be in the block set, but we may also have other loop
// blocks if they were already enqueued as predecessors of some other
// outer loop block.
for (auto *InnerBB : InnerL->blocks()) {
if (InnerBB == BB) {
assert(LoopBlockSet.count(InnerBB) &&
"Block should already be in the set!");
continue;
}
LoopBlockSet.insert(InnerBB);
}
// Add the preheader to the worklist so we will continue past the
// loop body.
Worklist.push_back(InnerPH);
continue;
}
// Insert any predecessors that were in the original loop into the new
// set, and if the insert is successful, add them to the worklist.
for (auto *Pred : predecessors(BB))
if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
Worklist.push_back(Pred);
}
assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
// We've found all the blocks participating in the loop, return our completed
// set.
return LoopBlockSet;
}
/// Rebuild a loop after unswitching removes some subset of blocks and edges.
///
/// The removal may have removed some child loops entirely but cannot have
/// disturbed any remaining child loops. However, they may need to be hoisted
/// to the parent loop (or to be top-level loops). The original loop may be
/// completely removed.
///
/// The sibling loops resulting from this update are returned. If the original
/// loop remains a valid loop, it will be the first entry in this list with all
/// of the newly sibling loops following it.
///
/// Returns true if the loop remains a loop after unswitching, and false if it
/// is no longer a loop after unswitching (and should not continue to be
/// referenced).
static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
LoopInfo &LI,
SmallVectorImpl<Loop *> &HoistedLoops,
ScalarEvolution *SE) {
auto *PH = L.getLoopPreheader();
// Compute the actual parent loop from the exit blocks. Because we may have
// pruned some exits the loop may be different from the original parent.
Loop *ParentL = nullptr;
SmallVector<Loop *, 4> ExitLoops;
SmallVector<BasicBlock *, 4> ExitsInLoops;
ExitsInLoops.reserve(ExitBlocks.size());
for (auto *ExitBB : ExitBlocks)
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoops.push_back(ExitL);
ExitsInLoops.push_back(ExitBB);
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
}
// Recompute the blocks participating in this loop. This may be empty if it
// is no longer a loop.
auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
// If we still have a loop, we need to re-set the loop's parent as the exit
// block set changing may have moved it within the loop nest. Note that this
// can only happen when this loop has a parent as it can only hoist the loop
// *up* the nest.
if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
// Remove this loop's (original) blocks from all of the intervening loops.
for (Loop *IL = L.getParentLoop(); IL != ParentL;
IL = IL->getParentLoop()) {
IL->getBlocksSet().erase(PH);
for (auto *BB : L.blocks())
IL->getBlocksSet().erase(BB);
llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
return BB == PH || L.contains(BB);
});
}
LI.changeLoopFor(PH, ParentL);
L.getParentLoop()->removeChildLoop(&L);
if (ParentL)
ParentL->addChildLoop(&L);
else
LI.addTopLevelLoop(&L);
}
// Now we update all the blocks which are no longer within the loop.
auto &Blocks = L.getBlocksVector();
auto BlocksSplitI =
LoopBlockSet.empty()
? Blocks.begin()
: std::stable_partition(
Blocks.begin(), Blocks.end(),
[&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
// Before we erase the list of unlooped blocks, build a set of them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
if (LoopBlockSet.empty())
UnloopedBlocks.insert(PH);
// Now erase these blocks from the loop.
for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
L.getBlocksSet().erase(BB);
Blocks.erase(BlocksSplitI, Blocks.end());
// Sort the exits in ascending loop depth, we'll work backwards across these
// to process them inside out.
llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
});
// We'll build up a set for each exit loop.
SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
auto RemoveUnloopedBlocksFromLoop =
[](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
for (auto *BB : UnloopedBlocks)
L.getBlocksSet().erase(BB);
llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
return UnloopedBlocks.count(BB);
});
};
SmallVector<BasicBlock *, 16> Worklist;
while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
// Grab the next exit block, in decreasing loop depth order.
BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
Loop &ExitL = *LI.getLoopFor(ExitBB);
assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
// Erase all of the unlooped blocks from the loops between the previous
// exit loop and this exit loop. This works because the ExitInLoops list is
// sorted in increasing order of loop depth and thus we visit loops in
// decreasing order of loop depth.
for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
Worklist.push_back(ExitBB);
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == PH)
continue;
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlocks.erase(PredBB)) {
assert((NewExitLoopBlocks.count(PredBB) ||
ExitL.contains(LI.getLoopFor(PredBB))) &&
"Predecessor not in a nested loop (or already visited)!");
continue;
}
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in a deterministic order rather
// than the predecessor-influenced visit order.
bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
(void)Inserted;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
Worklist.push_back(PredBB);
}
} while (!Worklist.empty());
// If blocks in this exit loop were directly part of the original loop (as
// opposed to a child loop) update the map to point to this exit loop. This
// just updates a map and so the fact that the order is unstable is fine.
for (auto *BB : NewExitLoopBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, &ExitL);
// We will remove the remaining unlooped blocks from this loop in the next
// iteration or below.
NewExitLoopBlocks.clear();
}
// Any remaining unlooped blocks are no longer part of any loop unless they
// are part of some child loop.
for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
for (auto *BB : UnloopedBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, nullptr);
// Sink all the child loops whose headers are no longer in the loop set to
// the parent (or to be top level loops). We reach into the loop and directly
// update its subloop vector to make this batch update efficient.
auto &SubLoops = L.getSubLoopsVector();
auto SubLoopsSplitI =
LoopBlockSet.empty()
? SubLoops.begin()
: std::stable_partition(
SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
return LoopBlockSet.count(SubL->getHeader());
});
for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
HoistedLoops.push_back(HoistedL);
HoistedL->setParentLoop(nullptr);
// To compute the new parent of this hoisted loop we look at where we
// placed the preheader above. We can't lookup the header itself because we
// retained the mapping from the header to the hoisted loop. But the
// preheader and header should have the exact same new parent computed
// based on the set of exit blocks from the original loop as the preheader
// is a predecessor of the header and so reached in the reverse walk. And
// because the loops were all in simplified form the preheader of the
// hoisted loop can't be part of some *other* loop.
if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
NewParentL->addChildLoop(HoistedL);
else
LI.addTopLevelLoop(HoistedL);
}
SubLoops.erase(SubLoopsSplitI, SubLoops.end());
// Actually delete the loop if nothing remained within it.
if (Blocks.empty()) {
assert(SubLoops.empty() &&
"Failed to remove all subloops from the original loop!");
if (Loop *ParentL = L.getParentLoop())
ParentL->removeChildLoop(llvm::find(*ParentL, &L));
else
LI.removeLoop(llvm::find(LI, &L));
// markLoopAsDeleted for L should be triggered by the caller (it is
// typically done within postUnswitch).
if (SE)
SE->forgetBlockAndLoopDispositions();
LI.destroy(&L);
return false;
}
return true;
}
/// Helper to visit a dominator subtree, invoking a callable on each node.
///
/// Returning false at any point will stop walking past that node of the tree.
template <typename CallableT>
void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
SmallVector<DomTreeNode *, 4> DomWorklist;
DomWorklist.push_back(DT[BB]);
#ifndef NDEBUG
SmallPtrSet<DomTreeNode *, 4> Visited;
Visited.insert(DT[BB]);
#endif
do {
DomTreeNode *N = DomWorklist.pop_back_val();
// Visit this node.
if (!Callable(N->getBlock()))
continue;
// Accumulate the child nodes.
for (DomTreeNode *ChildN : *N) {
assert(Visited.insert(ChildN).second &&
"Cannot visit a node twice when walking a tree!");
DomWorklist.push_back(ChildN);
}
} while (!DomWorklist.empty());
}
void postUnswitch(Loop &L, LPMUpdater &U, StringRef LoopName,
bool CurrentLoopValid, bool PartiallyInvariant,
bool InjectedCondition, ArrayRef<Loop *> NewLoops) {
// If we did a non-trivial unswitch, we have added new (cloned) loops.
if (!NewLoops.empty())
U.addSiblingLoops(NewLoops);
// If the current loop remains valid, we should revisit it to catch any
// other unswitch opportunities. Otherwise, we need to mark it as deleted.
if (CurrentLoopValid) {
if (PartiallyInvariant) {
// Mark the new loop as partially unswitched, to avoid unswitching on
// the same condition again.
