| //===- JumpThreading.cpp - Thread control through conditional blocks ------===// |
| // |
| // 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 |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements the Jump Threading pass. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar/JumpThreading.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/DenseSet.h" |
| #include "llvm/ADT/MapVector.h" |
| #include "llvm/ADT/Optional.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/BlockFrequencyInfo.h" |
| #include "llvm/Analysis/BranchProbabilityInfo.h" |
| #include "llvm/Analysis/CFG.h" |
| #include "llvm/Analysis/ConstantFolding.h" |
| #include "llvm/Analysis/DomTreeUpdater.h" |
| #include "llvm/Analysis/GlobalsModRef.h" |
| #include "llvm/Analysis/GuardUtils.h" |
| #include "llvm/Analysis/InstructionSimplify.h" |
| #include "llvm/Analysis/LazyValueInfo.h" |
| #include "llvm/Analysis/Loads.h" |
| #include "llvm/Analysis/LoopInfo.h" |
| #include "llvm/Analysis/MemoryLocation.h" |
| #include "llvm/Analysis/TargetLibraryInfo.h" |
| #include "llvm/Analysis/TargetTransformInfo.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/IR/BasicBlock.h" |
| #include "llvm/IR/CFG.h" |
| #include "llvm/IR/Constant.h" |
| #include "llvm/IR/ConstantRange.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/InstrTypes.h" |
| #include "llvm/IR/Instruction.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/IR/Intrinsics.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/IR/MDBuilder.h" |
| #include "llvm/IR/Metadata.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/IR/PassManager.h" |
| #include "llvm/IR/PatternMatch.h" |
| #include "llvm/IR/Type.h" |
| #include "llvm/IR/Use.h" |
| #include "llvm/IR/User.h" |
| #include "llvm/IR/Value.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/BlockFrequency.h" |
| #include "llvm/Support/BranchProbability.h" |
| #include "llvm/Support/Casting.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Cloning.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Transforms/Utils/SSAUpdater.h" |
| #include "llvm/Transforms/Utils/ValueMapper.h" |
| #include <algorithm> |
| #include <cassert> |
| #include <cstddef> |
| #include <cstdint> |
| #include <iterator> |
| #include <memory> |
| #include <utility> |
| |
| using namespace llvm; |
| using namespace jumpthreading; |
| |
| #define DEBUG_TYPE "jump-threading" |
| |
| STATISTIC(NumThreads, "Number of jumps threaded"); |
| STATISTIC(NumFolds, "Number of terminators folded"); |
| STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); |
| |
| static cl::opt<unsigned> |
| BBDuplicateThreshold("jump-threading-threshold", |
| cl::desc("Max block size to duplicate for jump threading"), |
| cl::init(6), cl::Hidden); |
| |
| static cl::opt<unsigned> |
| ImplicationSearchThreshold( |
| "jump-threading-implication-search-threshold", |
| cl::desc("The number of predecessors to search for a stronger " |
| "condition to use to thread over a weaker condition"), |
| cl::init(3), cl::Hidden); |
| |
| static cl::opt<bool> PrintLVIAfterJumpThreading( |
| "print-lvi-after-jump-threading", |
| cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false), |
| cl::Hidden); |
| |
| static cl::opt<bool> JumpThreadingFreezeSelectCond( |
| "jump-threading-freeze-select-cond", |
| cl::desc("Freeze the condition when unfolding select"), cl::init(false), |
| cl::Hidden); |
| |
| static cl::opt<bool> ThreadAcrossLoopHeaders( |
| "jump-threading-across-loop-headers", |
| cl::desc("Allow JumpThreading to thread across loop headers, for testing"), |
| cl::init(false), cl::Hidden); |
| |
| |
| namespace { |
| |
| /// This pass performs 'jump threading', which looks at blocks that have |
| /// multiple predecessors and multiple successors. If one or more of the |
| /// predecessors of the block can be proven to always jump to one of the |
| /// successors, we forward the edge from the predecessor to the successor by |
| /// duplicating the contents of this block. |
| /// |
| /// An example of when this can occur is code like this: |
| /// |
| /// if () { ... |
| /// X = 4; |
| /// } |
| /// if (X < 3) { |
| /// |
| /// In this case, the unconditional branch at the end of the first if can be |
| /// revectored to the false side of the second if. |
| class JumpThreading : public FunctionPass { |
| JumpThreadingPass Impl; |
| |
| public: |
| static char ID; // Pass identification |
| |
| JumpThreading(bool InsertFreezeWhenUnfoldingSelect = false, int T = -1) |
| : FunctionPass(ID), Impl(InsertFreezeWhenUnfoldingSelect, T) { |
| initializeJumpThreadingPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F) override; |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.addRequired<DominatorTreeWrapperPass>(); |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| AU.addRequired<AAResultsWrapperPass>(); |
| AU.addRequired<LazyValueInfoWrapperPass>(); |
| AU.addPreserved<LazyValueInfoWrapperPass>(); |
| AU.addPreserved<GlobalsAAWrapperPass>(); |
| AU.addRequired<TargetLibraryInfoWrapperPass>(); |
| AU.addRequired<TargetTransformInfoWrapperPass>(); |
| } |
| |
| void releaseMemory() override { Impl.releaseMemory(); } |
| }; |
| |
| } // end anonymous namespace |
| |
| char JumpThreading::ID = 0; |
| |
| INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading", |
| "Jump Threading", false, false) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) |
| INITIALIZE_PASS_END(JumpThreading, "jump-threading", |
| "Jump Threading", false, false) |
| |
| // Public interface to the Jump Threading pass |
| FunctionPass *llvm::createJumpThreadingPass(bool InsertFr, int Threshold) { |
| return new JumpThreading(InsertFr, Threshold); |
| } |
| |
| JumpThreadingPass::JumpThreadingPass(bool InsertFr, int T) { |
| InsertFreezeWhenUnfoldingSelect = JumpThreadingFreezeSelectCond | InsertFr; |
| DefaultBBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T); |
| } |
| |
| // Update branch probability information according to conditional |
| // branch probability. This is usually made possible for cloned branches |
| // in inline instances by the context specific profile in the caller. |
| // For instance, |
| // |
| // [Block PredBB] |
| // [Branch PredBr] |
| // if (t) { |
| // Block A; |
| // } else { |
| // Block B; |
| // } |
| // |
| // [Block BB] |
| // cond = PN([true, %A], [..., %B]); // PHI node |
| // [Branch CondBr] |
| // if (cond) { |
| // ... // P(cond == true) = 1% |
| // } |
| // |
| // Here we know that when block A is taken, cond must be true, which means |
| // P(cond == true | A) = 1 |
| // |
| // Given that P(cond == true) = P(cond == true | A) * P(A) + |
| // P(cond == true | B) * P(B) |
| // we get: |
| // P(cond == true ) = P(A) + P(cond == true | B) * P(B) |
| // |
| // which gives us: |
| // P(A) is less than P(cond == true), i.e. |
| // P(t == true) <= P(cond == true) |
| // |
| // In other words, if we know P(cond == true) is unlikely, we know |
| // that P(t == true) is also unlikely. |
| // |
| static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) { |
| BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); |
| if (!CondBr) |
| return; |
| |
| uint64_t TrueWeight, FalseWeight; |
| if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight)) |
| return; |
| |
| if (TrueWeight + FalseWeight == 0) |
| // Zero branch_weights do not give a hint for getting branch probabilities. |
| // Technically it would result in division by zero denominator, which is |
| // TrueWeight + FalseWeight. |
| return; |
| |
| // Returns the outgoing edge of the dominating predecessor block |
| // that leads to the PhiNode's incoming block: |
| auto GetPredOutEdge = |
| [](BasicBlock *IncomingBB, |
| BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> { |
| auto *PredBB = IncomingBB; |
| auto *SuccBB = PhiBB; |
| SmallPtrSet<BasicBlock *, 16> Visited; |
| while (true) { |
| BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()); |
| if (PredBr && PredBr->isConditional()) |
| return {PredBB, SuccBB}; |
| Visited.insert(PredBB); |
| auto *SinglePredBB = PredBB->getSinglePredecessor(); |
| if (!SinglePredBB) |
| return {nullptr, nullptr}; |
| |
| // Stop searching when SinglePredBB has been visited. It means we see |
| // an unreachable loop. |
| if (Visited.count(SinglePredBB)) |
| return {nullptr, nullptr}; |
| |
| SuccBB = PredBB; |
| PredBB = SinglePredBB; |
| } |
| }; |
| |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| Value *PhiOpnd = PN->getIncomingValue(i); |
| ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd); |
| |
| if (!CI || !CI->getType()->isIntegerTy(1)) |
| continue; |
| |
| BranchProbability BP = |
| (CI->isOne() ? BranchProbability::getBranchProbability( |
| TrueWeight, TrueWeight + FalseWeight) |
| : BranchProbability::getBranchProbability( |
| FalseWeight, TrueWeight + FalseWeight)); |
| |
| auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB); |
| if (!PredOutEdge.first) |
| return; |
| |
| BasicBlock *PredBB = PredOutEdge.first; |
| BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()); |
| if (!PredBr) |
| return; |
| |
| uint64_t PredTrueWeight, PredFalseWeight; |
| // FIXME: We currently only set the profile data when it is missing. |
| // With PGO, this can be used to refine even existing profile data with |
| // context information. This needs to be done after more performance |
| // testing. |
| if (PredBr->extractProfMetadata(PredTrueWeight, PredFalseWeight)) |
| continue; |
| |
| // We can not infer anything useful when BP >= 50%, because BP is the |
| // upper bound probability value. |
| if (BP >= BranchProbability(50, 100)) |
| continue; |
| |
| SmallVector<uint32_t, 2> Weights; |
| if (PredBr->getSuccessor(0) == PredOutEdge.second) { |
| Weights.push_back(BP.getNumerator()); |
| Weights.push_back(BP.getCompl().getNumerator()); |
| } else { |
| Weights.push_back(BP.getCompl().getNumerator()); |
| Weights.push_back(BP.getNumerator()); |
| } |
| PredBr->setMetadata(LLVMContext::MD_prof, |
| MDBuilder(PredBr->getParent()->getContext()) |
| .createBranchWeights(Weights)); |
| } |
| } |
| |
| /// runOnFunction - Toplevel algorithm. |
| bool JumpThreading::runOnFunction(Function &F) { |
| if (skipFunction(F)) |
| return false; |
| auto TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); |
| // Jump Threading has no sense for the targets with divergent CF |
| if (TTI->hasBranchDivergence()) |
| return false; |
| auto TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F); |
| auto DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| auto LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI(); |
| auto AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); |
| DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Lazy); |
| std::unique_ptr<BlockFrequencyInfo> BFI; |
| std::unique_ptr<BranchProbabilityInfo> BPI; |
| if (F.hasProfileData()) { |
| LoopInfo LI{DominatorTree(F)}; |
| BPI.reset(new BranchProbabilityInfo(F, LI, TLI)); |
| BFI.reset(new BlockFrequencyInfo(F, *BPI, LI)); |
| } |
| |
| bool Changed = Impl.runImpl(F, TLI, TTI, LVI, AA, &DTU, F.hasProfileData(), |
| std::move(BFI), std::move(BPI)); |
| if (PrintLVIAfterJumpThreading) { |
| dbgs() << "LVI for function '" << F.getName() << "':\n"; |
| LVI->printLVI(F, DTU.getDomTree(), dbgs()); |
| } |
| return Changed; |
| } |
| |
| PreservedAnalyses JumpThreadingPass::run(Function &F, |
| FunctionAnalysisManager &AM) { |
| auto &TTI = AM.getResult<TargetIRAnalysis>(F); |
| // Jump Threading has no sense for the targets with divergent CF |
| if (TTI.hasBranchDivergence()) |
| return PreservedAnalyses::all(); |
| auto &TLI = AM.getResult<TargetLibraryAnalysis>(F); |
| auto &DT = AM.getResult<DominatorTreeAnalysis>(F); |
| auto &LVI = AM.getResult<LazyValueAnalysis>(F); |
| auto &AA = AM.getResult<AAManager>(F); |
| DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); |
| |
| std::unique_ptr<BlockFrequencyInfo> BFI; |
| std::unique_ptr<BranchProbabilityInfo> BPI; |
| if (F.hasProfileData()) { |
| LoopInfo LI{DominatorTree(F)}; |
| BPI.reset(new BranchProbabilityInfo(F, LI, &TLI)); |
| BFI.reset(new BlockFrequencyInfo(F, *BPI, LI)); |
| } |
| |
| bool Changed = runImpl(F, &TLI, &TTI, &LVI, &AA, &DTU, F.hasProfileData(), |
| std::move(BFI), std::move(BPI)); |
| |
| if (PrintLVIAfterJumpThreading) { |
| dbgs() << "LVI for function '" << F.getName() << "':\n"; |
| LVI.printLVI(F, DTU.getDomTree(), dbgs()); |
| } |
| |
| if (!Changed) |
| return PreservedAnalyses::all(); |
| PreservedAnalyses PA; |
| PA.preserve<DominatorTreeAnalysis>(); |
| PA.preserve<LazyValueAnalysis>(); |
| return PA; |
| } |
| |
| bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_, |
| TargetTransformInfo *TTI_, LazyValueInfo *LVI_, |
| AliasAnalysis *AA_, DomTreeUpdater *DTU_, |
| bool HasProfileData_, |
| std::unique_ptr<BlockFrequencyInfo> BFI_, |
| std::unique_ptr<BranchProbabilityInfo> BPI_) { |
