4c82754704
While we do not want to form actual lookup tables early, we do want to perform some optimizations, as they may enable inlining of the much simpler form. Builds on https://github.com/llvm/llvm-project/pull/156477, which originally included this change as well. This PR makes two changes on top of it: * Do not perform the optimization early if it requires adding a mask check. These make the resulting IR less analyzable. * Add a new SimplifyCFG option that controls switch-to-arithmetic conversion separately from switch-to-lookup conversion. Enable the new flag at the end of the function simplification pipeline. This means that we attempt the arithmetic conversion before inlining, but avoid it in the early pipeline, where it may lose information.
444 lines
17 KiB
C++
444 lines
17 KiB
C++
//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements dead code elimination and basic block merging, along
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// with a collection of other peephole control flow optimizations. For example:
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//
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// * Removes basic blocks with no predecessors.
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// * Merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor.
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// * Eliminates a basic block that only contains an unconditional branch.
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// * Changes invoke instructions to nounwind functions to be calls.
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// * Change things like "if (x) if (y)" into "if (x&y)".
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// * etc..
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/MapVector.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/DomTreeUpdater.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/TargetTransformInfo.h"
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#include "llvm/IR/Attributes.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/ValueHandle.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Scalar/SimplifyCFG.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/SimplifyCFGOptions.h"
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#include <utility>
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using namespace llvm;
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#define DEBUG_TYPE "simplifycfg"
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static cl::opt<unsigned> UserBonusInstThreshold(
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"bonus-inst-threshold", cl::Hidden, cl::init(1),
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cl::desc("Control the number of bonus instructions (default = 1)"));
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static cl::opt<bool> UserKeepLoops(
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"keep-loops", cl::Hidden, cl::init(true),
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cl::desc("Preserve canonical loop structure (default = true)"));
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static cl::opt<bool> UserSwitchRangeToICmp(
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"switch-range-to-icmp", cl::Hidden, cl::init(false),
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cl::desc(
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"Convert switches into an integer range comparison (default = false)"));
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static cl::opt<bool> UserSwitchToLookup(
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"switch-to-lookup", cl::Hidden, cl::init(false),
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cl::desc("Convert switches to lookup tables (default = false)"));
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static cl::opt<bool> UserForwardSwitchCond(
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"forward-switch-cond", cl::Hidden, cl::init(false),
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cl::desc("Forward switch condition to phi ops (default = false)"));
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static cl::opt<bool> UserHoistCommonInsts(
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"hoist-common-insts", cl::Hidden, cl::init(false),
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cl::desc("hoist common instructions (default = false)"));
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static cl::opt<bool> UserHoistLoadsStoresWithCondFaulting(
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"hoist-loads-stores-with-cond-faulting", cl::Hidden, cl::init(false),
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cl::desc("Hoist loads/stores if the target supports conditional faulting "
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"(default = false)"));
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static cl::opt<bool> UserSinkCommonInsts(
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"sink-common-insts", cl::Hidden, cl::init(false),
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cl::desc("Sink common instructions (default = false)"));
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static cl::opt<bool> UserSpeculateUnpredictables(
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"speculate-unpredictables", cl::Hidden, cl::init(false),
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cl::desc("Speculate unpredictable branches (default = false)"));
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STATISTIC(NumSimpl, "Number of blocks simplified");
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static bool
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performBlockTailMerging(Function &F, ArrayRef<BasicBlock *> BBs,
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std::vector<DominatorTree::UpdateType> *Updates) {
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SmallVector<PHINode *, 1> NewOps;
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// We don't want to change IR just because we can.
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// Only do that if there are at least two blocks we'll tail-merge.
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if (BBs.size() < 2)
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return false;
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if (Updates)
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Updates->reserve(Updates->size() + BBs.size());
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BasicBlock *CanonicalBB;
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Instruction *CanonicalTerm;
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{
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auto *Term = BBs[0]->getTerminator();
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// Create a canonical block for this function terminator type now,
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// placing it *before* the first block that will branch to it.
