llvm-project/llvm/lib/Transforms/Utils/FixIrreducible.cpp

//===- FixIrreducible.cpp - Convert irreducible control-flow into loops ---===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// INPUT CFG: The blocks H and B form an irreducible cycle with two headers.
//
//                        Entry
//                       /     \
//                      v       v
//                      H ----> B
//                      ^      /|
//                       `----' |
//                              v
//                             Exit
//
// OUTPUT CFG: Converted to a natural loop with a new header N.
//
//                        Entry
//                          |
//                          v
//                          N <---.
//                         / \     \
//                        /   \     |
//                       v     v    /
//                       H --> B --'
//                             |
//                             v
//                            Exit
//
// To convert an irreducible cycle C to a natural loop L:
//
// 1. Add a new node N to C.
// 2. Redirect all external incoming edges through N.
// 3. Redirect all edges incident on header H through N.
//
// This is sufficient to ensure that:
//
// a. Every closed path in C also exists in L, with the modification that any
//    path passing through H now passes through N before reaching H.
// b. Every external path incident on any entry of C is now incident on N and
//    then redirected to the entry.
//
// Thus, L is a strongly connected component dominated by N, and hence L is a
// natural loop with header N.
//
// When an irreducible cycle C with header H is transformed into a loop, the
// following invariants hold:
//
// 1. No new subcycles are "discovered" in the set (C-H). The only internal
//    edges that are redirected by the transform are incident on H. Any subcycle
//    S in (C-H), already existed prior to this transform, and is already in the
//    list of children for this cycle C.
//
// 2. Subcycles of C are not modified by the transform. For some subcycle S of
//    C, edges incident on the entries of S are either internal to C, or they
//    are now redirected through N, which is outside of S. So the list of
//    entries to S does not change. Since the transform only adds a block
//    outside S, and redirects edges that are not internal to S, the list of
//    blocks in S does not change.
//
// 3. Similarly, any natural loop L included in C is not affected, with one
//    exception: L is "destroyed" by the transform iff its header is H. The
//    backedges of such a loop are now redirected to N instead, and hence the
//    body of this loop gets merged into the new loop with header N.
//
// The actual transformation is handled by the ControlFlowHub, which redirects
// specified control flow edges through a set of guard blocks. This also moves
// every PHINode in an outgoing block to the hub. Since the hub dominates all
// the outgoing blocks, each such PHINode continues to dominate its uses. Since
// every header in an SCC has at least two predecessors, every value used in the
// header (or later) but defined in a predecessor (or earlier) is represented by
// a PHINode in a header. Hence the above handling of PHINodes is sufficient and
// no further processing is required to restore SSA.
//
// Limitation: The pass cannot handle switch statements and indirect
//             branches. Both must be lowered to plain branches first.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/FixIrreducible.h"
#include "llvm/Analysis/CycleAnalysis.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/ControlFlowUtils.h"

#define DEBUG_TYPE "fix-irreducible"

using namespace llvm;

namespace {
struct FixIrreducible : public FunctionPass {
  static char ID;
  FixIrreducible() : FunctionPass(ID) {
    initializeFixIrreduciblePass(*PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addRequired<CycleInfoWrapperPass>();
    AU.addPreserved<DominatorTreeWrapperPass>();
    AU.addPreserved<CycleInfoWrapperPass>();
    AU.addPreserved<LoopInfoWrapperPass>();
  }

  bool runOnFunction(Function &F) override;
};
} // namespace

char FixIrreducible::ID = 0;

FunctionPass *llvm::createFixIrreduciblePass() { return new FixIrreducible(); }

INITIALIZE_PASS_BEGIN(FixIrreducible, "fix-irreducible",
                      "Convert irreducible control-flow into natural loops",
                      false /* Only looks at CFG */, false /* Analysis Pass */)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(FixIrreducible, "fix-irreducible",
                    "Convert irreducible control-flow into natural loops",
                    false /* Only looks at CFG */, false /* Analysis Pass */)