| LLVM_DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); |
| TLI = TLI_; |
| TTI = TTI_; |
| LVI = LVI_; |
| AA = AA_; |
| DTU = DTU_; |
| BFI.reset(); |
| BPI.reset(); |
| // When profile data is available, we need to update edge weights after |
| // successful jump threading, which requires both BPI and BFI being available. |
| HasProfileData = HasProfileData_; |
| auto *GuardDecl = F.getParent()->getFunction( |
| Intrinsic::getName(Intrinsic::experimental_guard)); |
| HasGuards = GuardDecl && !GuardDecl->use_empty(); |
| if (HasProfileData) { |
| BPI = std::move(BPI_); |
| BFI = std::move(BFI_); |
| } |
| |
| // Reduce the number of instructions duplicated when optimizing strictly for |
| // size. |
| if (BBDuplicateThreshold.getNumOccurrences()) |
| BBDupThreshold = BBDuplicateThreshold; |
| else if (F.hasFnAttribute(Attribute::MinSize)) |
| BBDupThreshold = 3; |
| else |
| BBDupThreshold = DefaultBBDupThreshold; |
| |
| // JumpThreading must not processes blocks unreachable from entry. It's a |
| // waste of compute time and can potentially lead to hangs. |
| SmallPtrSet<BasicBlock *, 16> Unreachable; |
| assert(DTU && "DTU isn't passed into JumpThreading before using it."); |
| assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed."); |
| DominatorTree &DT = DTU->getDomTree(); |
| for (auto &BB : F) |
| if (!DT.isReachableFromEntry(&BB)) |
| Unreachable.insert(&BB); |
| |
| if (!ThreadAcrossLoopHeaders) |
| findLoopHeaders(F); |
| |
| bool EverChanged = false; |
| bool Changed; |
| do { |
| Changed = false; |
| for (auto &BB : F) { |
| if (Unreachable.count(&BB)) |
| continue; |
| while (processBlock(&BB)) // Thread all of the branches we can over BB. |
| Changed = true; |
| |
| // Jump threading may have introduced redundant debug values into BB |
| // which should be removed. |
| if (Changed) |
| RemoveRedundantDbgInstrs(&BB); |
| |
| // Stop processing BB if it's the entry or is now deleted. The following |
| // routines attempt to eliminate BB and locating a suitable replacement |
| // for the entry is non-trivial. |
| if (&BB == &F.getEntryBlock() || DTU->isBBPendingDeletion(&BB)) |
| continue; |
| |
| if (pred_empty(&BB)) { |
| // When processBlock makes BB unreachable it doesn't bother to fix up |
| // the instructions in it. We must remove BB to prevent invalid IR. |
| LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB.getName() |
| << "' with terminator: " << *BB.getTerminator() |
| << '\n'); |
| LoopHeaders.erase(&BB); |
| LVI->eraseBlock(&BB); |
| DeleteDeadBlock(&BB, DTU); |
| Changed = true; |
| continue; |
| } |
| |
| // processBlock doesn't thread BBs with unconditional TIs. However, if BB |
| // is "almost empty", we attempt to merge BB with its sole successor. |
| auto *BI = dyn_cast<BranchInst>(BB.getTerminator()); |
| if (BI && BI->isUnconditional()) { |
| BasicBlock *Succ = BI->getSuccessor(0); |
| if ( |
| // The terminator must be the only non-phi instruction in BB. |
| BB.getFirstNonPHIOrDbg(true)->isTerminator() && |
| // Don't alter Loop headers and latches to ensure another pass can |
| // detect and transform nested loops later. |
| !LoopHeaders.count(&BB) && !LoopHeaders.count(Succ) && |
| TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU)) { |
| RemoveRedundantDbgInstrs(Succ); |
| // BB is valid for cleanup here because we passed in DTU. F remains |
| // BB's parent until a DTU->getDomTree() event. |
| LVI->eraseBlock(&BB); |
| Changed = true; |
| } |
| } |
| } |
| EverChanged |= Changed; |
| } while (Changed); |
| |
| LoopHeaders.clear(); |
| return EverChanged; |
| } |
| |
| // Replace uses of Cond with ToVal when safe to do so. If all uses are |
| // replaced, we can remove Cond. We cannot blindly replace all uses of Cond |
| // because we may incorrectly replace uses when guards/assumes are uses of |
| // of `Cond` and we used the guards/assume to reason about the `Cond` value |
| // at the end of block. RAUW unconditionally replaces all uses |
| // including the guards/assumes themselves and the uses before the |
| // guard/assume. |
| static void replaceFoldableUses(Instruction *Cond, Value *ToVal) { |
| assert(Cond->getType() == ToVal->getType()); |
| auto *BB = Cond->getParent(); |
| // We can unconditionally replace all uses in non-local blocks (i.e. uses |
| // strictly dominated by BB), since LVI information is true from the |
| // terminator of BB. |
| replaceNonLocalUsesWith(Cond, ToVal); |
| for (Instruction &I : reverse(*BB)) { |
| // Reached the Cond whose uses we are trying to replace, so there are no |
| // more uses. |
| if (&I == Cond) |
| break; |
| // We only replace uses in instructions that are guaranteed to reach the end |
| // of BB, where we know Cond is ToVal. |
| if (!isGuaranteedToTransferExecutionToSuccessor(&I)) |
| break; |
| I.replaceUsesOfWith(Cond, ToVal); |
| } |
| if (Cond->use_empty() && !Cond->mayHaveSideEffects()) |
| Cond->eraseFromParent(); |
| } |
| |
| /// Return the cost of duplicating a piece of this block from first non-phi |
| /// and before StopAt instruction to thread across it. Stop scanning the block |
| /// when exceeding the threshold. If duplication is impossible, returns ~0U. |
| static unsigned getJumpThreadDuplicationCost(const TargetTransformInfo *TTI, |
| BasicBlock *BB, |
| Instruction *StopAt, |
| unsigned Threshold) { |
| assert(StopAt->getParent() == BB && "Not an instruction from proper BB?"); |
| /// Ignore PHI nodes, these will be flattened when duplication happens. |
| BasicBlock::const_iterator I(BB->getFirstNonPHI()); |
| |
| // FIXME: THREADING will delete values that are just used to compute the |
| // branch, so they shouldn't count against the duplication cost. |
| |
| unsigned Bonus = 0; |
| if (BB->getTerminator() == StopAt) { |
| // Threading through a switch statement is particularly profitable. If this |
| // block ends in a switch, decrease its cost to make it more likely to |
| // happen. |
| if (isa<SwitchInst>(StopAt)) |
| Bonus = 6; |
| |
| // The same holds for indirect branches, but slightly more so. |
| if (isa<IndirectBrInst>(StopAt)) |
| Bonus = 8; |
| } |
| |
| // Bump the threshold up so the early exit from the loop doesn't skip the |
| // terminator-based Size adjustment at the end. |
| Threshold += Bonus; |
| |
| // Sum up the cost of each instruction until we get to the terminator. Don't |
| // include the terminator because the copy won't include it. |
| unsigned Size = 0; |
| for (; &*I != StopAt; ++I) { |
| |
| // Stop scanning the block if we've reached the threshold. |
| if (Size > Threshold) |
| return Size; |
| |
| // Bail out if this instruction gives back a token type, it is not possible |
| // to duplicate it if it is used outside this BB. |
| if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB)) |
| return ~0U; |
| |
| // Blocks with NoDuplicate are modelled as having infinite cost, so they |
| // are never duplicated. |
| if (const CallInst *CI = dyn_cast<CallInst>(I)) |
| if (CI->cannotDuplicate() || CI->isConvergent()) |
| return ~0U; |
| |
| if (TTI->getUserCost(&*I, TargetTransformInfo::TCK_SizeAndLatency) |
| == TargetTransformInfo::TCC_Free) |
| continue; |
| |
| // All other instructions count for at least one unit. |
| ++Size; |
| |
| // Calls are more expensive. If they are non-intrinsic calls, we model them |
| // as having cost of 4. If they are a non-vector intrinsic, we model them |
| // as having cost of 2 total, and if they are a vector intrinsic, we model |
| // them as having cost 1. |
| if (const CallInst *CI = dyn_cast<CallInst>(I)) { |
| if (!isa<IntrinsicInst>(CI)) |
| Size += 3; |
| else if (!CI->getType()->isVectorTy()) |
| Size += 1; |
| } |
| } |
| |
| return Size > Bonus ? Size - Bonus : 0; |
| } |
| |
| /// findLoopHeaders - We do not want jump threading to turn proper loop |
| /// structures into irreducible loops. Doing this breaks up the loop nesting |
| /// hierarchy and pessimizes later transformations. To prevent this from |
| /// happening, we first have to find the loop headers. Here we approximate this |
| /// by finding targets of backedges in the CFG. |
| /// |
| /// Note that there definitely are cases when we want to allow threading of |
| /// edges across a loop header. For example, threading a jump from outside the |
| /// loop (the preheader) to an exit block of the loop is definitely profitable. |
| /// It is also almost always profitable to thread backedges from within the loop |
| /// to exit blocks, and is often profitable to thread backedges to other blocks |
| /// within the loop (forming a nested loop). This simple analysis is not rich |
| /// enough to track all of these properties and keep it up-to-date as the CFG |
| /// mutates, so we don't allow any of these transformations. |
| void JumpThreadingPass::findLoopHeaders(Function &F) { |
| SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; |
| FindFunctionBackedges(F, Edges); |
| |
| for (const auto &Edge : Edges) |
| LoopHeaders.insert(Edge.second); |
| } |
| |
| /// getKnownConstant - Helper method to determine if we can thread over a |
| /// terminator with the given value as its condition, and if so what value to |
| /// use for that. What kind of value this is depends on whether we want an |
| /// integer or a block address, but an undef is always accepted. |
| /// Returns null if Val is null or not an appropriate constant. |
| static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) { |
| if (!Val) |
| return nullptr; |
| |
| // Undef is "known" enough. |
| if (UndefValue *U = dyn_cast<UndefValue>(Val)) |
| return U; |
| |
| if (Preference == WantBlockAddress) |
| return dyn_cast<BlockAddress>(Val->stripPointerCasts()); |
| |
| return dyn_cast<ConstantInt>(Val); |
| } |
| |
| /// computeValueKnownInPredecessors - Given a basic block BB and a value V, see |
| /// if we can infer that the value is a known ConstantInt/BlockAddress or undef |
| /// in any of our predecessors. If so, return the known list of value and pred |
| /// BB in the result vector. |
| /// |
| /// This returns true if there were any known values. |
| bool JumpThreadingPass::computeValueKnownInPredecessorsImpl( |
| Value *V, BasicBlock *BB, PredValueInfo &Result, |
| ConstantPreference Preference, DenseSet<Value *> &RecursionSet, |
| Instruction *CxtI) { |
| // This method walks up use-def chains recursively. Because of this, we could |
| // get into an infinite loop going around loops in the use-def chain. To |
| // prevent this, keep track of what (value, block) pairs we've already visited |
| // and terminate the search if we loop back to them |
| if (!RecursionSet.insert(V).second) |
| return false; |
| |
| // If V is a constant, then it is known in all predecessors. |
| if (Constant *KC = getKnownConstant(V, Preference)) { |
| for (BasicBlock *Pred : predecessors(BB)) |
| Result.emplace_back(KC, Pred); |
| |
| return !Result.empty(); |
| } |
| |
| // If V is a non-instruction value, or an instruction in a different block, |
| // then it can't be derived from a PHI. |
| Instruction *I = dyn_cast<Instruction>(V); |
| if (!I || I->getParent() != BB) { |
| |
| // Okay, if this is a live-in value, see if it has a known value at the end |
| // of any of our predecessors. |
| // |
| // FIXME: This should be an edge property, not a block end property. |
| /// TODO: Per PR2563, we could infer value range information about a |
| /// predecessor based on its terminator. |
| // |
| // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if |
| // "I" is a non-local compare-with-a-constant instruction. This would be |
| // able to handle value inequalities better, for example if the compare is |
| // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. |
| // Perhaps getConstantOnEdge should be smart enough to do this? |
| for (BasicBlock *P : predecessors(BB)) { |
| // If the value is known by LazyValueInfo to be a constant in a |
| // predecessor, use that information to try to thread this block. |
| Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI); |
| if (Constant *KC = getKnownConstant(PredCst, Preference)) |
| Result.emplace_back(KC, P); |
| } |
| |
| return !Result.empty(); |
| } |
| |
| /// If I is a PHI node, then we know the incoming values for any constants. |
| if (PHINode *PN = dyn_cast<PHINode>(I)) { |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| Value *InVal = PN->getIncomingValue(i); |
| if (Constant *KC = getKnownConstant(InVal, Preference)) { |
| Result.emplace_back(KC, PN->getIncomingBlock(i)); |
| } else { |
| Constant *CI = LVI->getConstantOnEdge(InVal, |
| PN->getIncomingBlock(i), |
| BB, CxtI); |
| if (Constant *KC = getKnownConstant(CI, Preference)) |
| Result.emplace_back(KC, PN->getIncomingBlock(i)); |
| } |
| } |
| |
| return !Result.empty(); |
| } |
| |
| // Handle Cast instructions. |
| if (CastInst *CI = dyn_cast<CastInst>(I)) { |
| Value *Source = CI->getOperand(0); |
| computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference, |