auto &Context = L.getHeader()->getContext();
MDNode *DisableUnswitchMD = MDNode::get(
Context,
MDString::get(Context, "llvm.loop.unswitch.partial.disable"));
MDNode *NewLoopID = makePostTransformationMetadata(
Context, L.getLoopID(), {"llvm.loop.unswitch.partial"},
{DisableUnswitchMD});
L.setLoopID(NewLoopID);
} else if (InjectedCondition) {
// Do the same for injection of invariant conditions.
auto &Context = L.getHeader()->getContext();
MDNode *DisableUnswitchMD = MDNode::get(
Context,
MDString::get(Context, "llvm.loop.unswitch.injection.disable"));
MDNode *NewLoopID = makePostTransformationMetadata(
Context, L.getLoopID(), {"llvm.loop.unswitch.injection"},
{DisableUnswitchMD});
L.setLoopID(NewLoopID);
} else
U.revisitCurrentLoop();
} else
U.markLoopAsDeleted(L, LoopName);
}
static void unswitchNontrivialInvariants(
Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
IVConditionInfo &PartialIVInfo, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
LPMUpdater &LoopUpdater, bool InsertFreeze, bool InjectedCondition) {
auto *ParentBB = TI.getParent();
BranchInst *BI = dyn_cast<BranchInst>(&TI);
SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI);
// Save the current loop name in a variable so that we can report it even
// after it has been deleted.
std::string LoopName(L.getName());
// We can only unswitch switches, conditional branches with an invariant
// condition, or combining invariant conditions with an instruction or
// partially invariant instructions.
assert((SI || (BI && BI->isConditional())) &&
"Can only unswitch switches and conditional branch!");
bool PartiallyInvariant = !PartialIVInfo.InstToDuplicate.empty();
bool FullUnswitch =
SI || (skipTrivialSelect(BI->getCondition()) == Invariants[0] &&
!PartiallyInvariant);
if (FullUnswitch)
assert(Invariants.size() == 1 &&
"Cannot have other invariants with full unswitching!");
else
assert(isa<Instruction>(skipTrivialSelect(BI->getCondition())) &&
"Partial unswitching requires an instruction as the condition!");
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Constant and BBs tracking the cloned and continuing successor. When we are
// unswitching the entire condition, this can just be trivially chosen to
// unswitch towards `true`. However, when we are unswitching a set of
// invariants combined with `and` or `or` or partially invariant instructions,
// the combining operation determines the best direction to unswitch: we want
// to unswitch the direction that will collapse the branch.
bool Direction = true;
int ClonedSucc = 0;
if (!FullUnswitch) {
Value *Cond = skipTrivialSelect(BI->getCondition());
(void)Cond;
assert(((match(Cond, m_LogicalAnd()) ^ match(Cond, m_LogicalOr())) ||
PartiallyInvariant) &&
"Only `or`, `and`, an `select`, partially invariant instructions "
"can combine invariants being unswitched.");
if (!match(Cond, m_LogicalOr())) {
if (match(Cond, m_LogicalAnd()) ||
(PartiallyInvariant && !PartialIVInfo.KnownValue->isOneValue())) {
Direction = false;
ClonedSucc = 1;
}
}
}
BasicBlock *RetainedSuccBB =
BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest();
SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
if (BI)
UnswitchedSuccBBs.insert(BI->getSuccessor(ClonedSucc));
else
for (auto Case : SI->cases())
if (Case.getCaseSuccessor() != RetainedSuccBB)
UnswitchedSuccBBs.insert(Case.getCaseSuccessor());
assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
"Should not unswitch the same successor we are retaining!");
// The branch should be in this exact loop. Any inner loop's invariant branch
// should be handled by unswitching that inner loop. The caller of this
// routine should filter out any candidates that remain (but were skipped for
// whatever reason).
assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
// Compute the parent loop now before we start hacking on things.
Loop *ParentL = L.getParentLoop();
// Get blocks in RPO order for MSSA update, before changing the CFG.
LoopBlocksRPO LBRPO(&L);
if (MSSAU)
LBRPO.perform(&LI);
// Compute the outer-most loop containing one of our exit blocks. This is the
// furthest up our loopnest which can be mutated, which we will use below to
// update things.
Loop *OuterExitL = &L;
SmallVector<BasicBlock *, 4> ExitBlocks;
L.getUniqueExitBlocks(ExitBlocks);
for (auto *ExitBB : ExitBlocks) {
// ExitBB can be an exit block for several levels in the loop nest. Make
// sure we find the top most.
Loop *NewOuterExitL = getTopMostExitingLoop(ExitBB, LI);
if (!NewOuterExitL) {
// We exited the entire nest with this block, so we're done.
OuterExitL = nullptr;
break;
}
if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
OuterExitL = NewOuterExitL;
}
// At this point, we're definitely going to unswitch something so invalidate
// any cached information in ScalarEvolution for the outer most loop
// containing an exit block and all nested loops.
if (SE) {
if (OuterExitL)
SE->forgetLoop(OuterExitL);
else
SE->forgetTopmostLoop(&L);
SE->forgetBlockAndLoopDispositions();
}
// If the edge from this terminator to a successor dominates that successor,
// store a map from each block in its dominator subtree to it. This lets us
// tell when cloning for a particular successor if a block is dominated by
// some *other* successor with a single data structure. We use this to
// significantly reduce cloning.
SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
for (auto *SuccBB : llvm::concat<BasicBlock *const>(ArrayRef(RetainedSuccBB),
UnswitchedSuccBBs))
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == ParentBB || DT.dominates(SuccBB, PredBB);
}))
visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) {
DominatingSucc[BB] = SuccBB;
return true;
});
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond. The original preheader will become the split point
// between the unswitched versions, and we will have a new preheader for the
// original loop.
BasicBlock *SplitBB = L.getLoopPreheader();
BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU);
// Keep track of the dominator tree updates needed.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
// Clone the loop for each unswitched successor.
SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
VMaps.reserve(UnswitchedSuccBBs.size());
SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
for (auto *SuccBB : UnswitchedSuccBBs) {
VMaps.emplace_back(new ValueToValueMapTy());
ClonedPHs[SuccBB] = buildClonedLoopBlocks(
L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU, SE);
}
// Drop metadata if we may break its semantics by moving this instr into the
// split block.
if (TI.getMetadata(LLVMContext::MD_make_implicit)) {
if (DropNonTrivialImplicitNullChecks)
// Do not spend time trying to understand if we can keep it, just drop it
// to save compile time.
TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
else {
// It is only legal to preserve make.implicit metadata if we are
// guaranteed no reach implicit null check after following this branch.
ICFLoopSafetyInfo SafetyInfo;
SafetyInfo.computeLoopSafetyInfo(&L);
if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
}
}
// The stitching of the branched code back together depends on whether we're
// doing full unswitching or not with the exception that we always want to
// nuke the initial terminator placed in the split block.
SplitBB->getTerminator()->eraseFromParent();
if (FullUnswitch) {
// Keep a clone of the terminator for MSSA updates.
Instruction *NewTI = TI.clone();
NewTI->insertInto(ParentBB, ParentBB->end());
// Splice the terminator from the original loop and rewrite its
// successors.
TI.moveBefore(*SplitBB, SplitBB->end());
TI.dropLocation();
// First wire up the moved terminator to the preheaders.
if (BI) {
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
BI->setSuccessor(ClonedSucc, ClonedPH);
BI->setSuccessor(1 - ClonedSucc, LoopPH);
Value *Cond = skipTrivialSelect(BI->getCondition());
if (InsertFreeze) {
// We don't give any debug location to the new freeze, because the
// BI (`dyn_cast<BranchInst>(TI)`) is an in-loop instruction hoisted
// out of the loop.
Cond = new FreezeInst(Cond, Cond->getName() + ".fr", BI->getIterator());
}
BI->setCondition(Cond);
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
} else {
assert(SI && "Must either be a branch or switch!");
// Walk the cases and directly update their successors.
assert(SI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
SI->setDefaultDest(LoopPH);
for (const auto &Case : SI->cases())
if (Case.getCaseSuccessor() == RetainedSuccBB)
Case.setSuccessor(LoopPH);
else
Case.setSuccessor(ClonedPHs.find(Case.getCaseSuccessor())->second);
if (InsertFreeze)
SI->setCondition(new FreezeInst(SI->getCondition(),
SI->getCondition()->getName() + ".fr",
SI->getIterator()));
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
DTUpdates.push_back(
{DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second});
}
if (MSSAU) {
DT.applyUpdates(DTUpdates);
DTUpdates.clear();
// Remove all but one edge to the retained block and all unswitched
// blocks. This is to avoid having duplicate entries in the cloned Phis,
// when we know we only keep a single edge for each case.