// When a new loop is created, existing children of the parent loop may now be
// fully inside the new loop. Reconnect these as children of the new loop.
static void reconnectChildLoops(LoopInfo &LI, Loop *ParentLoop, Loop *NewLoop,
                                BasicBlock *OldHeader) {
  auto &CandidateLoops = ParentLoop ? ParentLoop->getSubLoopsVector()
                                    : LI.getTopLevelLoopsVector();
  // Any candidate is a child iff its header is owned by the new loop. Move all
  // the children to a new vector.
  auto FirstChild = std::partition(
      CandidateLoops.begin(), CandidateLoops.end(), [&](Loop *L) {
        return NewLoop == L || !NewLoop->contains(L->getHeader());
      });
  SmallVector<Loop *, 8> ChildLoops(FirstChild, CandidateLoops.end());
  CandidateLoops.erase(FirstChild, CandidateLoops.end());

  for (Loop *Child : ChildLoops) {
    LLVM_DEBUG(dbgs() << "child loop: " << Child->getHeader()->getName()
                      << "\n");
    // A child loop whose header was the old cycle header gets destroyed since
    // its backedges are removed.
    if (Child->getHeader() == OldHeader) {
      for (auto *BB : Child->blocks()) {
        if (LI.getLoopFor(BB) != Child)
          continue;
        LI.changeLoopFor(BB, NewLoop);
        LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
                          << "\n");
      }
      std::vector<Loop *> GrandChildLoops;
      std::swap(GrandChildLoops, Child->getSubLoopsVector());
      for (auto *GrandChildLoop : GrandChildLoops) {
        GrandChildLoop->setParentLoop(nullptr);
        NewLoop->addChildLoop(GrandChildLoop);
      }
      LI.destroy(Child);
      LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
      continue;
    }

    Child->setParentLoop(nullptr);
    NewLoop->addChildLoop(Child);
    LLVM_DEBUG(dbgs() << "added child loop to new loop\n");
  }
}

static void updateLoopInfo(LoopInfo &LI, Cycle &C,
                           ArrayRef<BasicBlock *> GuardBlocks) {
  // The parent loop is a natural loop L mapped to the cycle header H as long as
  // H is not also the header of L. In the latter case, L is destroyed and we
  // seek its parent instead.
  BasicBlock *CycleHeader = C.getHeader();
  Loop *ParentLoop = LI.getLoopFor(CycleHeader);
  if (ParentLoop && ParentLoop->getHeader() == CycleHeader)
    ParentLoop = ParentLoop->getParentLoop();

  // Create a new loop from the now-transformed cycle
  auto *NewLoop = LI.AllocateLoop();
  if (ParentLoop) {
    ParentLoop->addChildLoop(NewLoop);
  } else {
    LI.addTopLevelLoop(NewLoop);
  }

  // Add the guard blocks to the new loop. The first guard block is
  // the head of all the backedges, and it is the first to be inserted
  // in the loop. This ensures that it is recognized as the
  // header. Since the new loop is already in LoopInfo, the new blocks
  // are also propagated up the chain of parent loops.
  for (auto *G : GuardBlocks) {
    LLVM_DEBUG(dbgs() << "added guard block to loop: " << G->getName() << "\n");
    NewLoop->addBasicBlockToLoop(G, LI);
  }

  for (auto *BB : C.blocks()) {
    NewLoop->addBlockEntry(BB);
    if (LI.getLoopFor(BB) == ParentLoop) {
      LLVM_DEBUG(dbgs() << "moved block from parent: " << BB->getName()
                        << "\n");
      LI.changeLoopFor(BB, NewLoop);
    } else {
      LLVM_DEBUG(dbgs() << "added block from child: " << BB->getName() << "\n");
    }
  }
  LLVM_DEBUG(dbgs() << "header for new loop: "
                    << NewLoop->getHeader()->getName() << "\n");

  reconnectChildLoops(LI, ParentLoop, NewLoop, C.getHeader());

  LLVM_DEBUG(dbgs() << "Verify new loop.\n"; NewLoop->print(dbgs()));
  NewLoop->verifyLoop();
  if (ParentLoop) {
    LLVM_DEBUG(dbgs() << "Verify parent loop.\n"; ParentLoop->print(dbgs()));
    ParentLoop->verifyLoop();
  }
}

// Given a set of blocks and headers in an irreducible SCC, convert it into a
// natural loop. Also insert this new loop at its appropriate place in the
// hierarchy of loops.
static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
                           LoopInfo *LI) {
  if (C.isReducible())
    return false;
  LLVM_DEBUG(dbgs() << "Processing cycle:\n" << CI.print(&C) << "\n";);

  ControlFlowHub CHub;
  SetVector<BasicBlock *> Predecessors;

  // Redirect internal edges incident on the header.
  BasicBlock *Header = C.getHeader();
  for (BasicBlock *P : predecessors(Header)) {
    if (C.contains(P))
      Predecessors.insert(P);
  }

  for (BasicBlock *P : Predecessors) {
    auto *Branch = cast<BranchInst>(P->getTerminator());
    // Exactly one of the two successors is the header.
    BasicBlock *Succ0 = Branch->getSuccessor(0) == Header ? Header : nullptr;
    BasicBlock *Succ1 = Succ0 ? nullptr : Header;
    if (!Succ0)
      assert(Branch->getSuccessor(1) == Header);
    assert(Succ0 || Succ1);
    CHub.addBranch(P, Succ0, Succ1);