| RecursionSet, CxtI); |
| if (Result.empty()) |
| return false; |
| |
| // Convert the known values. |
| for (auto &R : Result) |
| R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType()); |
| |
| return true; |
| } |
| |
| if (FreezeInst *FI = dyn_cast<FreezeInst>(I)) { |
| Value *Source = FI->getOperand(0); |
| computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference, |
| RecursionSet, CxtI); |
| |
| erase_if(Result, [](auto &Pair) { |
| return !isGuaranteedNotToBeUndefOrPoison(Pair.first); |
| }); |
| |
| return !Result.empty(); |
| } |
| |
| // Handle some boolean conditions. |
| if (I->getType()->getPrimitiveSizeInBits() == 1) { |
| using namespace PatternMatch; |
| |
| assert(Preference == WantInteger && "One-bit non-integer type?"); |
| // X | true -> true |
| // X & false -> false |
| Value *Op0, *Op1; |
| if (match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1))) || |
| match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) { |
| PredValueInfoTy LHSVals, RHSVals; |
| |
| computeValueKnownInPredecessorsImpl(Op0, BB, LHSVals, WantInteger, |
| RecursionSet, CxtI); |
| computeValueKnownInPredecessorsImpl(Op1, BB, RHSVals, WantInteger, |
| RecursionSet, CxtI); |
| |
| if (LHSVals.empty() && RHSVals.empty()) |
| return false; |
| |
| ConstantInt *InterestingVal; |
| if (match(I, m_LogicalOr())) |
| InterestingVal = ConstantInt::getTrue(I->getContext()); |
| else |
| InterestingVal = ConstantInt::getFalse(I->getContext()); |
| |
| SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; |
| |
| // Scan for the sentinel. If we find an undef, force it to the |
| // interesting value: x|undef -> true and x&undef -> false. |
| for (const auto &LHSVal : LHSVals) |
| if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) { |
| Result.emplace_back(InterestingVal, LHSVal.second); |
| LHSKnownBBs.insert(LHSVal.second); |
| } |
| for (const auto &RHSVal : RHSVals) |
| if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) { |
| // If we already inferred a value for this block on the LHS, don't |
| // re-add it. |
| if (!LHSKnownBBs.count(RHSVal.second)) |
| Result.emplace_back(InterestingVal, RHSVal.second); |
| } |
| |
| return !Result.empty(); |
| } |
| |
| // Handle the NOT form of XOR. |
| if (I->getOpcode() == Instruction::Xor && |
| isa<ConstantInt>(I->getOperand(1)) && |
| cast<ConstantInt>(I->getOperand(1))->isOne()) { |
| computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, Result, |
| WantInteger, RecursionSet, CxtI); |
| if (Result.empty()) |
| return false; |
| |
| // Invert the known values. |
| for (auto &R : Result) |
| R.first = ConstantExpr::getNot(R.first); |
| |
| return true; |
| } |
| |
| // Try to simplify some other binary operator values. |
| } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { |
| assert(Preference != WantBlockAddress |
| && "A binary operator creating a block address?"); |
| if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) { |
| PredValueInfoTy LHSVals; |
| computeValueKnownInPredecessorsImpl(BO->getOperand(0), BB, LHSVals, |
| WantInteger, RecursionSet, CxtI); |
| |
| // Try to use constant folding to simplify the binary operator. |
| for (const auto &LHSVal : LHSVals) { |
| Constant *V = LHSVal.first; |
| Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI); |
| |
| if (Constant *KC = getKnownConstant(Folded, WantInteger)) |
| Result.emplace_back(KC, LHSVal.second); |
| } |
| } |
| |
| return !Result.empty(); |
| } |
| |
| // Handle compare with phi operand, where the PHI is defined in this block. |
| if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { |
| assert(Preference == WantInteger && "Compares only produce integers"); |
| Type *CmpType = Cmp->getType(); |
| Value *CmpLHS = Cmp->getOperand(0); |
| Value *CmpRHS = Cmp->getOperand(1); |
| CmpInst::Predicate Pred = Cmp->getPredicate(); |
| |
| PHINode *PN = dyn_cast<PHINode>(CmpLHS); |
| if (!PN) |
| PN = dyn_cast<PHINode>(CmpRHS); |
| if (PN && PN->getParent() == BB) { |
| const DataLayout &DL = PN->getModule()->getDataLayout(); |
| // We can do this simplification if any comparisons fold to true or false. |
| // See if any do. |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| BasicBlock *PredBB = PN->getIncomingBlock(i); |
| Value *LHS, *RHS; |
| if (PN == CmpLHS) { |
| LHS = PN->getIncomingValue(i); |
| RHS = CmpRHS->DoPHITranslation(BB, PredBB); |
| } else { |
| LHS = CmpLHS->DoPHITranslation(BB, PredBB); |
| RHS = PN->getIncomingValue(i); |
| } |
| Value *Res = SimplifyCmpInst(Pred, LHS, RHS, {DL}); |
| if (!Res) { |
| if (!isa<Constant>(RHS)) |
| continue; |
| |
| // getPredicateOnEdge call will make no sense if LHS is defined in BB. |
| auto LHSInst = dyn_cast<Instruction>(LHS); |
| if (LHSInst && LHSInst->getParent() == BB) |
| continue; |
| |
| LazyValueInfo::Tristate |
| ResT = LVI->getPredicateOnEdge(Pred, LHS, |
| cast<Constant>(RHS), PredBB, BB, |
| CxtI ? CxtI : Cmp); |
| if (ResT == LazyValueInfo::Unknown) |
| continue; |
| Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); |
| } |
| |
| if (Constant *KC = getKnownConstant(Res, WantInteger)) |
| Result.emplace_back(KC, PredBB); |
| } |
| |
| return !Result.empty(); |
| } |
| |
| // If comparing a live-in value against a constant, see if we know the |
| // live-in value on any predecessors. |
| if (isa<Constant>(CmpRHS) && !CmpType->isVectorTy()) { |
| Constant *CmpConst = cast<Constant>(CmpRHS); |
| |
| if (!isa<Instruction>(CmpLHS) || |
| cast<Instruction>(CmpLHS)->getParent() != BB) { |
| for (BasicBlock *P : predecessors(BB)) { |
| // If the value is known by LazyValueInfo to be a constant in a |
| // predecessor, use that information to try to thread this block. |
| LazyValueInfo::Tristate Res = |
| LVI->getPredicateOnEdge(Pred, CmpLHS, |
| CmpConst, P, BB, CxtI ? CxtI : Cmp); |
| if (Res == LazyValueInfo::Unknown) |
| continue; |
| |
| Constant *ResC = ConstantInt::get(CmpType, Res); |
| Result.emplace_back(ResC, P); |
| } |
| |
| return !Result.empty(); |
| } |
| |
| // InstCombine can fold some forms of constant range checks into |
| // (icmp (add (x, C1)), C2). See if we have we have such a thing with |
| // x as a live-in. |
| { |
| using namespace PatternMatch; |
| |
| Value *AddLHS; |
| ConstantInt *AddConst; |
| if (isa<ConstantInt>(CmpConst) && |
| match(CmpLHS, m_Add(m_Value(AddLHS), m_ConstantInt(AddConst)))) { |
| if (!isa<Instruction>(AddLHS) || |
| cast<Instruction>(AddLHS)->getParent() != BB) { |
| for (BasicBlock *P : predecessors(BB)) { |
| // If the value is known by LazyValueInfo to be a ConstantRange in |
| // a predecessor, use that information to try to thread this |
| // block. |
| ConstantRange CR = LVI->getConstantRangeOnEdge( |
| AddLHS, P, BB, CxtI ? CxtI : cast<Instruction>(CmpLHS)); |
| // Propagate the range through the addition. |
| CR = CR.add(AddConst->getValue()); |
| |
| // Get the range where the compare returns true. |
| ConstantRange CmpRange = ConstantRange::makeExactICmpRegion( |
| Pred, cast<ConstantInt>(CmpConst)->getValue()); |
| |
| Constant *ResC; |
| if (CmpRange.contains(CR)) |
| ResC = ConstantInt::getTrue(CmpType); |
| else if (CmpRange.inverse().contains(CR)) |
| ResC = ConstantInt::getFalse(CmpType); |
| else |
| continue; |
| |
| Result.emplace_back(ResC, P); |
| } |
| |
| return !Result.empty(); |
| } |
| } |
| } |
| |
| // Try to find a constant value for the LHS of a comparison, |
| // and evaluate it statically if we can. |
| PredValueInfoTy LHSVals; |
| computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals, |
| WantInteger, RecursionSet, CxtI); |
| |
| for (const auto &LHSVal : LHSVals) { |
| Constant *V = LHSVal.first; |
| Constant *Folded = ConstantExpr::getCompare(Pred, V, CmpConst); |
| if (Constant *KC = getKnownConstant(Folded, WantInteger)) |
| Result.emplace_back(KC, LHSVal.second); |
| } |
| |
| return !Result.empty(); |
| } |
| } |
| |
| if (SelectInst *SI = dyn_cast<SelectInst>(I)) { |
| // Handle select instructions where at least one operand is a known constant |
| // and we can figure out the condition value for any predecessor block. |
| Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference); |
| Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference); |
| PredValueInfoTy Conds; |
| if ((TrueVal || FalseVal) && |
| computeValueKnownInPredecessorsImpl(SI->getCondition(), BB, Conds, |
| WantInteger, RecursionSet, CxtI)) { |
| for (auto &C : Conds) { |
| Constant *Cond = C.first; |
| |
| // Figure out what value to use for the condition. |
| bool KnownCond; |
| if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) { |
| // A known boolean. |
| KnownCond = CI->isOne(); |
| } else { |
| assert(isa<UndefValue>(Cond) && "Unexpected condition value"); |
| // Either operand will do, so be sure to pick the one that's a known |
| // constant. |
| // FIXME: Do this more cleverly if both values are known constants? |
| KnownCond = (TrueVal != nullptr); |
| } |
| |
| // See if the select has a known constant value for this predecessor. |
| if (Constant *Val = KnownCond ? TrueVal : FalseVal) |
| Result.emplace_back(Val, C.second); |
| } |
| |
| return !Result.empty(); |
| } |
| } |
| |
| // If all else fails, see if LVI can figure out a constant value for us. |
| assert(CxtI->getParent() == BB && "CxtI should be in BB"); |
| Constant *CI = LVI->getConstant(V, CxtI); |
| if (Constant *KC = getKnownConstant(CI, Preference)) { |
| for (BasicBlock *Pred : predecessors(BB)) |
| Result.emplace_back(KC, Pred); |
| } |
| |
| return !Result.empty(); |
| } |
| |
| /// GetBestDestForBranchOnUndef - If we determine that the specified block ends |
| /// in an undefined jump, decide which block is best to revector to. |
| /// |
| /// Since we can pick an arbitrary destination, we pick the successor with the |
| /// fewest predecessors. This should reduce the in-degree of the others. |
| static unsigned getBestDestForJumpOnUndef(BasicBlock *BB) { |
| Instruction *BBTerm = BB->getTerminator(); |
| unsigned MinSucc = 0; |
| BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); |
| // Compute the successor with the minimum number of predecessors. |
| unsigned MinNumPreds = pred_size(TestBB); |
| for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { |
| TestBB = BBTerm->getSuccessor(i); |
| unsigned NumPreds = pred_size(TestBB); |
| if (NumPreds < MinNumPreds) { |
| MinSucc = i; |
| MinNumPreds = NumPreds; |
| } |
| } |
| |
| return MinSucc; |
| } |
| |
| static bool hasAddressTakenAndUsed(BasicBlock *BB) { |
| if (!BB->hasAddressTaken()) return false; |
| |
| // If the block has its address taken, it may be a tree of dead constants |
| // hanging off of it. These shouldn't keep the block alive. |
| BlockAddress *BA = BlockAddress::get(BB); |
| BA->removeDeadConstantUsers(); |
| return !BA->use_empty(); |
| } |
| |
| /// processBlock - If there are any predecessors whose control can be threaded |
| /// through to a successor, transform them now. |
| bool JumpThreadingPass::processBlock(BasicBlock *BB) { |
| // If the block is trivially dead, just return and let the caller nuke it. |
| // This simplifies other transformations. |
| if (DTU->isBBPendingDeletion(BB) || |
| (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock())) |
| return false; |
| |
| // If this block has a single predecessor, and if that pred has a single |
| // successor, merge the blocks. This encourages recursive jump threading |
| // because now the condition in this block can be threaded through |
| // predecessors of our predecessor block. |
| if (maybeMergeBasicBlockIntoOnlyPred(BB)) |
| return true; |
| |
| if (tryToUnfoldSelectInCurrBB(BB)) |
| return true; |
| |
| // Look if we can propagate guards to predecessors. |
| if (HasGuards && processGuards(BB)) |
| return true; |
| |
| // What kind of constant we're looking for. |
| ConstantPreference Preference = WantInteger; |
| |
| // Look to see if the terminator is a conditional branch, switch or indirect |
| // branch, if not we can't thread it. |
| Value *Condition; |
| Instruction *Terminator = BB->getTerminator(); |
| if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) { |
| // Can't thread an unconditional jump. |
| if (BI->isUnconditional()) return false; |
| Condition = BI->getCondition(); |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) { |
| Condition = SI->getCondition(); |
| } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) { |
| // Can't thread indirect branch with no successors. |
| if (IB->getNumSuccessors() == 0) return false; |
| Condition = IB->getAddress()->stripPointerCasts(); |
| Preference = WantBlockAddress; |
| } else { |
| return false; // Must be an invoke or callbr. |
| } |
| |
| // Keep track if we constant folded the condition in this invocation. |
| bool ConstantFolded = false; |
| |
| // Run constant folding to see if we can reduce the condition to a simple |
| // constant. |