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB);
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB);
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
/*IgnoreIncomingWithNoClones=*/true);
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
// Remove all edges to unswitched blocks.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeEdge(ParentBB, SuccBB);
}
// Now unhook the successor relationship as we'll be replacing
// the terminator with a direct branch. This is much simpler for branches
// than switches so we handle those first.
if (BI) {
// Remove the parent as a predecessor of the unswitched successor.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
UnswitchedSuccBB->removePredecessor(ParentBB,
/*KeepOneInputPHIs*/ true);
DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
} else {
// Note that we actually want to remove the parent block as a predecessor
// of *every* case successor. The case successor is either unswitched,
// completely eliminating an edge from the parent to that successor, or it
// is a duplicate edge to the retained successor as the retained successor
// is always the default successor and as we'll replace this with a direct
// branch we no longer need the duplicate entries in the PHI nodes.
SwitchInst *NewSI = cast<SwitchInst>(NewTI);
assert(NewSI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
for (const auto &Case : NewSI->cases())
Case.getCaseSuccessor()->removePredecessor(
ParentBB,
/*KeepOneInputPHIs*/ true);
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
}
// Create a new unconditional branch to the continuing block (as opposed to
// the one cloned).
Instruction *NewBI = BranchInst::Create(RetainedSuccBB, ParentBB);
NewBI->setDebugLoc(NewTI->getDebugLoc());
// After MSSAU update, remove the cloned terminator instruction NewTI.
NewTI->eraseFromParent();
} else {
assert(BI && "Only branches have partial unswitching.");
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When doing a partial unswitch, we have to do a bit more work to build up
// the branch in the split block.
if (PartiallyInvariant)
buildPartialInvariantUnswitchConditionalBranch(
*SplitBB, Invariants, Direction, *ClonedPH, *LoopPH, L, MSSAU);
else {
buildPartialUnswitchConditionalBranch(
*SplitBB, Invariants, Direction, *ClonedPH, *LoopPH,
FreezeLoopUnswitchCond, BI, &AC, DT);
}
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
if (MSSAU) {
DT.applyUpdates(DTUpdates);
DTUpdates.clear();
// Perform MSSA cloning updates.
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
/*IgnoreIncomingWithNoClones=*/true);
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
}
}
// Apply the updates accumulated above to get an up-to-date dominator tree.
DT.applyUpdates(DTUpdates);
// Now that we have an accurate dominator tree, first delete the dead cloned
// blocks so that we can accurately build any cloned loops. It is important to
// not delete the blocks from the original loop yet because we still want to
// reference the original loop to understand the cloned loop's structure.
deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
// Build the cloned loop structure itself. This may be substantially
// different from the original structure due to the simplified CFG. This also
// handles inserting all the cloned blocks into the correct loops.
SmallVector<Loop *, 4> NonChildClonedLoops;
for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops);
// Now that our cloned loops have been built, we can update the original loop.
// First we delete the dead blocks from it and then we rebuild the loop
// structure taking these deletions into account.
deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU, SE, LoopUpdater);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
SmallVector<Loop *, 4> HoistedLoops;
bool IsStillLoop =
rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops, SE);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// This transformation has a high risk of corrupting the dominator tree, and
// the below steps to rebuild loop structures will result in hard to debug
// errors in that case so verify that the dominator tree is sane first.
// FIXME: Remove this when the bugs stop showing up and rely on existing
// verification steps.
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
if (BI && !PartiallyInvariant) {
// If we unswitched a branch which collapses the condition to a known
// constant we want to replace all the uses of the invariants within both
// the original and cloned blocks. We do this here so that we can use the
// now updated dominator tree to identify which side the users are on.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When considering multiple partially-unswitched invariants
// we cant just go replace them with constants in both branches.
//
// For 'AND' we infer that true branch ("continue") means true
// for each invariant operand.
// For 'OR' we can infer that false branch ("continue") means false
// for each invariant operand.
// So it happens that for multiple-partial case we dont replace
// in the unswitched branch.
bool ReplaceUnswitched =
FullUnswitch || (Invariants.size() == 1) || PartiallyInvariant;
ConstantInt *UnswitchedReplacement =
Direction ? ConstantInt::getTrue(BI->getContext())
: ConstantInt::getFalse(BI->getContext());
ConstantInt *ContinueReplacement =
Direction ? ConstantInt::getFalse(BI->getContext())
: ConstantInt::getTrue(BI->getContext());
for (Value *Invariant : Invariants) {
assert(!isa<Constant>(Invariant) &&
"Should not be replacing constant values!");
// Use make_early_inc_range here as set invalidates the iterator.
for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
Instruction *UserI = dyn_cast<Instruction>(U.getUser());
if (!UserI)
continue;
// Replace it with the 'continue' side if in the main loop body, and the
// unswitched if in the cloned blocks.
if (DT.dominates(LoopPH, UserI->getParent()))
U.set(ContinueReplacement);
else if (ReplaceUnswitched &&
DT.dominates(ClonedPH, UserI->getParent()))
U.set(UnswitchedReplacement);
}
}
}
// We can change which blocks are exit blocks of all the cloned sibling
// loops, the current loop, and any parent loops which shared exit blocks
// with the current loop. As a consequence, we need to re-form LCSSA for
// them. But we shouldn't need to re-form LCSSA for any child loops.
// FIXME: This could be made more efficient by tracking which exit blocks are
// new, and focusing on them, but that isn't likely to be necessary.
//
// In order to reasonably rebuild LCSSA we need to walk inside-out across the
// loop nest and update every loop that could have had its exits changed. We
// also need to cover any intervening loops. We add all of these loops to
// a list and sort them by loop depth to achieve this without updating
// unnecessary loops.
auto UpdateLoop = [&](Loop &UpdateL) {
#ifndef NDEBUG
UpdateL.verifyLoop();
for (Loop *ChildL : UpdateL) {
ChildL->verifyLoop();
assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
"Perturbed a child loop's LCSSA form!");
}
#endif
// First build LCSSA for this loop so that we can preserve it when
// forming dedicated exits. We don't want to perturb some other loop's
// LCSSA while doing that CFG edit.
formLCSSA(UpdateL, DT, &LI, SE);
// For loops reached by this loop's original exit blocks we may
// introduced new, non-dedicated exits. At least try to re-form dedicated
// exits for these loops. This may fail if they couldn't have dedicated
// exits to start with.
formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true);
};
// For non-child cloned loops and hoisted loops, we just need to update LCSSA
// and we can do it in any order as they don't nest relative to each other.
//
// Also check if any of the loops we have updated have become top-level loops
// as that will necessitate widening the outer loop scope.
for (Loop *UpdatedL :
llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) {
UpdateLoop(*UpdatedL);
if (UpdatedL->isOutermost())
OuterExitL = nullptr;
}
if (IsStillLoop) {
UpdateLoop(L);
if (L.isOutermost())
OuterExitL = nullptr;
}
// If the original loop had exit blocks, walk up through the outer most loop
// of those exit blocks to update LCSSA and form updated dedicated exits.
if (OuterExitL != &L)
for (Loop *OuterL = ParentL; OuterL != OuterExitL;
OuterL = OuterL->getParentLoop())
UpdateLoop(*OuterL);
#ifndef NDEBUG
// Verify the entire loop structure to catch any incorrect updates before we
// progress in the pass pipeline.
LI.verify(DT);
#endif
// Now that we've unswitched something, make callbacks to report the changes.
// For that we need to merge together the updated loops and the cloned loops
// and check whether the original loop survived.
SmallVector<Loop *, 4> SibLoops;
for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
if (UpdatedL->getParentLoop() == ParentL)
SibLoops.push_back(UpdatedL);
postUnswitch(L, LoopUpdater, LoopName, IsStillLoop, PartiallyInvariant,
InjectedCondition, SibLoops);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
if (BI)
++NumBranches;
else
++NumSwitches;
}
/// Recursively compute the cost of a dominator subtree based on the per-block
/// cost map provided.