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CanonicalBB = BasicBlock::Create(
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F.getContext(), Twine("common.") + Term->getOpcodeName(), &F, BBs[0]);
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// We'll also need a PHI node per each operand of the terminator.
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NewOps.resize(Term->getNumOperands());
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for (auto I : zip(Term->operands(), NewOps)) {
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std::get<1>(I) = PHINode::Create(std::get<0>(I)->getType(),
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/*NumReservedValues=*/BBs.size(),
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CanonicalBB->getName() + ".op");
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std::get<1>(I)->insertInto(CanonicalBB, CanonicalBB->end());
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}
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// Make it so that this canonical block actually has the right
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// terminator.
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CanonicalTerm = Term->clone();
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CanonicalTerm->insertInto(CanonicalBB, CanonicalBB->end());
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// If the canonical terminator has operands, rewrite it to take PHI's.
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for (auto I : zip(NewOps, CanonicalTerm->operands()))
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std::get<1>(I) = std::get<0>(I);
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}
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// Now, go through each block (with the current terminator type)
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// we've recorded, and rewrite it to branch to the new common block.
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DebugLoc CommonDebugLoc;
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for (BasicBlock *BB : BBs) {
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auto *Term = BB->getTerminator();
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assert(Term->getOpcode() == CanonicalTerm->getOpcode() &&
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"All blocks to be tail-merged must be the same "
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"(function-terminating) terminator type.");
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// Aha, found a new non-canonical function terminator. If it has operands,
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// forward them to the PHI nodes in the canonical block.
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for (auto I : zip(Term->operands(), NewOps))
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std::get<1>(I)->addIncoming(std::get<0>(I), BB);
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// Compute the debug location common to all the original terminators.
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if (!CommonDebugLoc)
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CommonDebugLoc = Term->getDebugLoc();
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else
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CommonDebugLoc =
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DebugLoc::getMergedLocation(CommonDebugLoc, Term->getDebugLoc());
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// And turn BB into a block that just unconditionally branches
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// to the canonical block.
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Instruction *BI = BranchInst::Create(CanonicalBB, BB);
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BI->setDebugLoc(Term->getDebugLoc());
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Term->eraseFromParent();
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if (Updates)
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Updates->push_back({DominatorTree::Insert, BB, CanonicalBB});
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}
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CanonicalTerm->setDebugLoc(CommonDebugLoc);
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return true;
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}
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static bool tailMergeBlocksWithSimilarFunctionTerminators(Function &F,
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DomTreeUpdater *DTU) {
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SmallMapVector<unsigned /*TerminatorOpcode*/, SmallVector<BasicBlock *, 2>, 4>
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Structure;
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// Scan all the blocks in the function, record the interesting-ones.
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for (BasicBlock &BB : F) {
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if (DTU && DTU->isBBPendingDeletion(&BB))
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continue;
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// We are only interested in function-terminating blocks.
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if (!succ_empty(&BB))
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continue;
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auto *Term = BB.getTerminator();
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// Fow now only support `ret`/`resume` function terminators.
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// FIXME: lift this restriction.
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switch (Term->getOpcode()) {
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case Instruction::Ret:
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case Instruction::Resume:
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break;
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default:
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continue;
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}
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// We can't tail-merge block that contains a musttail call.
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if (BB.getTerminatingMustTailCall())
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continue;
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// Calls to experimental_deoptimize must be followed by a return
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// of the value computed by experimental_deoptimize.
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// I.e., we can not change `ret` to `br` for this block.
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if (auto *CI = dyn_cast_or_null<CallInst>(Term->getPrevNode())) {
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if (Function *F = CI->getCalledFunction())
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if (Intrinsic::ID ID = F->getIntrinsicID())
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if (ID == Intrinsic::experimental_deoptimize)
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continue;
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}
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// PHI nodes cannot have token type, so if the terminator has an operand
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// with token type, we can not tail-merge this kind of function terminators.
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if (any_of(Term->operands(),
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[](Value *Op) { return Op->getType()->isTokenTy(); }))
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continue;
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// Canonical blocks are uniqued based on the terminator type (opcode).