| if (Instruction *I = dyn_cast<Instruction>(Condition)) { |
| Value *SimpleVal = |
| ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI); |
| if (SimpleVal) { |
| I->replaceAllUsesWith(SimpleVal); |
| if (isInstructionTriviallyDead(I, TLI)) |
| I->eraseFromParent(); |
| Condition = SimpleVal; |
| ConstantFolded = true; |
| } |
| } |
| |
| // If the terminator is branching on an undef or freeze undef, we can pick any |
| // of the successors to branch to. Let getBestDestForJumpOnUndef decide. |
| auto *FI = dyn_cast<FreezeInst>(Condition); |
| if (isa<UndefValue>(Condition) || |
| (FI && isa<UndefValue>(FI->getOperand(0)) && FI->hasOneUse())) { |
| unsigned BestSucc = getBestDestForJumpOnUndef(BB); |
| std::vector<DominatorTree::UpdateType> Updates; |
| |
| // Fold the branch/switch. |
| Instruction *BBTerm = BB->getTerminator(); |
| Updates.reserve(BBTerm->getNumSuccessors()); |
| for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { |
| if (i == BestSucc) continue; |
| BasicBlock *Succ = BBTerm->getSuccessor(i); |
| Succ->removePredecessor(BB, true); |
| Updates.push_back({DominatorTree::Delete, BB, Succ}); |
| } |
| |
| LLVM_DEBUG(dbgs() << " In block '" << BB->getName() |
| << "' folding undef terminator: " << *BBTerm << '\n'); |
| BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); |
| ++NumFolds; |
| BBTerm->eraseFromParent(); |
| DTU->applyUpdatesPermissive(Updates); |
| if (FI) |
| FI->eraseFromParent(); |
| return true; |
| } |
| |
| // If the terminator of this block is branching on a constant, simplify the |
| // terminator to an unconditional branch. This can occur due to threading in |
| // other blocks. |
| if (getKnownConstant(Condition, Preference)) { |
| LLVM_DEBUG(dbgs() << " In block '" << BB->getName() |
| << "' folding terminator: " << *BB->getTerminator() |
| << '\n'); |
| ++NumFolds; |
| ConstantFoldTerminator(BB, true, nullptr, DTU); |
| if (HasProfileData) |
| BPI->eraseBlock(BB); |
| return true; |
| } |
| |
| Instruction *CondInst = dyn_cast<Instruction>(Condition); |
| |
| // All the rest of our checks depend on the condition being an instruction. |
| if (!CondInst) { |
| // FIXME: Unify this with code below. |
| if (processThreadableEdges(Condition, BB, Preference, Terminator)) |
| return true; |
| return ConstantFolded; |
| } |
| |
| if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { |
| // If we're branching on a conditional, LVI might be able to determine |
| // it's value at the branch instruction. We only handle comparisons |
| // against a constant at this time. |
| // TODO: This should be extended to handle switches as well. |
| BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); |
| Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); |
| if (CondBr && CondConst) { |
| // We should have returned as soon as we turn a conditional branch to |
| // unconditional. Because its no longer interesting as far as jump |
| // threading is concerned. |
| assert(CondBr->isConditional() && "Threading on unconditional terminator"); |
| |
| LazyValueInfo::Tristate Ret = |
| LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0), |
| CondConst, CondBr, /*UseBlockValue=*/false); |
| if (Ret != LazyValueInfo::Unknown) { |
| unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0; |
| unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1; |
| BasicBlock *ToRemoveSucc = CondBr->getSuccessor(ToRemove); |
| ToRemoveSucc->removePredecessor(BB, true); |
| BranchInst *UncondBr = |
| BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); |
| UncondBr->setDebugLoc(CondBr->getDebugLoc()); |
| ++NumFolds; |
| CondBr->eraseFromParent(); |
| if (CondCmp->use_empty()) |
| CondCmp->eraseFromParent(); |
| // We can safely replace *some* uses of the CondInst if it has |
| // exactly one value as returned by LVI. RAUW is incorrect in the |
| // presence of guards and assumes, that have the `Cond` as the use. This |
| // is because we use the guards/assume to reason about the `Cond` value |
| // at the end of block, but RAUW unconditionally replaces all uses |
| // including the guards/assumes themselves and the uses before the |
| // guard/assume. |
| else if (CondCmp->getParent() == BB) { |
| auto *CI = Ret == LazyValueInfo::True ? |
| ConstantInt::getTrue(CondCmp->getType()) : |
| ConstantInt::getFalse(CondCmp->getType()); |
| replaceFoldableUses(CondCmp, CI); |
| } |
| DTU->applyUpdatesPermissive( |
| {{DominatorTree::Delete, BB, ToRemoveSucc}}); |
| if (HasProfileData) |
| BPI->eraseBlock(BB); |
| return true; |
| } |
| |
| // We did not manage to simplify this branch, try to see whether |
| // CondCmp depends on a known phi-select pattern. |
| if (tryToUnfoldSelect(CondCmp, BB)) |
| return true; |
| } |
| } |
| |
| if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) |
| if (tryToUnfoldSelect(SI, BB)) |
| return true; |
| |
| // Check for some cases that are worth simplifying. Right now we want to look |
| // for loads that are used by a switch or by the condition for the branch. If |
| // we see one, check to see if it's partially redundant. If so, insert a PHI |
| // which can then be used to thread the values. |
| Value *SimplifyValue = CondInst; |
| |
| if (auto *FI = dyn_cast<FreezeInst>(SimplifyValue)) |
| // Look into freeze's operand |
| SimplifyValue = FI->getOperand(0); |
| |
| if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) |
| if (isa<Constant>(CondCmp->getOperand(1))) |
| SimplifyValue = CondCmp->getOperand(0); |
| |
| // TODO: There are other places where load PRE would be profitable, such as |
| // more complex comparisons. |
| if (LoadInst *LoadI = dyn_cast<LoadInst>(SimplifyValue)) |
| if (simplifyPartiallyRedundantLoad(LoadI)) |
| return true; |
| |
| // Before threading, try to propagate profile data backwards: |
| if (PHINode *PN = dyn_cast<PHINode>(CondInst)) |
| if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) |
| updatePredecessorProfileMetadata(PN, BB); |
| |
| // Handle a variety of cases where we are branching on something derived from |
| // a PHI node in the current block. If we can prove that any predecessors |
| // compute a predictable value based on a PHI node, thread those predecessors. |
| if (processThreadableEdges(CondInst, BB, Preference, Terminator)) |
| return true; |
| |
| // If this is an otherwise-unfoldable branch on a phi node or freeze(phi) in |
| // the current block, see if we can simplify. |
| PHINode *PN = dyn_cast<PHINode>( |
| isa<FreezeInst>(CondInst) ? cast<FreezeInst>(CondInst)->getOperand(0) |
| : CondInst); |
| |
| if (PN && PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) |
| return processBranchOnPHI(PN); |
| |
| // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. |
| if (CondInst->getOpcode() == Instruction::Xor && |
| CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) |
| return processBranchOnXOR(cast<BinaryOperator>(CondInst)); |
| |
| // Search for a stronger dominating condition that can be used to simplify a |
| // conditional branch leaving BB. |
| if (processImpliedCondition(BB)) |
| return true; |
| |
| return false; |
| } |
| |
| bool JumpThreadingPass::processImpliedCondition(BasicBlock *BB) { |
| auto *BI = dyn_cast<BranchInst>(BB->getTerminator()); |
| if (!BI || !BI->isConditional()) |
| return false; |
| |
| Value *Cond = BI->getCondition(); |
| BasicBlock *CurrentBB = BB; |
| BasicBlock *CurrentPred = BB->getSinglePredecessor(); |
| unsigned Iter = 0; |
| |
| auto &DL = BB->getModule()->getDataLayout(); |
| |
| while (CurrentPred && Iter++ < ImplicationSearchThreshold) { |
| auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator()); |
| if (!PBI || !PBI->isConditional()) |
| return false; |
| if (PBI->getSuccessor(0) != CurrentBB && PBI->getSuccessor(1) != CurrentBB) |
| return false; |
| |
| bool CondIsTrue = PBI->getSuccessor(0) == CurrentBB; |
| Optional<bool> Implication = |
| isImpliedCondition(PBI->getCondition(), Cond, DL, CondIsTrue); |
| if (Implication) { |
| BasicBlock *KeepSucc = BI->getSuccessor(*Implication ? 0 : 1); |
| BasicBlock *RemoveSucc = BI->getSuccessor(*Implication ? 1 : 0); |
| RemoveSucc->removePredecessor(BB); |
| BranchInst *UncondBI = BranchInst::Create(KeepSucc, BI); |
| UncondBI->setDebugLoc(BI->getDebugLoc()); |
| ++NumFolds; |
| BI->eraseFromParent(); |
| DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}}); |
| if (HasProfileData) |
| BPI->eraseBlock(BB); |
| return true; |
| } |
| CurrentBB = CurrentPred; |
| CurrentPred = CurrentBB->getSinglePredecessor(); |
| } |
| |
| return false; |
| } |
| |
| /// Return true if Op is an instruction defined in the given block. |
| static bool isOpDefinedInBlock(Value *Op, BasicBlock *BB) { |
| if (Instruction *OpInst = dyn_cast<Instruction>(Op)) |
| if (OpInst->getParent() == BB) |
| return true; |
| return false; |
| } |
| |
| /// simplifyPartiallyRedundantLoad - If LoadI is an obviously partially |
| /// redundant load instruction, eliminate it by replacing it with a PHI node. |
| /// This is an important optimization that encourages jump threading, and needs |
| /// to be run interlaced with other jump threading tasks. |
| bool JumpThreadingPass::simplifyPartiallyRedundantLoad(LoadInst *LoadI) { |
| // Don't hack volatile and ordered loads. |
| if (!LoadI->isUnordered()) return false; |
| |
| // If the load is defined in a block with exactly one predecessor, it can't be |
| // partially redundant. |
| BasicBlock *LoadBB = LoadI->getParent(); |
| if (LoadBB->getSinglePredecessor()) |
| return false; |
| |
| // If the load is defined in an EH pad, it can't be partially redundant, |
| // because the edges between the invoke and the EH pad cannot have other |
| // instructions between them. |
| if (LoadBB->isEHPad()) |
| return false; |
| |
| Value *LoadedPtr = LoadI->getOperand(0); |
| |
| // If the loaded operand is defined in the LoadBB and its not a phi, |
| // it can't be available in predecessors. |
| if (isOpDefinedInBlock(LoadedPtr, LoadBB) && !isa<PHINode>(LoadedPtr)) |
| return false; |
| |
| // Scan a few instructions up from the load, to see if it is obviously live at |
| // the entry to its block. |
| BasicBlock::iterator BBIt(LoadI); |
| bool IsLoadCSE; |
| if (Value *AvailableVal = FindAvailableLoadedValue( |
| LoadI, LoadBB, BBIt, DefMaxInstsToScan, AA, &IsLoadCSE)) { |
| // If the value of the load is locally available within the block, just use |
| // it. This frequently occurs for reg2mem'd allocas. |
| |
| if (IsLoadCSE) { |
| LoadInst *NLoadI = cast<LoadInst>(AvailableVal); |
| combineMetadataForCSE(NLoadI, LoadI, false); |
| }; |
| |
| // If the returned value is the load itself, replace with an undef. This can |
| // only happen in dead loops. |
| if (AvailableVal == LoadI) |
| AvailableVal = UndefValue::get(LoadI->getType()); |
| if (AvailableVal->getType() != LoadI->getType()) |
| AvailableVal = CastInst::CreateBitOrPointerCast( |
| AvailableVal, LoadI->getType(), "", LoadI); |
| LoadI->replaceAllUsesWith(AvailableVal); |
| LoadI->eraseFromParent(); |
| return true; |
| } |
| |
| // Otherwise, if we scanned the whole block and got to the top of the block, |
| // we know the block is locally transparent to the load. If not, something |
| // might clobber its value. |
| if (BBIt != LoadBB->begin()) |
| return false; |
| |
| // If all of the loads and stores that feed the value have the same AA tags, |
| // then we can propagate them onto any newly inserted loads. |
| AAMDNodes AATags = LoadI->getAAMetadata(); |
| |
| SmallPtrSet<BasicBlock*, 8> PredsScanned; |
| |
| using AvailablePredsTy = SmallVector<std::pair<BasicBlock *, Value *>, 8>; |
| |
| AvailablePredsTy AvailablePreds; |
| BasicBlock *OneUnavailablePred = nullptr; |
| SmallVector<LoadInst*, 8> CSELoads; |
| |
| // If we got here, the loaded value is transparent through to the start of the |
| // block. Check to see if it is available in any of the predecessor blocks. |
| for (BasicBlock *PredBB : predecessors(LoadBB)) { |
| // If we already scanned this predecessor, skip it. |
| if (!PredsScanned.insert(PredBB).second) |
| continue; |
| |
| BBIt = PredBB->end(); |
| unsigned NumScanedInst = 0; |
| Value *PredAvailable = nullptr; |
| // NOTE: We don't CSE load that is volatile or anything stronger than |
| // unordered, that should have been checked when we entered the function. |
| assert(LoadI->isUnordered() && |
| "Attempting to CSE volatile or atomic loads"); |
| // If this is a load on a phi pointer, phi-translate it and search |
| // for available load/store to the pointer in predecessors. |
| Type *AccessTy = LoadI->getType(); |
| const auto &DL = LoadI->getModule()->getDataLayout(); |
| MemoryLocation Loc(LoadedPtr->DoPHITranslation(LoadBB, PredBB), |
| LocationSize::precise(DL.getTypeStoreSize(AccessTy)), |
| AATags); |
| PredAvailable = findAvailablePtrLoadStore(Loc, AccessTy, LoadI->isAtomic(), |
| PredBB, BBIt, DefMaxInstsToScan, |
| AA, &IsLoadCSE, &NumScanedInst); |