///
/// The recursive computation is memozied into the provided DT-indexed cost map
/// to allow querying it for most nodes in the domtree without it becoming
/// quadratic.
static InstructionCost computeDomSubtreeCost(
DomTreeNode &N,
const SmallDenseMap<BasicBlock *, InstructionCost, 4> &BBCostMap,
SmallDenseMap<DomTreeNode *, InstructionCost, 4> &DTCostMap) {
// Don't accumulate cost (or recurse through) blocks not in our block cost
// map and thus not part of the duplication cost being considered.
auto BBCostIt = BBCostMap.find(N.getBlock());
if (BBCostIt == BBCostMap.end())
return 0;
// Lookup this node to see if we already computed its cost.
auto DTCostIt = DTCostMap.find(&N);
if (DTCostIt != DTCostMap.end())
return DTCostIt->second;
// If not, we have to compute it. We can't use insert above and update
// because computing the cost may insert more things into the map.
InstructionCost Cost = std::accumulate(
N.begin(), N.end(), BBCostIt->second,
[&](InstructionCost Sum, DomTreeNode *ChildN) -> InstructionCost {
return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
});
bool Inserted = DTCostMap.insert({&N, Cost}).second;
(void)Inserted;
assert(Inserted && "Should not insert a node while visiting children!");
return Cost;
}
/// Turns a select instruction into implicit control flow branch,
/// making the following replacement:
///
/// head:
/// --code before select--
/// select %cond, %trueval, %falseval
/// --code after select--
///
/// into
///
/// head:
/// --code before select--
/// br i1 %cond, label %then, label %tail
///
/// then:
/// br %tail
///
/// tail:
/// phi [ %trueval, %then ], [ %falseval, %head]
/// unreachable
///
/// It also makes all relevant DT and LI updates, so that all structures are in
/// valid state after this transform.
static BranchInst *turnSelectIntoBranch(SelectInst *SI, DominatorTree &DT,
LoopInfo &LI, MemorySSAUpdater *MSSAU,
AssumptionCache *AC) {
LLVM_DEBUG(dbgs() << "Turning " << *SI << " into a branch.\n");
BasicBlock *HeadBB = SI->getParent();
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
SplitBlockAndInsertIfThen(SI->getCondition(), SI, false,
SI->getMetadata(LLVMContext::MD_prof), &DTU, &LI);
auto *CondBr = cast<BranchInst>(HeadBB->getTerminator());
BasicBlock *ThenBB = CondBr->getSuccessor(0),
*TailBB = CondBr->getSuccessor(1);
if (MSSAU)
MSSAU->moveAllAfterSpliceBlocks(HeadBB, TailBB, SI);
PHINode *Phi =
PHINode::Create(SI->getType(), 2, "unswitched.select", SI->getIterator());
Phi->addIncoming(SI->getTrueValue(), ThenBB);
Phi->addIncoming(SI->getFalseValue(), HeadBB);
Phi->setDebugLoc(SI->getDebugLoc());
SI->replaceAllUsesWith(Phi);
SI->eraseFromParent();
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
++NumSelects;
return CondBr;
}
/// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
/// making the following replacement:
///
/// --code before guard--
/// call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
/// --code after guard--
///
/// into
///
/// --code before guard--
/// br i1 %cond, label %guarded, label %deopt
///
/// guarded:
/// --code after guard--
///
/// deopt:
/// call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
/// unreachable
///
/// It also makes all relevant DT and LI updates, so that all structures are in
/// valid state after this transform.
static BranchInst *turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU) {
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
BasicBlock *CheckBB = GI->getParent();
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
Instruction *DeoptBlockTerm =
SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true,
GI->getMetadata(LLVMContext::MD_prof), &DTU, &LI);
BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
// SplitBlockAndInsertIfThen inserts control flow that branches to
// DeoptBlockTerm if the condition is true. We want the opposite.
CheckBI->swapSuccessors();
BasicBlock *GuardedBlock = CheckBI->getSuccessor(0);
GuardedBlock->setName("guarded");
CheckBI->getSuccessor(1)->setName("deopt");
BasicBlock *DeoptBlock = CheckBI->getSuccessor(1);
if (MSSAU)
MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI);
GI->moveBefore(DeoptBlockTerm->getIterator());
GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext()));
if (MSSAU) {
MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI));
MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::BeforeTerminator);
if (VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
}
if (VerifyLoopInfo)
LI.verify(DT);
++NumGuards;
return CheckBI;
}
/// Cost multiplier is a way to limit potentially exponential behavior
/// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch
/// candidates available. Also accounting for the number of "sibling" loops with
/// the idea to account for previous unswitches that already happened on this
/// cluster of loops. There was an attempt to keep this formula simple,
/// just enough to limit the worst case behavior. Even if it is not that simple
/// now it is still not an attempt to provide a detailed heuristic size
/// prediction.
///
/// TODO: Make a proper accounting of "explosion" effect for all kinds of
/// unswitch candidates, making adequate predictions instead of wild guesses.
/// That requires knowing not just the number of "remaining" candidates but
/// also costs of unswitching for each of these candidates.
static int CalculateUnswitchCostMultiplier(
const Instruction &TI, const Loop &L, const LoopInfo &LI,
const DominatorTree &DT,
ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates) {
// Guards and other exiting conditions do not contribute to exponential
// explosion as soon as they dominate the latch (otherwise there might be
// another path to the latch remaining that does not allow to eliminate the
// loop copy on unswitch).
const BasicBlock *Latch = L.getLoopLatch();
const BasicBlock *CondBlock = TI.getParent();
if (DT.dominates(CondBlock, Latch) &&
(isGuard(&TI) ||
(TI.isTerminator() &&
llvm::count_if(successors(&TI), [&L](const BasicBlock *SuccBB) {
return L.contains(SuccBB);
}) <= 1))) {
NumCostMultiplierSkipped++;
return 1;
}
auto *ParentL = L.getParentLoop();
int SiblingsCount = (ParentL ? ParentL->getSubLoopsVector().size()
: std::distance(LI.begin(), LI.end()));
// Count amount of clones that all the candidates might cause during
// unswitching. Branch/guard/select counts as 1, switch counts as log2 of its
// cases.
int UnswitchedClones = 0;
for (const auto &Candidate : UnswitchCandidates) {
const Instruction *CI = Candidate.TI;
const BasicBlock *CondBlock = CI->getParent();
bool SkipExitingSuccessors = DT.dominates(CondBlock, Latch);
if (isa<SelectInst>(CI)) {
UnswitchedClones++;
continue;
}
if (isGuard(CI)) {
if (!SkipExitingSuccessors)
UnswitchedClones++;
continue;
}
int NonExitingSuccessors =
llvm::count_if(successors(CondBlock),
[SkipExitingSuccessors, &L](const BasicBlock *SuccBB) {
return !SkipExitingSuccessors || L.contains(SuccBB);
});
UnswitchedClones += Log2_32(NonExitingSuccessors);
}
// Ignore up to the "unscaled candidates" number of unswitch candidates
// when calculating the power-of-two scaling of the cost. The main idea
// with this control is to allow a small number of unswitches to happen
// and rely more on siblings multiplier (see below) when the number
// of candidates is small.
unsigned ClonesPower =
std::max(UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, 0);
// Allowing top-level loops to spread a bit more than nested ones.
int SiblingsMultiplier =
std::max((ParentL ? SiblingsCount
: SiblingsCount / (int)UnswitchSiblingsToplevelDiv),
1);
// Compute the cost multiplier in a way that won't overflow by saturating
// at an upper bound.
int CostMultiplier;
if (ClonesPower > Log2_32(UnswitchThreshold) ||
SiblingsMultiplier > UnswitchThreshold)
CostMultiplier = UnswitchThreshold;
else
CostMultiplier = std::min(SiblingsMultiplier * (1 << ClonesPower),
(int)UnswitchThreshold);
LLVM_DEBUG(dbgs() << " Computed multiplier " << CostMultiplier
<< " (siblings " << SiblingsMultiplier << " * clones "
<< (1 << ClonesPower) << ")"
<< " for unswitch candidate: " << TI << "\n");
return CostMultiplier;
}
static bool collectUnswitchCandidates(
SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates,
IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch,
const Loop &L, const LoopInfo &LI, AAResults &AA,
const MemorySSAUpdater *MSSAU) {
assert(UnswitchCandidates.empty() && "Should be!");
auto AddUnswitchCandidatesForInst = [&](Instruction *I, Value *Cond) {
Cond = skipTrivialSelect(Cond);
if (isa<Constant>(Cond))
return;
if (L.isLoopInvariant(Cond)) {
UnswitchCandidates.push_back({I, {Cond}});
return;
}
if (match(Cond, m_CombineOr(m_LogicalAnd(), m_LogicalOr()))) {
TinyPtrVector<Value *> Invariants =
collectHomogenousInstGraphLoopInvariants(
L, *static_cast<Instruction *>(Cond), LI);
if (!Invariants.empty())
UnswitchCandidates.push_back({I, std::move(Invariants)});
}
};
// Whether or not we should also collect guards in the loop.