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Structure[Term->getOpcode()].emplace_back(&BB);
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}
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bool Changed = false;
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std::vector<DominatorTree::UpdateType> Updates;
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for (ArrayRef<BasicBlock *> BBs : make_second_range(Structure))
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Changed |= performBlockTailMerging(F, BBs, DTU ? &Updates : nullptr);
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if (DTU)
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DTU->applyUpdates(Updates);
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return Changed;
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}
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/// Call SimplifyCFG on all the blocks in the function,
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/// iterating until no more changes are made.
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static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
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DomTreeUpdater *DTU,
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const SimplifyCFGOptions &Options) {
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bool Changed = false;
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bool LocalChange = true;
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SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
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FindFunctionBackedges(F, Edges);
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SmallPtrSet<BasicBlock *, 16> UniqueLoopHeaders;
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for (const auto &Edge : Edges)
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UniqueLoopHeaders.insert(const_cast<BasicBlock *>(Edge.second));
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SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(),
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UniqueLoopHeaders.end());
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unsigned IterCnt = 0;
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(void)IterCnt;
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while (LocalChange) {
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assert(IterCnt++ < 1000 && "Iterative simplification didn't converge!");
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LocalChange = false;
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// Loop over all of the basic blocks and remove them if they are unneeded.
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for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
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BasicBlock &BB = *BBIt++;
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if (DTU) {
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assert(
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!DTU->isBBPendingDeletion(&BB) &&
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"Should not end up trying to simplify blocks marked for removal.");
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// Make sure that the advanced iterator does not point at the blocks
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// that are marked for removal, skip over all such blocks.
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while (BBIt != F.end() && DTU->isBBPendingDeletion(&*BBIt))
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++BBIt;
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}
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if (simplifyCFG(&BB, TTI, DTU, Options, LoopHeaders)) {
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LocalChange = true;
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++NumSimpl;
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}
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI,
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DominatorTree *DT,
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const SimplifyCFGOptions &Options) {
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DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
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bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr);
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EverChanged |=
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tailMergeBlocksWithSimilarFunctionTerminators(F, DT ? &DTU : nullptr);
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EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options);
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// If neither pass changed anything, we're done.
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if (!EverChanged) return false;
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// iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
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// removeUnreachableBlocks is needed to nuke them, which means we should
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// iterate between the two optimizations. We structure the code like this to
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// avoid rerunning iterativelySimplifyCFG if the second pass of
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// removeUnreachableBlocks doesn't do anything.
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if (!removeUnreachableBlocks(F, DT ? &DTU : nullptr))
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return true;
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do {
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EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options);
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EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr);
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} while (EverChanged);
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return true;
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}
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static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
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DominatorTree *DT,
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const SimplifyCFGOptions &Options) {
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assert((!RequireAndPreserveDomTree ||
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(DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
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"Original domtree is invalid?");
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bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
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assert((!RequireAndPreserveDomTree ||
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(DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
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"Failed to maintain validity of domtree!");
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return Changed;
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}
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// Command-line settings override compile-time settings.