| |
| // If PredBB has a single predecessor, continue scanning through the |
| // single predecessor. |
| BasicBlock *SinglePredBB = PredBB; |
| while (!PredAvailable && SinglePredBB && BBIt == SinglePredBB->begin() && |
| NumScanedInst < DefMaxInstsToScan) { |
| SinglePredBB = SinglePredBB->getSinglePredecessor(); |
| if (SinglePredBB) { |
| BBIt = SinglePredBB->end(); |
| PredAvailable = findAvailablePtrLoadStore( |
| Loc, AccessTy, LoadI->isAtomic(), SinglePredBB, BBIt, |
| (DefMaxInstsToScan - NumScanedInst), AA, &IsLoadCSE, |
| &NumScanedInst); |
| } |
| } |
| |
| if (!PredAvailable) { |
| OneUnavailablePred = PredBB; |
| continue; |
| } |
| |
| if (IsLoadCSE) |
| CSELoads.push_back(cast<LoadInst>(PredAvailable)); |
| |
| // If so, this load is partially redundant. Remember this info so that we |
| // can create a PHI node. |
| AvailablePreds.emplace_back(PredBB, PredAvailable); |
| } |
| |
| // If the loaded value isn't available in any predecessor, it isn't partially |
| // redundant. |
| if (AvailablePreds.empty()) return false; |
| |
| // Okay, the loaded value is available in at least one (and maybe all!) |
| // predecessors. If the value is unavailable in more than one unique |
| // predecessor, we want to insert a merge block for those common predecessors. |
| // This ensures that we only have to insert one reload, thus not increasing |
| // code size. |
| BasicBlock *UnavailablePred = nullptr; |
| |
| // If the value is unavailable in one of predecessors, we will end up |
| // inserting a new instruction into them. It is only valid if all the |
| // instructions before LoadI are guaranteed to pass execution to its |
| // successor, or if LoadI is safe to speculate. |
| // TODO: If this logic becomes more complex, and we will perform PRE insertion |
| // farther than to a predecessor, we need to reuse the code from GVN's PRE. |
| // It requires domination tree analysis, so for this simple case it is an |
| // overkill. |
| if (PredsScanned.size() != AvailablePreds.size() && |
| !isSafeToSpeculativelyExecute(LoadI)) |
| for (auto I = LoadBB->begin(); &*I != LoadI; ++I) |
| if (!isGuaranteedToTransferExecutionToSuccessor(&*I)) |
| return false; |
| |
| // If there is exactly one predecessor where the value is unavailable, the |
| // already computed 'OneUnavailablePred' block is it. If it ends in an |
| // unconditional branch, we know that it isn't a critical edge. |
| if (PredsScanned.size() == AvailablePreds.size()+1 && |
| OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { |
| UnavailablePred = OneUnavailablePred; |
| } else if (PredsScanned.size() != AvailablePreds.size()) { |
| // Otherwise, we had multiple unavailable predecessors or we had a critical |
| // edge from the one. |
| SmallVector<BasicBlock*, 8> PredsToSplit; |
| SmallPtrSet<BasicBlock*, 8> AvailablePredSet; |
| |
| for (const auto &AvailablePred : AvailablePreds) |
| AvailablePredSet.insert(AvailablePred.first); |
| |
| // Add all the unavailable predecessors to the PredsToSplit list. |
| for (BasicBlock *P : predecessors(LoadBB)) { |
| // If the predecessor is an indirect goto, we can't split the edge. |
| // Same for CallBr. |
| if (isa<IndirectBrInst>(P->getTerminator()) || |
| isa<CallBrInst>(P->getTerminator())) |
| return false; |
| |
| if (!AvailablePredSet.count(P)) |
| PredsToSplit.push_back(P); |
| } |
| |
| // Split them out to their own block. |
| UnavailablePred = splitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split"); |
| } |
| |
| // If the value isn't available in all predecessors, then there will be |
| // exactly one where it isn't available. Insert a load on that edge and add |
| // it to the AvailablePreds list. |
| if (UnavailablePred) { |
| assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && |
| "Can't handle critical edge here!"); |
| LoadInst *NewVal = new LoadInst( |
| LoadI->getType(), LoadedPtr->DoPHITranslation(LoadBB, UnavailablePred), |
| LoadI->getName() + ".pr", false, LoadI->getAlign(), |
| LoadI->getOrdering(), LoadI->getSyncScopeID(), |
| UnavailablePred->getTerminator()); |
| NewVal->setDebugLoc(LoadI->getDebugLoc()); |
| if (AATags) |
| NewVal->setAAMetadata(AATags); |
| |
| AvailablePreds.emplace_back(UnavailablePred, NewVal); |
| } |
| |
| // Now we know that each predecessor of this block has a value in |
| // AvailablePreds, sort them for efficient access as we're walking the preds. |
| array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); |
| |
| // Create a PHI node at the start of the block for the PRE'd load value. |
| pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB); |
| PHINode *PN = PHINode::Create(LoadI->getType(), std::distance(PB, PE), "", |
| &LoadBB->front()); |
| PN->takeName(LoadI); |
| PN->setDebugLoc(LoadI->getDebugLoc()); |
| |
| // Insert new entries into the PHI for each predecessor. A single block may |
| // have multiple entries here. |
| for (pred_iterator PI = PB; PI != PE; ++PI) { |
| BasicBlock *P = *PI; |
| AvailablePredsTy::iterator I = |
| llvm::lower_bound(AvailablePreds, std::make_pair(P, (Value *)nullptr)); |
| |
| assert(I != AvailablePreds.end() && I->first == P && |
| "Didn't find entry for predecessor!"); |
| |
| // If we have an available predecessor but it requires casting, insert the |
| // cast in the predecessor and use the cast. Note that we have to update the |
| // AvailablePreds vector as we go so that all of the PHI entries for this |
| // predecessor use the same bitcast. |
| Value *&PredV = I->second; |
| if (PredV->getType() != LoadI->getType()) |
| PredV = CastInst::CreateBitOrPointerCast(PredV, LoadI->getType(), "", |
| P->getTerminator()); |
| |
| PN->addIncoming(PredV, I->first); |
| } |
| |
| for (LoadInst *PredLoadI : CSELoads) { |
| combineMetadataForCSE(PredLoadI, LoadI, true); |
| } |
| |
| LoadI->replaceAllUsesWith(PN); |
| LoadI->eraseFromParent(); |
| |
| return true; |
| } |
| |
| /// findMostPopularDest - The specified list contains multiple possible |
| /// threadable destinations. Pick the one that occurs the most frequently in |
| /// the list. |
| static BasicBlock * |
| findMostPopularDest(BasicBlock *BB, |
| const SmallVectorImpl<std::pair<BasicBlock *, |
| BasicBlock *>> &PredToDestList) { |
| assert(!PredToDestList.empty()); |
| |
| // Determine popularity. If there are multiple possible destinations, we |
| // explicitly choose to ignore 'undef' destinations. We prefer to thread |
| // blocks with known and real destinations to threading undef. We'll handle |
| // them later if interesting. |
| MapVector<BasicBlock *, unsigned> DestPopularity; |
| |
| // Populate DestPopularity with the successors in the order they appear in the |
| // successor list. This way, we ensure determinism by iterating it in the |
| // same order in std::max_element below. We map nullptr to 0 so that we can |
| // return nullptr when PredToDestList contains nullptr only. |
| DestPopularity[nullptr] = 0; |
| for (auto *SuccBB : successors(BB)) |
| DestPopularity[SuccBB] = 0; |
| |
| for (const auto &PredToDest : PredToDestList) |
| if (PredToDest.second) |
| DestPopularity[PredToDest.second]++; |
| |
| // Find the most popular dest. |
| using VT = decltype(DestPopularity)::value_type; |
| auto MostPopular = std::max_element( |
| DestPopularity.begin(), DestPopularity.end(), |
| [](const VT &L, const VT &R) { return L.second < R.second; }); |
| |
| // Okay, we have finally picked the most popular destination. |
| return MostPopular->first; |
| } |
| |
| // Try to evaluate the value of V when the control flows from PredPredBB to |
| // BB->getSinglePredecessor() and then on to BB. |
| Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB, |
| BasicBlock *PredPredBB, |
| Value *V) { |
| BasicBlock *PredBB = BB->getSinglePredecessor(); |
| assert(PredBB && "Expected a single predecessor"); |
| |
| if (Constant *Cst = dyn_cast<Constant>(V)) { |
| return Cst; |
| } |
| |
| // Consult LVI if V is not an instruction in BB or PredBB. |
| Instruction *I = dyn_cast<Instruction>(V); |
| if (!I || (I->getParent() != BB && I->getParent() != PredBB)) { |
| return LVI->getConstantOnEdge(V, PredPredBB, PredBB, nullptr); |
| } |
| |
| // Look into a PHI argument. |
| if (PHINode *PHI = dyn_cast<PHINode>(V)) { |
| if (PHI->getParent() == PredBB) |
| return dyn_cast<Constant>(PHI->getIncomingValueForBlock(PredPredBB)); |
| return nullptr; |
| } |
| |
| // If we have a CmpInst, try to fold it for each incoming edge into PredBB. |
| if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) { |
| if (CondCmp->getParent() == BB) { |
| Constant *Op0 = |
| evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0)); |
| Constant *Op1 = |
| evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1)); |
| if (Op0 && Op1) { |
| return ConstantExpr::getCompare(CondCmp->getPredicate(), Op0, Op1); |
| } |
| } |
| return nullptr; |
| } |
| |
| return nullptr; |
| } |
| |
| bool JumpThreadingPass::processThreadableEdges(Value *Cond, BasicBlock *BB, |
| ConstantPreference Preference, |
| Instruction *CxtI) { |
| // If threading this would thread across a loop header, don't even try to |
| // thread the edge. |
| if (LoopHeaders.count(BB)) |
| return false; |
| |
| PredValueInfoTy PredValues; |
| if (!computeValueKnownInPredecessors(Cond, BB, PredValues, Preference, |
| CxtI)) { |
| // We don't have known values in predecessors. See if we can thread through |
| // BB and its sole predecessor. |
| return maybethreadThroughTwoBasicBlocks(BB, Cond); |
| } |
| |
| assert(!PredValues.empty() && |
| "computeValueKnownInPredecessors returned true with no values"); |
| |
| LLVM_DEBUG(dbgs() << "IN BB: " << *BB; |
| for (const auto &PredValue : PredValues) { |
| dbgs() << " BB '" << BB->getName() |
| << "': FOUND condition = " << *PredValue.first |
| << " for pred '" << PredValue.second->getName() << "'.\n"; |
| }); |
| |
| // Decide what we want to thread through. Convert our list of known values to |
| // a list of known destinations for each pred. This also discards duplicate |
| // predecessors and keeps track of the undefined inputs (which are represented |
| // as a null dest in the PredToDestList). |
| SmallPtrSet<BasicBlock*, 16> SeenPreds; |
| SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; |
| |
| BasicBlock *OnlyDest = nullptr; |
| BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; |
| Constant *OnlyVal = nullptr; |
| Constant *MultipleVal = (Constant *)(intptr_t)~0ULL; |
| |
| for (const auto &PredValue : PredValues) { |
| BasicBlock *Pred = PredValue.second; |
| if (!SeenPreds.insert(Pred).second) |
| continue; // Duplicate predecessor entry. |
| |
| Constant *Val = PredValue.first; |
| |
| BasicBlock *DestBB; |
| if (isa<UndefValue>(Val)) |
| DestBB = nullptr; |
| else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { |
| assert(isa<ConstantInt>(Val) && "Expecting a constant integer"); |
| DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero()); |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { |
| assert(isa<ConstantInt>(Val) && "Expecting a constant integer"); |
| DestBB = SI->findCaseValue(cast<ConstantInt>(Val))->getCaseSuccessor(); |
| } else { |
| assert(isa<IndirectBrInst>(BB->getTerminator()) |
| && "Unexpected terminator"); |
| assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress"); |
| DestBB = cast<BlockAddress>(Val)->getBasicBlock(); |
| } |
| |
| // If we have exactly one destination, remember it for efficiency below. |
| if (PredToDestList.empty()) { |
| OnlyDest = DestBB; |
| OnlyVal = Val; |
| } else { |
| if (OnlyDest != DestBB) |
| OnlyDest = MultipleDestSentinel; |
| // It possible we have same destination, but different value, e.g. default |
| // case in switchinst. |
| if (Val != OnlyVal) |
| OnlyVal = MultipleVal; |
| } |
| |
| // If the predecessor ends with an indirect goto, we can't change its |
| // destination. Same for CallBr. |
| if (isa<IndirectBrInst>(Pred->getTerminator()) || |
| isa<CallBrInst>(Pred->getTerminator())) |
| continue; |
| |
| PredToDestList.emplace_back(Pred, DestBB); |
| } |
| |
| // If all edges were unthreadable, we fail. |
| if (PredToDestList.empty()) |
| return false; |
| |
| // If all the predecessors go to a single known successor, we want to fold, |
| // not thread. By doing so, we do not need to duplicate the current block and |
| // also miss potential opportunities in case we dont/cant duplicate. |
| if (OnlyDest && OnlyDest != MultipleDestSentinel) { |
| if (BB->hasNPredecessors(PredToDestList.size())) { |
| bool SeenFirstBranchToOnlyDest = false; |
| std::vector <DominatorTree::UpdateType> Updates; |
| Updates.reserve(BB->getTerminator()->getNumSuccessors() - 1); |
| for (BasicBlock *SuccBB : successors(BB)) { |
| if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest) { |
| SeenFirstBranchToOnlyDest = true; // Don't modify the first branch. |
| } else { |
| SuccBB->removePredecessor(BB, true); // This is unreachable successor. |