bool CollectGuards = false;
if (UnswitchGuards) {
auto *GuardDecl = Intrinsic::getDeclarationIfExists(
L.getHeader()->getParent()->getParent(), Intrinsic::experimental_guard);
if (GuardDecl && !GuardDecl->use_empty())
CollectGuards = true;
}
for (auto *BB : L.blocks()) {
if (LI.getLoopFor(BB) != &L)
continue;
for (auto &I : *BB) {
if (auto *SI = dyn_cast<SelectInst>(&I)) {
auto *Cond = SI->getCondition();
// Do not unswitch vector selects and logical and/or selects
if (Cond->getType()->isIntegerTy(1) && !SI->getType()->isIntegerTy(1))
AddUnswitchCandidatesForInst(SI, Cond);
} else if (CollectGuards && isGuard(&I)) {
auto *Cond =
skipTrivialSelect(cast<IntrinsicInst>(&I)->getArgOperand(0));
// TODO: Support AND, OR conditions and partial unswitching.
if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond))
UnswitchCandidates.push_back({&I, {Cond}});
}
}
if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
// We can only consider fully loop-invariant switch conditions as we need
// to completely eliminate the switch after unswitching.
if (!isa<Constant>(SI->getCondition()) &&
L.isLoopInvariant(SI->getCondition()) && !BB->getUniqueSuccessor())
UnswitchCandidates.push_back({SI, {SI->getCondition()}});
continue;
}
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional() ||
BI->getSuccessor(0) == BI->getSuccessor(1))
continue;
AddUnswitchCandidatesForInst(BI, BI->getCondition());
}
if (MSSAU && !findOptionMDForLoop(&L, "llvm.loop.unswitch.partial.disable") &&
!any_of(UnswitchCandidates, [&L](auto &TerminatorAndInvariants) {
return TerminatorAndInvariants.TI == L.getHeader()->getTerminator();
})) {
MemorySSA *MSSA = MSSAU->getMemorySSA();
if (auto Info = hasPartialIVCondition(L, MSSAThreshold, *MSSA, AA)) {
LLVM_DEBUG(
dbgs() << "simple-loop-unswitch: Found partially invariant condition "
<< *Info->InstToDuplicate[0] << "\n");
PartialIVInfo = *Info;
PartialIVCondBranch = L.getHeader()->getTerminator();
TinyPtrVector<Value *> ValsToDuplicate;
llvm::append_range(ValsToDuplicate, Info->InstToDuplicate);
UnswitchCandidates.push_back(
{L.getHeader()->getTerminator(), std::move(ValsToDuplicate)});
}
}
return !UnswitchCandidates.empty();
}
/// Tries to canonicalize condition described by:
///
/// br (LHS pred RHS), label IfTrue, label IfFalse
///
/// into its equivalent where `Pred` is something that we support for injected
/// invariants (so far it is limited to ult), LHS in canonicalized form is
/// non-invariant and RHS is an invariant.
static void canonicalizeForInvariantConditionInjection(CmpPredicate &Pred,
Value *&LHS, Value *&RHS,
BasicBlock *&IfTrue,
BasicBlock *&IfFalse,
const Loop &L) {
if (!L.contains(IfTrue)) {
Pred = ICmpInst::getInversePredicate(Pred);
std::swap(IfTrue, IfFalse);
}
// Move loop-invariant argument to RHS position.
if (L.isLoopInvariant(LHS)) {
Pred = ICmpInst::getSwappedPredicate(Pred);
std::swap(LHS, RHS);
}
if (Pred == ICmpInst::ICMP_SGE && match(RHS, m_Zero())) {
// Turn "x >=s 0" into "x <u UMIN_INT"
Pred = ICmpInst::ICMP_ULT;
RHS = ConstantInt::get(
RHS->getContext(),
APInt::getSignedMinValue(RHS->getType()->getIntegerBitWidth()));
}
}
/// Returns true, if predicate described by ( \p Pred, \p LHS, \p RHS )
/// succeeding into blocks ( \p IfTrue, \p IfFalse) can be optimized by
/// injecting a loop-invariant condition.
static bool shouldTryInjectInvariantCondition(
const ICmpInst::Predicate Pred, const Value *LHS, const Value *RHS,
const BasicBlock *IfTrue, const BasicBlock *IfFalse, const Loop &L) {
if (L.isLoopInvariant(LHS) || !L.isLoopInvariant(RHS))
return false;
// TODO: Support other predicates.
if (Pred != ICmpInst::ICMP_ULT)
return false;
// TODO: Support non-loop-exiting branches?
if (!L.contains(IfTrue) || L.contains(IfFalse))
return false;
// FIXME: For some reason this causes problems with MSSA updates, need to
// investigate why. So far, just don't unswitch latch.
if (L.getHeader() == IfTrue)
return false;
return true;
}
/// Returns true, if metadata on \p BI allows us to optimize branching into \p
/// TakenSucc via injection of invariant conditions. The branch should be not
/// enough and not previously unswitched, the information about this comes from
/// the metadata.
bool shouldTryInjectBasingOnMetadata(const BranchInst *BI,
const BasicBlock *TakenSucc) {
SmallVector<uint32_t> Weights;
if (!extractBranchWeights(*BI, Weights))
return false;
unsigned T = InjectInvariantConditionHotnesThreshold;
BranchProbability LikelyTaken(T - 1, T);
assert(Weights.size() == 2 && "Unexpected profile data!");
size_t Idx = BI->getSuccessor(0) == TakenSucc ? 0 : 1;
auto Num = Weights[Idx];
auto Denom = Weights[0] + Weights[1];
// Degenerate or overflowed metadata.
if (Denom == 0 || Num > Denom)
return false;
BranchProbability ActualTaken(Num, Denom);
if (LikelyTaken > ActualTaken)
return false;
return true;
}
/// Materialize pending invariant condition of the given candidate into IR. The
/// injected loop-invariant condition implies the original loop-variant branch
/// condition, so the materialization turns
///
/// loop_block:
/// ...
/// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc
///
/// into
///
/// preheader:
/// %invariant_cond = LHS pred RHS
/// ...
/// loop_block:
/// br i1 %invariant_cond, label InLoopSucc, label OriginalCheck
/// OriginalCheck:
/// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc
/// ...
static NonTrivialUnswitchCandidate
injectPendingInvariantConditions(NonTrivialUnswitchCandidate Candidate, Loop &L,
DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, MemorySSAUpdater *MSSAU) {
assert(Candidate.hasPendingInjection() && "Nothing to inject!");
BasicBlock *Preheader = L.getLoopPreheader();
assert(Preheader && "Loop is not in simplified form?");
assert(LI.getLoopFor(Candidate.TI->getParent()) == &L &&
"Unswitching branch of inner loop!");
auto Pred = Candidate.PendingInjection->Pred;
auto *LHS = Candidate.PendingInjection->LHS;
auto *RHS = Candidate.PendingInjection->RHS;
auto *InLoopSucc = Candidate.PendingInjection->InLoopSucc;
auto *TI = cast<BranchInst>(Candidate.TI);
auto *BB = Candidate.TI->getParent();
auto *OutOfLoopSucc = InLoopSucc == TI->getSuccessor(0) ? TI->getSuccessor(1)
: TI->getSuccessor(0);
// FIXME: Remove this once limitation on successors is lifted.