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static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
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if (UserBonusInstThreshold.getNumOccurrences())
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Options.BonusInstThreshold = UserBonusInstThreshold;
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if (UserForwardSwitchCond.getNumOccurrences())
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Options.ForwardSwitchCondToPhi = UserForwardSwitchCond;
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if (UserSwitchRangeToICmp.getNumOccurrences())
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Options.ConvertSwitchRangeToICmp = UserSwitchRangeToICmp;
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if (UserSwitchToLookup.getNumOccurrences())
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Options.ConvertSwitchToLookupTable = UserSwitchToLookup;
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if (UserKeepLoops.getNumOccurrences())
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Options.NeedCanonicalLoop = UserKeepLoops;
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if (UserHoistCommonInsts.getNumOccurrences())
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Options.HoistCommonInsts = UserHoistCommonInsts;
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if (UserHoistLoadsStoresWithCondFaulting.getNumOccurrences())
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Options.HoistLoadsStoresWithCondFaulting =
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UserHoistLoadsStoresWithCondFaulting;
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if (UserSinkCommonInsts.getNumOccurrences())
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Options.SinkCommonInsts = UserSinkCommonInsts;
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if (UserSpeculateUnpredictables.getNumOccurrences())
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Options.SpeculateUnpredictables = UserSpeculateUnpredictables;
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}
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SimplifyCFGPass::SimplifyCFGPass() {
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applyCommandLineOverridesToOptions(Options);
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}
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SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts)
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: Options(Opts) {
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applyCommandLineOverridesToOptions(Options);
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}
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void SimplifyCFGPass::printPipeline(
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raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
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static_cast<PassInfoMixin<SimplifyCFGPass> *>(this)->printPipeline(
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OS, MapClassName2PassName);
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OS << '<';
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OS << "bonus-inst-threshold=" << Options.BonusInstThreshold << ';';
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OS << (Options.ForwardSwitchCondToPhi ? "" : "no-") << "forward-switch-cond;";
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OS << (Options.ConvertSwitchRangeToICmp ? "" : "no-")
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<< "switch-range-to-icmp;";
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OS << (Options.ConvertSwitchToArithmetic ? "" : "no-")
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<< "switch-to-arithmetic;";
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OS << (Options.ConvertSwitchToLookupTable ? "" : "no-")
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<< "switch-to-lookup;";
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OS << (Options.NeedCanonicalLoop ? "" : "no-") << "keep-loops;";
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OS << (Options.HoistCommonInsts ? "" : "no-") << "hoist-common-insts;";
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OS << (Options.HoistLoadsStoresWithCondFaulting ? "" : "no-")
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<< "hoist-loads-stores-with-cond-faulting;";
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OS << (Options.SinkCommonInsts ? "" : "no-") << "sink-common-insts;";
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OS << (Options.SpeculateBlocks ? "" : "no-") << "speculate-blocks;";
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OS << (Options.SimplifyCondBranch ? "" : "no-") << "simplify-cond-branch;";
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OS << (Options.SpeculateUnpredictables ? "" : "no-")
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<< "speculate-unpredictables";
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OS << '>';
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}
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PreservedAnalyses SimplifyCFGPass::run(Function &F,
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FunctionAnalysisManager &AM) {
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auto &TTI = AM.getResult<TargetIRAnalysis>(F);
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Options.AC = &AM.getResult<AssumptionAnalysis>(F);
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DominatorTree *DT = nullptr;
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if (RequireAndPreserveDomTree)
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DT = &AM.getResult<DominatorTreeAnalysis>(F);
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if (!simplifyFunctionCFG(F, TTI, DT, Options))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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if (RequireAndPreserveDomTree)
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PA.preserve<DominatorTreeAnalysis>();
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return PA;
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}
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namespace {
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struct CFGSimplifyPass : public FunctionPass {
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static char ID;
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SimplifyCFGOptions Options;
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std::function<bool(const Function &)> PredicateFtor;
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CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(),
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std::function<bool(const Function &)> Ftor = nullptr)
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: FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) {
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initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
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// Check for command-line overrides of options for debug/customization.
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applyCommandLineOverridesToOptions(Options);
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}
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bool runOnFunction(Function &F) override {
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if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
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return false;
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Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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DominatorTree *DT = nullptr;
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if (RequireAndPreserveDomTree)
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DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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return simplifyFunctionCFG(F, TTI, DT, Options);
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<AssumptionCacheTracker>();
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if (RequireAndPreserveDomTree)
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<TargetTransformInfoWrapperPass>();
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if (RequireAndPreserveDomTree)
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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}
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};
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}
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char CFGSimplifyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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false)
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INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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false)
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// Public interface to the CFGSimplification pass
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FunctionPass *
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llvm::createCFGSimplificationPass(SimplifyCFGOptions Options,
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std::function<bool(const Function &)> Ftor) {
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return new CFGSimplifyPass(Options, std::move(Ftor));
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}
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