    LLVM_DEBUG(dbgs() << "Added internal branch: " << P->getName() << " -> "
                      << (Succ0 ? Succ0->getName() : "") << " "
                      << (Succ1 ? Succ1->getName() : "") << "\n");
  }

  // Redirect external incoming edges. This includes the edges on the header.
  Predecessors.clear();
  for (BasicBlock *E : C.entries()) {
    for (BasicBlock *P : predecessors(E)) {
      if (!C.contains(P))
        Predecessors.insert(P);
    }
  }

  for (BasicBlock *P : Predecessors) {
    auto *Branch = cast<BranchInst>(P->getTerminator());
    BasicBlock *Succ0 = Branch->getSuccessor(0);
    Succ0 = C.contains(Succ0) ? Succ0 : nullptr;
    BasicBlock *Succ1 =
        Branch->isUnconditional() ? nullptr : Branch->getSuccessor(1);
    Succ1 = Succ1 && C.contains(Succ1) ? Succ1 : nullptr;
    CHub.addBranch(P, Succ0, Succ1);

    LLVM_DEBUG(dbgs() << "Added external branch: " << P->getName() << " -> "
                      << (Succ0 ? Succ0->getName() : "") << " "
                      << (Succ1 ? Succ1->getName() : "") << "\n");
  }

  // Redirect all the backedges through a "hub" consisting of a series
  // of guard blocks that manage the flow of control from the
  // predecessors to the headers.
  SmallVector<BasicBlock *> GuardBlocks;

  // Minor optimization: The cycle entries are discovered in an order that is
  // the opposite of the order in which these blocks appear as branch targets.
  // This results in a lot of condition inversions in the control flow out of
  // the new ControlFlowHub, which can be mitigated if the orders match. So we
  // reverse the entries when adding them to the hub.
  SetVector<BasicBlock *> Entries;
  Entries.insert(C.entry_rbegin(), C.entry_rend());

  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
  CHub.finalize(&DTU, GuardBlocks, "irr");
#if defined(EXPENSIVE_CHECKS)
  assert(DT.verify(DominatorTree::VerificationLevel::Full));
#else
  assert(DT.verify(DominatorTree::VerificationLevel::Fast));
#endif

  // If we are updating LoopInfo, do that now before modifying the cycle. This
  // ensures that the first guard block is the header of a new natural loop.
  if (LI)
    updateLoopInfo(*LI, C, GuardBlocks);

  for (auto *G : GuardBlocks) {
    LLVM_DEBUG(dbgs() << "added guard block to cycle: " << G->getName()
                      << "\n");
    CI.addBlockToCycle(G, &C);
  }
  C.setSingleEntry(GuardBlocks[0]);

  C.verifyCycle();
  if (Cycle *Parent = C.getParentCycle())
    Parent->verifyCycle();

  LLVM_DEBUG(dbgs() << "Finished one cycle:\n"; CI.print(dbgs()););
  return true;
}

static bool FixIrreducibleImpl(Function &F, CycleInfo &CI, DominatorTree &DT,
                               LoopInfo *LI) {
  LLVM_DEBUG(dbgs() << "===== Fix irreducible control-flow in function: "
                    << F.getName() << "\n");

  assert(hasOnlySimpleTerminator(F) && "Unsupported block terminator.");

  bool Changed = false;
  for (Cycle *TopCycle : CI.toplevel_cycles()) {
    for (Cycle *C : depth_first(TopCycle)) {
      Changed |= fixIrreducible(*C, CI, DT, LI);
    }
  }

  if (!Changed)
    return false;

#if defined(EXPENSIVE_CHECKS)
  CI.verify();
  if (LI) {
    LI->verify(DT);
  }
#endif // EXPENSIVE_CHECKS

  return true;
}

bool FixIrreducible::runOnFunction(Function &F) {
  auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
  LoopInfo *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
  auto &CI = getAnalysis<CycleInfoWrapperPass>().getResult();
  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  return FixIrreducibleImpl(F, CI, DT, LI);
}

PreservedAnalyses FixIrreduciblePass::run(Function &F,
                                          FunctionAnalysisManager &AM) {
  auto *LI = AM.getCachedResult<LoopAnalysis>(F);
  auto &CI = AM.getResult<CycleAnalysis>(F);
  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);

  if (!FixIrreducibleImpl(F, CI, DT, LI))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserve<LoopAnalysis>();
  PA.preserve<CycleAnalysis>();
  PA.preserve<DominatorTreeAnalysis>();
  return PA;
}