assert(L.contains(InLoopSucc) && "Not supported yet!");
assert(!L.contains(OutOfLoopSucc) && "Not supported yet!");
auto &Ctx = BB->getContext();
IRBuilder<> Builder(Preheader->getTerminator());
assert(ICmpInst::isUnsigned(Pred) && "Not supported yet!");
if (LHS->getType() != RHS->getType()) {
if (LHS->getType()->getIntegerBitWidth() <
RHS->getType()->getIntegerBitWidth())
LHS = Builder.CreateZExt(LHS, RHS->getType(), LHS->getName() + ".wide");
else
RHS = Builder.CreateZExt(RHS, LHS->getType(), RHS->getName() + ".wide");
}
// Do not use builder here: CreateICmp may simplify this into a constant and
// unswitching will break. Better optimize it away later.
auto *InjectedCond =
ICmpInst::Create(Instruction::ICmp, Pred, LHS, RHS, "injected.cond",
Preheader->getTerminator()->getIterator());
BasicBlock *CheckBlock = BasicBlock::Create(Ctx, BB->getName() + ".check",
BB->getParent(), InLoopSucc);
Builder.SetInsertPoint(TI);
auto *InvariantBr =
Builder.CreateCondBr(InjectedCond, InLoopSucc, CheckBlock);
Builder.SetInsertPoint(CheckBlock);
Builder.CreateCondBr(TI->getCondition(), TI->getSuccessor(0),
TI->getSuccessor(1));
TI->eraseFromParent();
// Fixup phis.
for (auto &I : *InLoopSucc) {
auto *PN = dyn_cast<PHINode>(&I);
if (!PN)
break;
auto *Inc = PN->getIncomingValueForBlock(BB);
PN->addIncoming(Inc, CheckBlock);
}
OutOfLoopSucc->replacePhiUsesWith(BB, CheckBlock);
SmallVector<DominatorTree::UpdateType, 4> DTUpdates = {
{ DominatorTree::Insert, BB, CheckBlock },
{ DominatorTree::Insert, CheckBlock, InLoopSucc },
{ DominatorTree::Insert, CheckBlock, OutOfLoopSucc },
{ DominatorTree::Delete, BB, OutOfLoopSucc }
};
DT.applyUpdates(DTUpdates);
if (MSSAU)
MSSAU->applyUpdates(DTUpdates, DT);
L.addBasicBlockToLoop(CheckBlock, LI);
#ifndef NDEBUG
DT.verify();
LI.verify(DT);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
#endif
// TODO: In fact, cost of unswitching a new invariant candidate is *slightly*
// higher because we have just inserted a new block. Need to think how to
// adjust the cost of injected candidates when it was first computed.
LLVM_DEBUG(dbgs() << "Injected a new loop-invariant branch " << *InvariantBr
<< " and considering it for unswitching.");
++NumInvariantConditionsInjected;
return NonTrivialUnswitchCandidate(InvariantBr, { InjectedCond },
Candidate.Cost);
}
/// Given chain of loop branch conditions looking like:
/// br (Variant < Invariant1)
/// br (Variant < Invariant2)
/// br (Variant < Invariant3)
/// ...
/// collect set of invariant conditions on which we want to unswitch, which
/// look like:
/// Invariant1 <= Invariant2
/// Invariant2 <= Invariant3
/// ...
/// Though they might not immediately exist in the IR, we can still inject them.
static bool insertCandidatesWithPendingInjections(
SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates, Loop &L,
ICmpInst::Predicate Pred, ArrayRef<CompareDesc> Compares,
const DominatorTree &DT) {
assert(ICmpInst::isRelational(Pred));
assert(ICmpInst::isStrictPredicate(Pred));
if (Compares.size() < 2)
return false;
ICmpInst::Predicate NonStrictPred = ICmpInst::getNonStrictPredicate(Pred);
for (auto Prev = Compares.begin(), Next = Compares.begin() + 1;
Next != Compares.end(); ++Prev, ++Next) {
Value *LHS = Next->Invariant;
Value *RHS = Prev->Invariant;
BasicBlock *InLoopSucc = Prev->InLoopSucc;
InjectedInvariant ToInject(NonStrictPred, LHS, RHS, InLoopSucc);
NonTrivialUnswitchCandidate Candidate(Prev->Term, { LHS, RHS },
std::nullopt, std::move(ToInject));
UnswitchCandidates.push_back(std::move(Candidate));
}
return true;
}
/// Collect unswitch candidates by invariant conditions that are not immediately
/// present in the loop. However, they can be injected into the code if we
/// decide it's profitable.
/// An example of such conditions is following:
///
/// for (...) {
/// x = load ...
/// if (! x <u C1) break;
/// if (! x <u C2) break;
/// <do something>
/// }
///
/// We can unswitch by condition "C1 <=u C2". If that is true, then "x <u C1 <=
/// C2" automatically implies "x <u C2", so we can get rid of one of
/// loop-variant checks in unswitched loop version.
static bool collectUnswitchCandidatesWithInjections(
SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates,
IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch, Loop &L,
const DominatorTree &DT, const LoopInfo &LI, AAResults &AA,
const MemorySSAUpdater *MSSAU) {
if (!InjectInvariantConditions)
return false;
if (!DT.isReachableFromEntry(L.getHeader()))
return false;
auto *Latch = L.getLoopLatch();
// Need to have a single latch and a preheader.
if (!Latch)
return false;
assert(L.getLoopPreheader() && "Must have a preheader!");
DenseMap<Value *, SmallVector<CompareDesc, 4> > CandidatesULT;
// Traverse the conditions that dominate latch (and therefore dominate each
// other).
for (auto *DTN = DT.getNode(Latch); L.contains(DTN->getBlock());
DTN = DTN->getIDom()) {
CmpPredicate Pred;
Value *LHS = nullptr, *RHS = nullptr;
BasicBlock *IfTrue = nullptr, *IfFalse = nullptr;
auto *BB = DTN->getBlock();
// Ignore inner loops.
if (LI.getLoopFor(BB) != &L)
continue;
auto *Term = BB->getTerminator();
if (!match(Term, m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)),
m_BasicBlock(IfTrue), m_BasicBlock(IfFalse))))
continue;
if (!LHS->getType()->isIntegerTy())
continue;
canonicalizeForInvariantConditionInjection(Pred, LHS, RHS, IfTrue, IfFalse,
L);
if (!shouldTryInjectInvariantCondition(Pred, LHS, RHS, IfTrue, IfFalse, L))
continue;
if (!shouldTryInjectBasingOnMetadata(cast<BranchInst>(Term), IfTrue))
continue;
// Strip ZEXT for unsigned predicate.
// TODO: once signed predicates are supported, also strip SEXT.
CompareDesc Desc(cast<BranchInst>(Term), RHS, IfTrue);
while (auto *Zext = dyn_cast<ZExtInst>(LHS))
LHS = Zext->getOperand(0);
CandidatesULT[LHS].push_back(Desc);
}
bool Found = false;
for (auto &It : CandidatesULT)
Found |= insertCandidatesWithPendingInjections(
UnswitchCandidates, L, ICmpInst::ICMP_ULT, It.second, DT);
return Found;
}
static bool isSafeForNoNTrivialUnswitching(Loop &L, LoopInfo &LI) {
if (!L.isSafeToClone())
return false;
for (auto *BB : L.blocks())
for (auto &I : *BB) {
if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
return false;
if (auto *CB = dyn_cast<CallBase>(&I)) {
assert(!CB->cannotDuplicate() && "Checked by L.isSafeToClone().");
if (CB->isConvergent())
return false;
}
}
// Check if there are irreducible CFG cycles in this loop. If so, we cannot
// easily unswitch non-trivial edges out of the loop. Doing so might turn the
// irreducible control flow into reducible control flow and introduce new
// loops "out of thin air". If we ever discover important use cases for doing
// this, we can add support to loop unswitch, but it is a lot of complexity
// for what seems little or no real world benefit.
LoopBlocksRPO RPOT(&L);
RPOT.perform(&LI);
if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
return false;
SmallVector<BasicBlock *, 4> ExitBlocks;
L.getUniqueExitBlocks(ExitBlocks);
// We cannot unswitch if exit blocks contain a cleanuppad/catchswitch
// instruction as we don't know how to split those exit blocks.