| Updates.push_back({DominatorTree::Delete, BB, SuccBB}); |
| } |
| } |
| |
| // Finally update the terminator. |
| Instruction *Term = BB->getTerminator(); |
| BranchInst::Create(OnlyDest, Term); |
| ++NumFolds; |
| Term->eraseFromParent(); |
| DTU->applyUpdatesPermissive(Updates); |
| if (HasProfileData) |
| BPI->eraseBlock(BB); |
| |
| // If the condition is now dead due to the removal of the old terminator, |
| // erase it. |
| if (auto *CondInst = dyn_cast<Instruction>(Cond)) { |
| if (CondInst->use_empty() && !CondInst->mayHaveSideEffects()) |
| CondInst->eraseFromParent(); |
| // We can safely replace *some* uses of the CondInst if it has |
| // exactly one value as returned by LVI. RAUW is incorrect in the |
| // presence of guards and assumes, that have the `Cond` as the use. This |
| // is because we use the guards/assume to reason about the `Cond` value |
| // at the end of block, but RAUW unconditionally replaces all uses |
| // including the guards/assumes themselves and the uses before the |
| // guard/assume. |
| else if (OnlyVal && OnlyVal != MultipleVal && |
| CondInst->getParent() == BB) |
| replaceFoldableUses(CondInst, OnlyVal); |
| } |
| return true; |
| } |
| } |
| |
| // Determine which is the most common successor. If we have many inputs and |
| // this block is a switch, we want to start by threading the batch that goes |
| // to the most popular destination first. If we only know about one |
| // threadable destination (the common case) we can avoid this. |
| BasicBlock *MostPopularDest = OnlyDest; |
| |
| if (MostPopularDest == MultipleDestSentinel) { |
| // Remove any loop headers from the Dest list, threadEdge conservatively |
| // won't process them, but we might have other destination that are eligible |
| // and we still want to process. |
| erase_if(PredToDestList, |
| [&](const std::pair<BasicBlock *, BasicBlock *> &PredToDest) { |
| return LoopHeaders.contains(PredToDest.second); |
| }); |
| |
| if (PredToDestList.empty()) |
| return false; |
| |
| MostPopularDest = findMostPopularDest(BB, PredToDestList); |
| } |
| |
| // Now that we know what the most popular destination is, factor all |
| // predecessors that will jump to it into a single predecessor. |
| SmallVector<BasicBlock*, 16> PredsToFactor; |
| for (const auto &PredToDest : PredToDestList) |
| if (PredToDest.second == MostPopularDest) { |
| BasicBlock *Pred = PredToDest.first; |
| |
| // This predecessor may be a switch or something else that has multiple |
| // edges to the block. Factor each of these edges by listing them |
| // according to # occurrences in PredsToFactor. |
| for (BasicBlock *Succ : successors(Pred)) |
| if (Succ == BB) |
| PredsToFactor.push_back(Pred); |
| } |
| |
| // If the threadable edges are branching on an undefined value, we get to pick |
| // the destination that these predecessors should get to. |
| if (!MostPopularDest) |
| MostPopularDest = BB->getTerminator()-> |
| getSuccessor(getBestDestForJumpOnUndef(BB)); |
| |
| // Ok, try to thread it! |
| return tryThreadEdge(BB, PredsToFactor, MostPopularDest); |
| } |
| |
| /// processBranchOnPHI - We have an otherwise unthreadable conditional branch on |
| /// a PHI node (or freeze PHI) in the current block. See if there are any |
| /// simplifications we can do based on inputs to the phi node. |
| bool JumpThreadingPass::processBranchOnPHI(PHINode *PN) { |
| BasicBlock *BB = PN->getParent(); |
| |
| // TODO: We could make use of this to do it once for blocks with common PHI |
| // values. |
| SmallVector<BasicBlock*, 1> PredBBs; |
| PredBBs.resize(1); |
| |
| // If any of the predecessor blocks end in an unconditional branch, we can |
| // *duplicate* the conditional branch into that block in order to further |
| // encourage jump threading and to eliminate cases where we have branch on a |
| // phi of an icmp (branch on icmp is much better). |
| // This is still beneficial when a frozen phi is used as the branch condition |
| // because it allows CodeGenPrepare to further canonicalize br(freeze(icmp)) |
| // to br(icmp(freeze ...)). |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| BasicBlock *PredBB = PN->getIncomingBlock(i); |
| if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) |
| if (PredBr->isUnconditional()) { |
| PredBBs[0] = PredBB; |
| // Try to duplicate BB into PredBB. |
| if (duplicateCondBranchOnPHIIntoPred(BB, PredBBs)) |
| return true; |
| } |
| } |
| |
| return false; |
| } |
| |
| /// processBranchOnXOR - We have an otherwise unthreadable conditional branch on |
| /// a xor instruction in the current block. See if there are any |
| /// simplifications we can do based on inputs to the xor. |
| bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) { |
| BasicBlock *BB = BO->getParent(); |
| |
| // If either the LHS or RHS of the xor is a constant, don't do this |
| // optimization. |
| if (isa<ConstantInt>(BO->getOperand(0)) || |
| isa<ConstantInt>(BO->getOperand(1))) |
| return false; |
| |
| // If the first instruction in BB isn't a phi, we won't be able to infer |
| // anything special about any particular predecessor. |
| if (!isa<PHINode>(BB->front())) |
| return false; |
| |
| // If this BB is a landing pad, we won't be able to split the edge into it. |
| if (BB->isEHPad()) |
| return false; |
| |
| // If we have a xor as the branch input to this block, and we know that the |
| // LHS or RHS of the xor in any predecessor is true/false, then we can clone |
| // the condition into the predecessor and fix that value to true, saving some |
| // logical ops on that path and encouraging other paths to simplify. |
| // |
| // This copies something like this: |
| // |
| // BB: |
| // %X = phi i1 [1], [%X'] |
| // %Y = icmp eq i32 %A, %B |
| // %Z = xor i1 %X, %Y |
| // br i1 %Z, ... |
| // |
| // Into: |
| // BB': |
| // %Y = icmp ne i32 %A, %B |
| // br i1 %Y, ... |
| |
| PredValueInfoTy XorOpValues; |
| bool isLHS = true; |
| if (!computeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues, |
| WantInteger, BO)) { |
| assert(XorOpValues.empty()); |
| if (!computeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues, |
| WantInteger, BO)) |
| return false; |
| isLHS = false; |
| } |
| |
| assert(!XorOpValues.empty() && |
| "computeValueKnownInPredecessors returned true with no values"); |
| |
| // Scan the information to see which is most popular: true or false. The |
| // predecessors can be of the set true, false, or undef. |
| unsigned NumTrue = 0, NumFalse = 0; |
| for (const auto &XorOpValue : XorOpValues) { |
| if (isa<UndefValue>(XorOpValue.first)) |
| // Ignore undefs for the count. |
| continue; |
| if (cast<ConstantInt>(XorOpValue.first)->isZero()) |
| ++NumFalse; |
| else |
| ++NumTrue; |
| } |
| |
| // Determine which value to split on, true, false, or undef if neither. |
| ConstantInt *SplitVal = nullptr; |
| if (NumTrue > NumFalse) |
| SplitVal = ConstantInt::getTrue(BB->getContext()); |
| else if (NumTrue != 0 || NumFalse != 0) |
| SplitVal = ConstantInt::getFalse(BB->getContext()); |
| |
| // Collect all of the blocks that this can be folded into so that we can |
| // factor this once and clone it once. |
| SmallVector<BasicBlock*, 8> BlocksToFoldInto; |
| for (const auto &XorOpValue : XorOpValues) { |
| if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first)) |
| continue; |
| |
| BlocksToFoldInto.push_back(XorOpValue.second); |
| } |
| |
| // If we inferred a value for all of the predecessors, then duplication won't |
| // help us. However, we can just replace the LHS or RHS with the constant. |
| if (BlocksToFoldInto.size() == |
| cast<PHINode>(BB->front()).getNumIncomingValues()) { |
| if (!SplitVal) { |
| // If all preds provide undef, just nuke the xor, because it is undef too. |
| BO->replaceAllUsesWith(UndefValue::get(BO->getType())); |
| BO->eraseFromParent(); |
| } else if (SplitVal->isZero()) { |
| // If all preds provide 0, replace the xor with the other input. |
| BO->replaceAllUsesWith(BO->getOperand(isLHS)); |
| BO->eraseFromParent(); |
| } else { |
| // If all preds provide 1, set the computed value to 1. |
| BO->setOperand(!isLHS, SplitVal); |
| } |
| |
| return true; |
| } |
| |
| // If any of predecessors end with an indirect goto, we can't change its |
| // destination. Same for CallBr. |
| if (any_of(BlocksToFoldInto, [](BasicBlock *Pred) { |
| return isa<IndirectBrInst>(Pred->getTerminator()) || |
| isa<CallBrInst>(Pred->getTerminator()); |
| })) |
| return false; |
| |
| // Try to duplicate BB into PredBB. |
| return duplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); |
| } |
| |
| /// addPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new |
| /// predecessor to the PHIBB block. If it has PHI nodes, add entries for |
| /// NewPred using the entries from OldPred (suitably mapped). |
| static void addPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, |
| BasicBlock *OldPred, |
| BasicBlock *NewPred, |
| DenseMap<Instruction*, Value*> &ValueMap) { |
| for (PHINode &PN : PHIBB->phis()) { |
| // Ok, we have a PHI node. Figure out what the incoming value was for the |
| // DestBlock. |
| Value *IV = PN.getIncomingValueForBlock(OldPred); |
| |
| // Remap the value if necessary. |
| if (Instruction *Inst = dyn_cast<Instruction>(IV)) { |
| DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); |
| if (I != ValueMap.end()) |
| IV = I->second; |
| } |
| |
| PN.addIncoming(IV, NewPred); |
| } |
| } |
| |
| /// Merge basic block BB into its sole predecessor if possible. |
| bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) { |
| BasicBlock *SinglePred = BB->getSinglePredecessor(); |
| if (!SinglePred) |
| return false; |
| |
| const Instruction *TI = SinglePred->getTerminator(); |
| if (TI->isExceptionalTerminator() || TI->getNumSuccessors() != 1 || |
| SinglePred == BB || hasAddressTakenAndUsed(BB)) |
| return false; |
| |
| // If SinglePred was a loop header, BB becomes one. |
| if (LoopHeaders.erase(SinglePred)) |
| LoopHeaders.insert(BB); |
| |
| LVI->eraseBlock(SinglePred); |
| MergeBasicBlockIntoOnlyPred(BB, DTU); |
| |
| // Now that BB is merged into SinglePred (i.e. SinglePred code followed by |
| // BB code within one basic block `BB`), we need to invalidate the LVI |
| // information associated with BB, because the LVI information need not be |
| // true for all of BB after the merge. For example, |
| // Before the merge, LVI info and code is as follows: |
| // SinglePred: <LVI info1 for %p val> |
| // %y = use of %p |
| // call @exit() // need not transfer execution to successor. |
| // assume(%p) // from this point on %p is true |
| // br label %BB |
| // BB: <LVI info2 for %p val, i.e. %p is true> |
| // %x = use of %p |
| // br label exit |
| // |
| // Note that this LVI info for blocks BB and SinglPred is correct for %p |
| // (info2 and info1 respectively). After the merge and the deletion of the |
| // LVI info1 for SinglePred. We have the following code: |
| // BB: <LVI info2 for %p val> |
| // %y = use of %p |
| // call @exit() |
| // assume(%p) |
| // %x = use of %p <-- LVI info2 is correct from here onwards. |
| // br label exit |
| // LVI info2 for BB is incorrect at the beginning of BB. |
| |
| // Invalidate LVI information for BB if the LVI is not provably true for |
| // all of BB. |
| if (!isGuaranteedToTransferExecutionToSuccessor(BB)) |
| LVI->eraseBlock(BB); |
| return true; |
| } |
| |
| /// Update the SSA form. NewBB contains instructions that are copied from BB. |
| /// ValueMapping maps old values in BB to new ones in NewBB. |
| void JumpThreadingPass::updateSSA( |
| BasicBlock *BB, BasicBlock *NewBB, |
| DenseMap<Instruction *, Value *> &ValueMapping) { |
| // If there were values defined in BB that are used outside the block, then we |
| // now have to update all uses of the value to use either the original value, |
| // the cloned value, or some PHI derived value. This can require arbitrary |
| // PHI insertion, of which we are prepared to do, clean these up now. |
| SSAUpdater SSAUpdate; |
| SmallVector<Use *, 16> UsesToRename; |
| |
| for (Instruction &I : *BB) { |
| // Scan all uses of this instruction to see if it is used outside of its |
| // block, and if so, record them in UsesToRename. |
| for (Use &U : I.uses()) { |
| Instruction *User = cast<Instruction>(U.getUser()); |
| if (PHINode *UserPN = dyn_cast<PHINode>(User)) { |
| if (UserPN->getIncomingBlock(U) == BB) |
| continue; |
| } else if (User->getParent() == BB) |
| continue; |
| |
| UsesToRename.push_back(&U); |
| } |
| |
| // If there are no uses outside the block, we're done with this instruction. |
| if (UsesToRename.empty()) |
| continue; |
| LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n"); |
| |
| // We found a use of I outside of BB. Rename all uses of I that are outside |
| // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks |
| // with the two values we know. |