// FIXME: We should teach SplitBlock to handle this and remove this
// restriction.
for (auto *ExitBB : ExitBlocks) {
auto It = ExitBB->getFirstNonPHIIt();
if (isa<CleanupPadInst>(It) || isa<CatchSwitchInst>(It)) {
LLVM_DEBUG(dbgs() << "Cannot unswitch because of cleanuppad/catchswitch "
"in exit block\n");
return false;
}
}
return true;
}
static NonTrivialUnswitchCandidate findBestNonTrivialUnswitchCandidate(
ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates, const Loop &L,
const DominatorTree &DT, const LoopInfo &LI, AssumptionCache &AC,
const TargetTransformInfo &TTI, const IVConditionInfo &PartialIVInfo) {
// Given that unswitching these terminators will require duplicating parts of
// the loop, so we need to be able to model that cost. Compute the ephemeral
// values and set up a data structure to hold per-BB costs. We cache each
// block's cost so that we don't recompute this when considering different
// subsets of the loop for duplication during unswitching.
SmallPtrSet<const Value *, 4> EphValues;
CodeMetrics::collectEphemeralValues(&L, &AC, EphValues);
SmallDenseMap<BasicBlock *, InstructionCost, 4> BBCostMap;
// Compute the cost of each block, as well as the total loop cost. Also, bail
// out if we see instructions which are incompatible with loop unswitching
// (convergent, noduplicate, or cross-basic-block tokens).
// FIXME: We might be able to safely handle some of these in non-duplicated
// regions.
TargetTransformInfo::TargetCostKind CostKind =
L.getHeader()->getParent()->hasMinSize()
? TargetTransformInfo::TCK_CodeSize
: TargetTransformInfo::TCK_SizeAndLatency;
InstructionCost LoopCost = 0;
for (auto *BB : L.blocks()) {
InstructionCost Cost = 0;
for (auto &I : *BB) {
if (EphValues.count(&I))
continue;
Cost += TTI.getInstructionCost(&I, CostKind);
}
assert(Cost >= 0 && "Must not have negative costs!");
LoopCost += Cost;
assert(LoopCost >= 0 && "Must not have negative loop costs!");
BBCostMap[BB] = Cost;
}
LLVM_DEBUG(dbgs() << " Total loop cost: " << LoopCost << "\n");
// Now we find the best candidate by searching for the one with the following
// properties in order:
//
// 1) An unswitching cost below the threshold
// 2) The smallest number of duplicated unswitch candidates (to avoid
// creating redundant subsequent unswitching)
// 3) The smallest cost after unswitching.
//
// We prioritize reducing fanout of unswitch candidates provided the cost
// remains below the threshold because this has a multiplicative effect.
//
// This requires memoizing each dominator subtree to avoid redundant work.
//
// FIXME: Need to actually do the number of candidates part above.
SmallDenseMap<DomTreeNode *, InstructionCost, 4> DTCostMap;
// Given a terminator which might be unswitched, computes the non-duplicated
// cost for that terminator.
auto ComputeUnswitchedCost = [&](Instruction &TI,
bool FullUnswitch) -> InstructionCost {
// Unswitching selects unswitches the entire loop.
if (isa<SelectInst>(TI))
return LoopCost;
BasicBlock &BB = *TI.getParent();
SmallPtrSet<BasicBlock *, 4> Visited;
InstructionCost Cost = 0;
for (BasicBlock *SuccBB : successors(&BB)) {
// Don't count successors more than once.
if (!Visited.insert(SuccBB).second)
continue;
// If this is a partial unswitch candidate, then it must be a conditional
// branch with a condition of either `or`, `and`, their corresponding
// select forms or partially invariant instructions. In that case, one of
// the successors is necessarily duplicated, so don't even try to remove
// its cost.
if (!FullUnswitch) {
auto &BI = cast<BranchInst>(TI);
Value *Cond = skipTrivialSelect(BI.getCondition());
if (match(Cond, m_LogicalAnd())) {
if (SuccBB == BI.getSuccessor(1))
continue;
} else if (match(Cond, m_LogicalOr())) {
if (SuccBB == BI.getSuccessor(0))
continue;
} else if ((PartialIVInfo.KnownValue->isOneValue() &&
SuccBB == BI.getSuccessor(0)) ||
(!PartialIVInfo.KnownValue->isOneValue() &&
SuccBB == BI.getSuccessor(1)))
continue;
}
// This successor's domtree will not need to be duplicated after
// unswitching if the edge to the successor dominates it (and thus the
// entire tree). This essentially means there is no other path into this
// subtree and so it will end up live in only one clone of the loop.
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == &BB || DT.dominates(SuccBB, PredBB);
})) {
Cost += computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap);
assert(Cost <= LoopCost &&
"Non-duplicated cost should never exceed total loop cost!");
}
}
// Now scale the cost by the number of unique successors minus one. We
// subtract one because there is already at least one copy of the entire
// loop. This is computing the new cost of unswitching a condition.
// Note that guards always have 2 unique successors that are implicit and
// will be materialized if we decide to unswitch it.
int SuccessorsCount = isGuard(&TI) ? 2 : Visited.size();
assert(SuccessorsCount > 1 &&
"Cannot unswitch a condition without multiple distinct successors!");
return (LoopCost - Cost) * (SuccessorsCount - 1);
};
std::optional<NonTrivialUnswitchCandidate> Best;
for (auto &Candidate : UnswitchCandidates) {
Instruction &TI = *Candidate.TI;
ArrayRef<Value *> Invariants = Candidate.Invariants;
BranchInst *BI = dyn_cast<BranchInst>(&TI);
bool FullUnswitch =
!BI || Candidate.hasPendingInjection() ||
(Invariants.size() == 1 &&
Invariants[0] == skipTrivialSelect(BI->getCondition()));
InstructionCost CandidateCost = ComputeUnswitchedCost(TI, FullUnswitch);
// Calculate cost multiplier which is a tool to limit potentially
// exponential behavior of loop-unswitch.
if (EnableUnswitchCostMultiplier) {
int CostMultiplier =
CalculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates);
assert(
(CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) &&
"cost multiplier needs to be in the range of 1..UnswitchThreshold");
CandidateCost *= CostMultiplier;
LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
<< " (multiplier: " << CostMultiplier << ")"
<< " for unswitch candidate: " << TI << "\n");
} else {
LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
<< " for unswitch candidate: " << TI << "\n");
}
if (!Best || CandidateCost < Best->Cost) {
Best = Candidate;
Best->Cost = CandidateCost;
}
}
assert(Best && "Must be!");
return *Best;
}
// Insert a freeze on an unswitched branch if all is true:
// 1. freeze-loop-unswitch-cond option is true
// 2. The branch may not execute in the loop pre-transformation. If a branch may
// not execute and could cause UB, it would always cause UB if it is hoisted outside
// of the loop. Insert a freeze to prevent this case.
// 3. The branch condition may be poison or undef
static bool shouldInsertFreeze(Loop &L, Instruction &TI, DominatorTree &DT,
AssumptionCache &AC) {
assert(isa<BranchInst>(TI) || isa<SwitchInst>(TI));
if (!FreezeLoopUnswitchCond)
return false;
ICFLoopSafetyInfo SafetyInfo;
SafetyInfo.computeLoopSafetyInfo(&L);
if (SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
return false;
Value *Cond;
if (BranchInst *BI = dyn_cast<BranchInst>(&TI))
Cond = skipTrivialSelect(BI->getCondition());
else
Cond = skipTrivialSelect(cast<SwitchInst>(&TI)->getCondition());
return !isGuaranteedNotToBeUndefOrPoison(
Cond, &AC, L.getLoopPreheader()->getTerminator(), &DT);
}
static bool unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, AAResults &AA,
TargetTransformInfo &TTI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU,
LPMUpdater &LoopUpdater) {
// Collect all invariant conditions within this loop (as opposed to an inner
// loop which would be handled when visiting that inner loop).
SmallVector<NonTrivialUnswitchCandidate, 4> UnswitchCandidates;
IVConditionInfo PartialIVInfo;
Instruction *PartialIVCondBranch = nullptr;
collectUnswitchCandidates(UnswitchCandidates, PartialIVInfo,
PartialIVCondBranch, L, LI, AA, MSSAU);
if (!findOptionMDForLoop(&L, "llvm.loop.unswitch.injection.disable"))
collectUnswitchCandidatesWithInjections(UnswitchCandidates, PartialIVInfo,
PartialIVCondBranch, L, DT, LI, AA,
MSSAU);
// If we didn't find any candidates, we're done.
if (UnswitchCandidates.empty())
return false;
LLVM_DEBUG(
dbgs() << "Considering " << UnswitchCandidates.size()
<< " non-trivial loop invariant conditions for unswitching.\n");
NonTrivialUnswitchCandidate Best = findBestNonTrivialUnswitchCandidate(
UnswitchCandidates, L, DT, LI, AC, TTI, PartialIVInfo);
assert(Best.TI && "Failed to find loop unswitch candidate");
assert(Best.Cost && "Failed to compute cost");
if (*Best.Cost >= UnswitchThreshold) {
LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: " << *Best.Cost
<< "\n");
return false;
}
bool InjectedCondition = false;
if (Best.hasPendingInjection()) {
Best = injectPendingInvariantConditions(Best, L, DT, LI, AC, MSSAU);
InjectedCondition = true;
}
assert(!Best.hasPendingInjection() &&
"All injections should have been done by now!");
if (Best.TI != PartialIVCondBranch)
PartialIVInfo.InstToDuplicate.clear();
bool InsertFreeze;
if (auto *SI = dyn_cast<SelectInst>(Best.TI)) {
// If the best candidate is a select, turn it into a branch. Select
// instructions with a poison conditional do not propagate poison, but
// branching on poison causes UB. Insert a freeze on the select
// conditional to prevent UB after turning the select into a branch.
InsertFreeze = !isGuaranteedNotToBeUndefOrPoison(
SI->getCondition(), &AC, L.getLoopPreheader()->getTerminator(), &DT);
Best.TI = turnSelectIntoBranch(SI, DT, LI, MSSAU, &AC);
} else {
// If the best candidate is a guard, turn it into a branch.
if (isGuard(Best.TI))
Best.TI =
turnGuardIntoBranch(cast<IntrinsicInst>(Best.TI), L, DT, LI, MSSAU);
InsertFreeze = shouldInsertFreeze(L, *Best.TI, DT, AC);
}
LLVM_DEBUG(dbgs() << " Unswitching non-trivial (cost = " << Best.Cost
<< ") terminator: " << *Best.TI << "\n");
unswitchNontrivialInvariants(L, *Best.TI, Best.Invariants, PartialIVInfo, DT,
LI, AC, SE, MSSAU, LoopUpdater, InsertFreeze,
InjectedCondition);
return true;
}
/// Unswitch control flow predicated on loop invariant conditions.
///
/// This first hoists all branches or switches which are trivial (IE, do not
/// require duplicating any part of the loop) out of the loop body. It then
/// looks at other loop invariant control flows and tries to unswitch those as
/// well by cloning the loop if the result is small enough.
///
/// The `DT`, `LI`, `AC`, `AA`, `TTI` parameters are required analyses that are
/// also updated based on the unswitch. The `MSSA` analysis is also updated if
/// valid (i.e. its use is enabled).
///
/// If either `NonTrivial` is true or the flag `EnableNonTrivialUnswitch` is
/// true, we will attempt to do non-trivial unswitching as well as trivial
/// unswitching.
///
/// The `postUnswitch` function will be run after unswitching is complete
/// with information on whether or not the provided loop remains a loop and
/// a list of new sibling loops created.
///
/// If `SE` is non-null, we will update that analysis based on the unswitching
/// done.
static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, AAResults &AA,
TargetTransformInfo &TTI, bool Trivial,
bool NonTrivial, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU, ProfileSummaryInfo *PSI,
BlockFrequencyInfo *BFI, LPMUpdater &LoopUpdater) {
assert(L.isRecursivelyLCSSAForm(DT, LI) &&
"Loops must be in LCSSA form before unswitching.");
// Must be in loop simplified form: we need a preheader and dedicated exits.
if (!L.isLoopSimplifyForm())
return false;
// Try trivial unswitch first before loop over other basic blocks in the loop.
if (Trivial && unswitchAllTrivialConditions(L, DT, LI, SE, MSSAU)) {
// If we unswitched successfully we will want to clean up the loop before
// processing it further so just mark it as unswitched and return.
postUnswitch(L, LoopUpdater, L.getName(),
/*CurrentLoopValid*/ true, /*PartiallyInvariant*/ false,
/*InjectedCondition*/ false, {});
return true;
}
const Function *F = L.getHeader()->getParent();
// Check whether we should continue with non-trivial conditions.
// EnableNonTrivialUnswitch: Global variable that forces non-trivial
// unswitching for testing and debugging.
// NonTrivial: Parameter that enables non-trivial unswitching for this
// invocation of the transform. But this should be allowed only
// for targets without branch divergence.
//
// FIXME: If divergence analysis becomes available to a loop
// transform, we should allow unswitching for non-trivial uniform
// branches even on targets that have divergence.
// https://bugs.llvm.org/show_bug.cgi?id=48819
bool ContinueWithNonTrivial =
EnableNonTrivialUnswitch || (NonTrivial && !TTI.hasBranchDivergence(F));
if (!ContinueWithNonTrivial)
return false;
// Skip non-trivial unswitching for optsize functions.
if (F->hasOptSize())
return false;
// Returns true if Loop L's loop nest is cold, i.e. if the headers of L,
// of the loops L is nested in, and of the loops nested in L are all cold.
auto IsLoopNestCold = [&](const Loop *L) {
// Check L and all of its parent loops.
auto *Parent = L;
while (Parent) {
if (!PSI->isColdBlock(Parent->getHeader(), BFI))
return false;
Parent = Parent->getParentLoop();
}
// Next check all loops nested within L.
SmallVector<const Loop *, 4> Worklist;
Worklist.insert(Worklist.end(), L->getSubLoops().begin(),
L->getSubLoops().end());
while (!Worklist.empty()) {
auto *CurLoop = Worklist.pop_back_val();
if (!PSI->isColdBlock(CurLoop->getHeader(), BFI))
return false;
Worklist.insert(Worklist.end(), CurLoop->getSubLoops().begin(),
CurLoop->getSubLoops().end());
}
return true;
};
// Skip cold loops in cold loop nests, as unswitching them brings little
// benefit but increases the code size
if (PSI && PSI->hasProfileSummary() && BFI && IsLoopNestCold(&L)) {
LLVM_DEBUG(dbgs() << " Skip cold loop: " << L << "\n");
return false;
}
// Perform legality checks.
if (!isSafeForNoNTrivialUnswitching(L, LI))
return false;
// For non-trivial unswitching, because it often creates new loops, we rely on
// the pass manager to iterate on the loops rather than trying to immediately
// reach a fixed point. There is no substantial advantage to iterating
// internally, and if any of the new loops are simplified enough to contain
// trivial unswitching we want to prefer those.
// Try to unswitch the best invariant condition. We prefer this full unswitch to
// a partial unswitch when possible below the threshold.
if (unswitchBestCondition(L, DT, LI, AC, AA, TTI, SE, MSSAU, LoopUpdater))
return true;
// No other opportunities to unswitch.
return false;
}
PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &U) {
Function &F = *L.getHeader()->getParent();
(void)F;
ProfileSummaryInfo *PSI = nullptr;
if (auto OuterProxy =
AM.getResult<FunctionAnalysisManagerLoopProxy>(L, AR)
.getCachedResult<ModuleAnalysisManagerFunctionProxy>(F))
PSI = OuterProxy->getCachedResult<ProfileSummaryAnalysis>(*F.getParent());
LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L
<< "\n");
std::optional<MemorySSAUpdater> MSSAU;
if (AR.MSSA) {
MSSAU = MemorySSAUpdater(AR.MSSA);
if (VerifyMemorySSA)
AR.MSSA->verifyMemorySSA();
}
if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.AA, AR.TTI, Trivial, NonTrivial,
&AR.SE, MSSAU ? &*MSSAU : nullptr, PSI, AR.BFI, U))
return PreservedAnalyses::all();
if (AR.MSSA && VerifyMemorySSA)
AR.MSSA->verifyMemorySSA();
// Historically this pass has had issues with the dominator tree so verify it
// in asserts builds.
assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
auto PA = getLoopPassPreservedAnalyses();
if (AR.MSSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
void SimpleLoopUnswitchPass::printPipeline(
raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
static_cast<PassInfoMixin<SimpleLoopUnswitchPass> *>(this)->printPipeline(
OS, MapClassName2PassName);
OS << '<';
OS << (NonTrivial ? "" : "no-") << "nontrivial;";
OS << (Trivial ? "" : "no-") << "trivial";
OS << '>';
}
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