| SSAUpdate.Initialize(I.getType(), I.getName()); |
| SSAUpdate.AddAvailableValue(BB, &I); |
| SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]); |
| |
| while (!UsesToRename.empty()) |
| SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); |
| LLVM_DEBUG(dbgs() << "\n"); |
| } |
| } |
| |
| /// Clone instructions in range [BI, BE) to NewBB. For PHI nodes, we only clone |
| /// arguments that come from PredBB. Return the map from the variables in the |
| /// source basic block to the variables in the newly created basic block. |
| DenseMap<Instruction *, Value *> |
| JumpThreadingPass::cloneInstructions(BasicBlock::iterator BI, |
| BasicBlock::iterator BE, BasicBlock *NewBB, |
| BasicBlock *PredBB) { |
| // We are going to have to map operands from the source basic block to the new |
| // copy of the block 'NewBB'. If there are PHI nodes in the source basic |
| // block, evaluate them to account for entry from PredBB. |
| DenseMap<Instruction *, Value *> ValueMapping; |
| |
| // Clone the phi nodes of the source basic block into NewBB. The resulting |
| // phi nodes are trivial since NewBB only has one predecessor, but SSAUpdater |
| // might need to rewrite the operand of the cloned phi. |
| for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) { |
| PHINode *NewPN = PHINode::Create(PN->getType(), 1, PN->getName(), NewBB); |
| NewPN->addIncoming(PN->getIncomingValueForBlock(PredBB), PredBB); |
| ValueMapping[PN] = NewPN; |
| } |
| |
| // Clone noalias scope declarations in the threaded block. When threading a |
| // loop exit, we would otherwise end up with two idential scope declarations |
| // visible at the same time. |
| SmallVector<MDNode *> NoAliasScopes; |
| DenseMap<MDNode *, MDNode *> ClonedScopes; |
| LLVMContext &Context = PredBB->getContext(); |
| identifyNoAliasScopesToClone(BI, BE, NoAliasScopes); |
| cloneNoAliasScopes(NoAliasScopes, ClonedScopes, "thread", Context); |
| |
| // Clone the non-phi instructions of the source basic block into NewBB, |
| // keeping track of the mapping and using it to remap operands in the cloned |
| // instructions. |
| for (; BI != BE; ++BI) { |
| Instruction *New = BI->clone(); |
| New->setName(BI->getName()); |
| NewBB->getInstList().push_back(New); |
| ValueMapping[&*BI] = New; |
| adaptNoAliasScopes(New, ClonedScopes, Context); |
| |
| // Remap operands to patch up intra-block references. |
| for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) |
| if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { |
| DenseMap<Instruction *, Value *>::iterator I = ValueMapping.find(Inst); |
| if (I != ValueMapping.end()) |
| New->setOperand(i, I->second); |
| } |
| } |
| |
| return ValueMapping; |
| } |
| |
| /// Attempt to thread through two successive basic blocks. |
| bool JumpThreadingPass::maybethreadThroughTwoBasicBlocks(BasicBlock *BB, |
| Value *Cond) { |
| // Consider: |
| // |
| // PredBB: |
| // %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ] |
| // %tobool = icmp eq i32 %cond, 0 |
| // br i1 %tobool, label %BB, label ... |
| // |
| // BB: |
| // %cmp = icmp eq i32* %var, null |
| // br i1 %cmp, label ..., label ... |
| // |
| // We don't know the value of %var at BB even if we know which incoming edge |
| // we take to BB. However, once we duplicate PredBB for each of its incoming |
| // edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of |
| // PredBB. Then we can thread edges PredBB1->BB and PredBB2->BB through BB. |
| |
| // Require that BB end with a Branch for simplicity. |
| BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); |
| if (!CondBr) |
| return false; |
| |
| // BB must have exactly one predecessor. |
| BasicBlock *PredBB = BB->getSinglePredecessor(); |
| if (!PredBB) |
| return false; |
| |
| // Require that PredBB end with a conditional Branch. If PredBB ends with an |
| // unconditional branch, we should be merging PredBB and BB instead. For |
| // simplicity, we don't deal with a switch. |
| BranchInst *PredBBBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); |
| if (!PredBBBranch || PredBBBranch->isUnconditional()) |
| return false; |
| |
| // If PredBB has exactly one incoming edge, we don't gain anything by copying |
| // PredBB. |
| if (PredBB->getSinglePredecessor()) |
| return false; |
| |
| // Don't thread through PredBB if it contains a successor edge to itself, in |
| // which case we would infinite loop. Suppose we are threading an edge from |
| // PredPredBB through PredBB and BB to SuccBB with PredBB containing a |
| // successor edge to itself. If we allowed jump threading in this case, we |
| // could duplicate PredBB and BB as, say, PredBB.thread and BB.thread. Since |
| // PredBB.thread has a successor edge to PredBB, we would immediately come up |
| // with another jump threading opportunity from PredBB.thread through PredBB |
| // and BB to SuccBB. This jump threading would repeatedly occur. That is, we |
| // would keep peeling one iteration from PredBB. |
| if (llvm::is_contained(successors(PredBB), PredBB)) |
| return false; |
| |
| // Don't thread across a loop header. |
| if (LoopHeaders.count(PredBB)) |
| return false; |
| |
| // Avoid complication with duplicating EH pads. |
| if (PredBB->isEHPad()) |
| return false; |
| |
| // Find a predecessor that we can thread. For simplicity, we only consider a |
| // successor edge out of BB to which we thread exactly one incoming edge into |
| // PredBB. |
| unsigned ZeroCount = 0; |
| unsigned OneCount = 0; |
| BasicBlock *ZeroPred = nullptr; |
| BasicBlock *OnePred = nullptr; |
| for (BasicBlock *P : predecessors(PredBB)) { |
| if (ConstantInt *CI = dyn_cast_or_null<ConstantInt>( |
| evaluateOnPredecessorEdge(BB, P, Cond))) { |
| if (CI->isZero()) { |
| ZeroCount++; |
| ZeroPred = P; |
| } else if (CI->isOne()) { |
| OneCount++; |
| OnePred = P; |
| } |
| } |
| } |
| |
| // Disregard complicated cases where we have to thread multiple edges. |
| BasicBlock *PredPredBB; |
| if (ZeroCount == 1) { |
| PredPredBB = ZeroPred; |
| } else if (OneCount == 1) { |
| PredPredBB = OnePred; |
| } else { |
| return false; |
| } |
| |
| BasicBlock *SuccBB = CondBr->getSuccessor(PredPredBB == ZeroPred); |
| |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName() |
| << "' - would thread to self!\n"); |
| return false; |
| } |
| |
| // If threading this would thread across a loop header, don't thread the edge. |
| // See the comments above findLoopHeaders for justifications and caveats. |
| if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) { |
| LLVM_DEBUG({ |
| bool BBIsHeader = LoopHeaders.count(BB); |
| bool SuccIsHeader = LoopHeaders.count(SuccBB); |
| dbgs() << " Not threading across " |
| << (BBIsHeader ? "loop header BB '" : "block BB '") |
| << BB->getName() << "' to dest " |
| << (SuccIsHeader ? "loop header BB '" : "block BB '") |
| << SuccBB->getName() |
| << "' - it might create an irreducible loop!\n"; |
| }); |
| return false; |
| } |
| |
| // Compute the cost of duplicating BB and PredBB. |
| unsigned BBCost = getJumpThreadDuplicationCost( |
| TTI, BB, BB->getTerminator(), BBDupThreshold); |
| unsigned PredBBCost = getJumpThreadDuplicationCost( |
| TTI, PredBB, PredBB->getTerminator(), BBDupThreshold); |
| |
| // Give up if costs are too high. We need to check BBCost and PredBBCost |
| // individually before checking their sum because getJumpThreadDuplicationCost |
| // return (unsigned)~0 for those basic blocks that cannot be duplicated. |
| if (BBCost > BBDupThreshold || PredBBCost > BBDupThreshold || |
| BBCost + PredBBCost > BBDupThreshold) { |
| LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName() |
| << "' - Cost is too high: " << PredBBCost |
| << " for PredBB, " << BBCost << "for BB\n"); |
| return false; |
| } |
| |
| // Now we are ready to duplicate PredBB. |
| threadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB); |
| return true; |
| } |
| |
| void JumpThreadingPass::threadThroughTwoBasicBlocks(BasicBlock *PredPredBB, |
| BasicBlock *PredBB, |
| BasicBlock *BB, |
| BasicBlock *SuccBB) { |
| LLVM_DEBUG(dbgs() << " Threading through '" << PredBB->getName() << "' and '" |
| << BB->getName() << "'\n"); |
| |
| BranchInst *CondBr = cast<BranchInst>(BB->getTerminator()); |
| BranchInst *PredBBBranch = cast<BranchInst>(PredBB->getTerminator()); |
| |
| BasicBlock *NewBB = |
| BasicBlock::Create(PredBB->getContext(), PredBB->getName() + ".thread", |
| PredBB->getParent(), PredBB); |
| NewBB->moveAfter(PredBB); |
| |
| // Set the block frequency of NewBB. |
| if (HasProfileData) { |
| auto NewBBFreq = BFI->getBlockFreq(PredPredBB) * |
| BPI->getEdgeProbability(PredPredBB, PredBB); |
| BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency()); |
| } |
| |
| // We are going to have to map operands from the original BB block to the new |
| // copy of the block 'NewBB'. If there are PHI nodes in PredBB, evaluate them |
| // to account for entry from PredPredBB. |
| DenseMap<Instruction *, Value *> ValueMapping = |
| cloneInstructions(PredBB->begin(), PredBB->end(), NewBB, PredPredBB); |
| |
| // Copy the edge probabilities from PredBB to NewBB. |
| if (HasProfileData) |
| BPI->copyEdgeProbabilities(PredBB, NewBB); |
| |
| // Update the terminator of PredPredBB to jump to NewBB instead of PredBB. |
| // This eliminates predecessors from PredPredBB, which requires us to simplify |
| // any PHI nodes in PredBB. |
| Instruction *PredPredTerm = PredPredBB->getTerminator(); |
| for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i) |
| if (PredPredTerm->getSuccessor(i) == PredBB) { |
| PredBB->removePredecessor(PredPredBB, true); |
| PredPredTerm->setSuccessor(i, NewBB); |
| } |
| |
| addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(0), PredBB, NewBB, |
| ValueMapping); |
| addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB, |
| ValueMapping); |
| |
| DTU->applyUpdatesPermissive( |
| {{DominatorTree::Insert, NewBB, CondBr->getSuccessor(0)}, |
| {DominatorTree::Insert, NewBB, CondBr->getSuccessor(1)}, |
| {DominatorTree::Insert, PredPredBB, NewBB}, |
| {DominatorTree::Delete, PredPredBB, PredBB}}); |
| |
| updateSSA(PredBB, NewBB, ValueMapping); |
| |
| // Clean up things like PHI nodes with single operands, dead instructions, |
| // etc. |
| SimplifyInstructionsInBlock(NewBB, TLI); |
| SimplifyInstructionsInBlock(PredBB, TLI); |
| |
| SmallVector<BasicBlock *, 1> PredsToFactor; |
| PredsToFactor.push_back(NewBB); |
| threadEdge(BB, PredsToFactor, SuccBB); |
| } |
| |
| /// tryThreadEdge - Thread an edge if it's safe and profitable to do so. |
| bool JumpThreadingPass::tryThreadEdge( |
| BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs, |
| BasicBlock *SuccBB) { |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName() |
| << "' - would thread to self!\n"); |
| return false; |
| } |
| |
| // If threading this would thread across a loop header, don't thread the edge. |
| // See the comments above findLoopHeaders for justifications and caveats. |
| if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) { |
| LLVM_DEBUG({ |
| bool BBIsHeader = LoopHeaders.count(BB); |
| bool SuccIsHeader = LoopHeaders.count(SuccBB); |
| dbgs() << " Not threading across " |
| << (BBIsHeader ? "loop header BB '" : "block BB '") << BB->getName() |
| << "' to dest " << (SuccIsHeader ? "loop header BB '" : "block BB '") |
| << SuccBB->getName() << "' - it might create an irreducible loop!\n"; |
| }); |
| return false; |
| } |
| |
| unsigned JumpThreadCost = getJumpThreadDuplicationCost( |
| TTI, BB, BB->getTerminator(), BBDupThreshold); |
| if (JumpThreadCost > BBDupThreshold) { |
| LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName() |
| << "' - Cost is too high: " << JumpThreadCost << "\n"); |
| return false; |
| } |
| |
| threadEdge(BB, PredBBs, SuccBB); |
| return true; |
| } |
| |
| /// threadEdge - We have decided that it is safe and profitable to factor the |
| /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB |
| /// across BB. Transform the IR to reflect this change. |
| void JumpThreadingPass::threadEdge(BasicBlock *BB, |
| const SmallVectorImpl<BasicBlock *> &PredBBs, |
| BasicBlock *SuccBB) { |
| assert(SuccBB != BB && "Don't create an infinite loop"); |
| |
| assert(!LoopHeaders.count(BB) && !LoopHeaders.count(SuccBB) && |
| "Don't thread across loop headers"); |
| |
| // And finally, do it! Start by factoring the predecessors if needed. |
| BasicBlock *PredBB; |
| if (PredBBs.size() == 1) |
| PredBB = PredBBs[0]; |
| else { |
| LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size() |
| << " common predecessors.\n"); |
| PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm"); |
| } |
| |
| // And finally, do it! |
| LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() |
| << "' to '" << SuccBB->getName() |
| << ", across block:\n " << *BB << "\n"); |
| |
| LVI->threadEdge(PredBB, BB, SuccBB); |
| |
| BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), |
| BB->getName()+".thread", |
| BB->getParent(), BB); |
| NewBB->moveAfter(PredBB); |
| |
| // Set the block frequency of NewBB. |
| if (HasProfileData) { |
| auto NewBBFreq = |
| BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB); |
| BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency()); |
| } |
| |
| // Copy all the instructions from BB to NewBB except the terminator. |
| DenseMap<Instruction *, Value *> ValueMapping = |
| cloneInstructions(BB->begin(), std::prev(BB->end()), NewBB, PredBB); |
| |
| // We didn't copy the terminator from BB over to NewBB, because there is now |
| // an unconditional jump to SuccBB. Insert the unconditional jump. |
| BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB); |
| NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc()); |
| |
| // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the |
| // PHI nodes for NewBB now. |
| addPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); |
| |
| // Update the terminator of PredBB to jump to NewBB instead of BB. This |
| // eliminates predecessors from BB, which requires us to simplify any PHI |
| // nodes in BB. |
| Instruction *PredTerm = PredBB->getTerminator(); |
| for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) |
| if (PredTerm->getSuccessor(i) == BB) { |
| BB->removePredecessor(PredBB, true); |
| PredTerm->setSuccessor(i, NewBB); |
| } |
| |
| // Enqueue required DT updates. |
| DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, SuccBB}, |
| {DominatorTree::Insert, PredBB, NewBB}, |
| {DominatorTree::Delete, PredBB, BB}}); |
| |
| updateSSA(BB, NewBB, ValueMapping); |
| |
| // At this point, the IR is fully up to date and consistent. Do a quick scan |
| // over the new instructions and zap any that are constants or dead. This |
| // frequently happens because of phi translation. |
| SimplifyInstructionsInBlock(NewBB, TLI); |
| |
| // Update the edge weight from BB to SuccBB, which should be less than before. |
| updateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB); |
| |
| // Threaded an edge! |
| ++NumThreads; |
| } |
| |
| /// Create a new basic block that will be the predecessor of BB and successor of |
| /// all blocks in Preds. When profile data is available, update the frequency of |
| /// this new block. |
| BasicBlock *JumpThreadingPass::splitBlockPreds(BasicBlock *BB, |
| ArrayRef<BasicBlock *> Preds, |
| const char *Suffix) { |
| SmallVector<BasicBlock *, 2> NewBBs; |
| |
| // Collect the frequencies of all predecessors of BB, which will be used to |
| // update the edge weight of the result of splitting predecessors. |
| DenseMap<BasicBlock *, BlockFrequency> FreqMap; |
| if (HasProfileData) |
| for (auto Pred : Preds) |
| FreqMap.insert(std::make_pair( |
| Pred, BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB))); |
| |
| // In the case when BB is a LandingPad block we create 2 new predecessors |
| // instead of just one. |
| if (BB->isLandingPad()) { |
| std::string NewName = std::string(Suffix) + ".split-lp"; |
| SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs); |
| } else { |
| NewBBs.push_back(SplitBlockPredecessors(BB, Preds, Suffix)); |
| } |
| |
| std::vector<DominatorTree::UpdateType> Updates; |
| Updates.reserve((2 * Preds.size()) + NewBBs.size()); |
| for (auto NewBB : NewBBs) { |
| BlockFrequency NewBBFreq(0); |
| Updates.push_back({DominatorTree::Insert, NewBB, BB}); |
| for (auto Pred : predecessors(NewBB)) { |
| Updates.push_back({DominatorTree::Delete, Pred, BB}); |
| Updates.push_back({DominatorTree::Insert, Pred, NewBB}); |
| if (HasProfileData) // Update frequencies between Pred -> NewBB. |
| NewBBFreq += FreqMap.lookup(Pred); |
| } |
| if (HasProfileData) // Apply the summed frequency to NewBB. |
| BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency()); |
| } |
| |
| DTU->applyUpdatesPermissive(Updates); |
| return NewBBs[0]; |
| } |
| |
| bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock *BB) { |
| const Instruction *TI = BB->getTerminator(); |
| assert(TI->getNumSuccessors() > 1 && "not a split"); |
| |
| MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof); |
| if (!WeightsNode) |
| return false; |
| |
| MDString *MDName = cast<MDString>(WeightsNode->getOperand(0)); |
| if (MDName->getString() != "branch_weights") |
| return false; |
| |
| // Ensure there are weights for all of the successors. Note that the first |
| // operand to the metadata node is a name, not a weight. |
| return WeightsNode->getNumOperands() == TI->getNumSuccessors() + 1; |
| } |
| |
| /// Update the block frequency of BB and branch weight and the metadata on the |
| /// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 - |
| /// Freq(PredBB->BB) / Freq(BB->SuccBB). |
| void JumpThreadingPass::updateBlockFreqAndEdgeWeight(BasicBlock *PredBB, |
| BasicBlock *BB, |
| BasicBlock *NewBB, |
| BasicBlock *SuccBB) { |
| if (!HasProfileData) |
| return; |
| |
| assert(BFI && BPI && "BFI & BPI should have been created here"); |
| |
| // As the edge from PredBB to BB is deleted, we have to update the block |
| // frequency of BB. |
| auto BBOrigFreq = BFI->getBlockFreq(BB); |
| auto NewBBFreq = BFI->getBlockFreq(NewBB); |
| auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB); |
| auto BBNewFreq = BBOrigFreq - NewBBFreq; |
| BFI->setBlockFreq(BB, BBNewFreq.getFrequency()); |
| |
| // Collect updated outgoing edges' frequencies from BB and use them to update |
| // edge probabilities. |
| SmallVector<uint64_t, 4> BBSuccFreq; |
| for (BasicBlock *Succ : successors(BB)) { |
| auto SuccFreq = (Succ == SuccBB) |
| ? BB2SuccBBFreq - NewBBFreq |
| : BBOrigFreq * BPI->getEdgeProbability(BB, Succ); |
| BBSuccFreq.push_back(SuccFreq.getFrequency()); |
| } |
| |
| uint64_t MaxBBSuccFreq = |
| *std::max_element(BBSuccFreq.begin(), BBSuccFreq.end()); |
| |
| SmallVector<BranchProbability, 4> BBSuccProbs; |
| if (MaxBBSuccFreq == 0) |
| BBSuccProbs.assign(BBSuccFreq.size(), |
| {1, static_cast<unsigned>(BBSuccFreq.size())}); |
| else { |
| for (uint64_t Freq : BBSuccFreq) |
| BBSuccProbs.push_back( |
| BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq)); |
| // Normalize edge probabilities so that they sum up to one. |
| BranchProbability::normalizeProbabilities(BBSuccProbs.begin(), |
| BBSuccProbs.end()); |
| } |
| |
| // Update edge probabilities in BPI. |
| BPI->setEdgeProbability(BB, BBSuccProbs); |
| |
| // Update the profile metadata as well. |
| // |
| // Don't do this if the profile of the transformed blocks was statically |
| // estimated. (This could occur despite the function having an entry |
| // frequency in completely cold parts of the CFG.) |
| // |
| // In this case we don't want to suggest to subsequent passes that the |
| // calculated weights are fully consistent. Consider this graph: |
| // |
| // check_1 |
| // 50% / | |
| // eq_1 | 50% |
| // \ | |
| // check_2 |
| // 50% / | |
| // eq_2 | 50% |
| // \ | |
| // check_3 |
| // 50% / | |
| // eq_3 | 50% |
| // \ | |
| // |
| // Assuming the blocks check_* all compare the same value against 1, 2 and 3, |
| // the overall probabilities are inconsistent; the total probability that the |
| // value is either 1, 2 or 3 is 150%. |
| // |
| // As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3 |
| // becomes 0%. This is even worse if the edge whose probability becomes 0% is |
| // the loop exit edge. Then based solely on static estimation we would assume |
| // the loop was extremely hot. |
| // |
| // FIXME this locally as well so that BPI and BFI are consistent as well. We |
| // shouldn't make edges extremely likely or unlikely based solely on static |
| // estimation. |
| if (BBSuccProbs.size() >= 2 && doesBlockHaveProfileData(BB)) { |
| SmallVector<uint32_t, 4> Weights; |
| for (auto Prob : BBSuccProbs) |
| Weights.push_back(Prob.getNumerator()); |
| |
| auto TI = BB->getTerminator(); |
| TI->setMetadata( |
| LLVMContext::MD_prof, |
| MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights)); |
| } |
| } |
| |
| /// duplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch |
| /// to BB which contains an i1 PHI node and a conditional branch on that PHI. |
| /// If we can duplicate the contents of BB up into PredBB do so now, this |
| /// improves the odds that the branch will be on an analyzable instruction like |
| /// a compare. |
| bool JumpThreadingPass::duplicateCondBranchOnPHIIntoPred( |
| BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) { |
| assert(!PredBBs.empty() && "Can't handle an empty set"); |
| |
| // If BB is a loop header, then duplicating this block outside the loop would |
| // cause us to transform this into an irreducible loop, don't do this. |
| // See the comments above findLoopHeaders for justifications and caveats. |
| if (LoopHeaders.count(BB)) { |
| LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() |
| << "' into predecessor block '" << PredBBs[0]->getName() |
| << "' - it might create an irreducible loop!\n"); |
| return false; |
| } |
| |
| unsigned DuplicationCost = getJumpThreadDuplicationCost( |
| TTI, BB, BB->getTerminator(), BBDupThreshold); |
| if (DuplicationCost > BBDupThreshold) { |
| LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() |
| << "' - Cost is too high: " << DuplicationCost << "\n"); |
| return false; |
| } |
| |
| // And finally, do it! Start by factoring the predecessors if needed. |
| std::vector<DominatorTree::UpdateType> Updates; |
| BasicBlock *PredBB; |
| if (PredBBs.size() == 1) |
| PredBB = PredBBs[0]; |
| else { |
| LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size() |
| << " common predecessors.\n"); |
| PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm"); |
| } |
| Updates.push_back({DominatorTree::Delete, PredBB, BB}); |
| |
| // Okay, we decided to do this! Clone all the instructions in BB onto the end |
| // of PredBB. |
| LLVM_DEBUG(dbgs() << " Duplicating block '" << BB->getName() |
| << "' into end of '" << PredBB->getName() |
| << "' to eliminate branch on phi. Cost: " |
| << DuplicationCost << " block is:" << *BB << "\n"); |
| |
| // Unless PredBB ends with an unconditional branch, split the edge so that we |
| // can just clone the bits from BB into the end of the new PredBB. |
| BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); |
| |
| if (!OldPredBranch || !OldPredBranch->isUnconditional()) { |
| BasicBlock *OldPredBB = PredBB; |
| PredBB = SplitEdge(OldPredBB, BB); |
| Updates.push_back({DominatorTree::Insert, OldPredBB, PredBB}); |
| Updates.push_back({DominatorTree::Insert, PredBB, BB}); |
| Updates.push_back({DominatorTree::Delete, OldPredBB, BB}); |
| OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); |
| } |
| |
| // We are going to have to map operands from the original BB block into the |
| // PredBB block. Evaluate PHI nodes in BB. |
| DenseMap<Instruction*, Value*> ValueMapping; |
| |
| BasicBlock::iterator BI = BB->begin(); |
| for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) |
| ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); |
| // Clone the non-phi instructions of BB into PredBB, keeping track of the |
| // mapping and using it to remap operands in the cloned instructions. |
| for (; BI != BB->end(); ++BI) { |
| Instruction *New = BI->clone(); |
| |
| // Remap operands to patch up intra-block references. |
| for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) |
| if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { |
| DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); |
| if (I != ValueMapping.end()) |
| New->setOperand(i, I->second); |
| } |
| |
| // If this instruction can be simplified after the operands are updated, |
| // just use the simplified value instead. This frequently happens due to |
| // phi translation. |
| if (Value *IV = SimplifyInstruction( |
| New, |
| {BB->getModule()->getDataLayout(), TLI, nullptr, nullptr, New})) { |
| ValueMapping[&*BI] = IV; |
| if (!New->mayHaveSideEffects()) { |
| New->deleteValue(); |
| New = nullptr; |
| } |
| } else { |
| ValueMapping[&*BI] = New; |
| } |
| if (New) { |
| // Otherwise, insert the new instruction into the block. |
| New->setName(BI->getName()); |
| PredBB->getInstList().insert(OldPredBranch->getIterator(), New); |
| // Update Dominance from simplified New instruction operands. |
| for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) |
| if (BasicBlock *SuccBB = dyn_cast<BasicBlock>(New->getOperand(i))) |
| Updates.push_back({DominatorTree::Insert, PredBB, SuccBB}); |
| } |
| } |
| |
| // Check to see if the targets of the branch had PHI nodes. If so, we need to |
| // add entries to the PHI nodes for branch from PredBB now. |
| BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); |
| addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, |
| ValueMapping); |
| addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, |
| ValueMapping); |
| |
| updateSSA(BB, PredBB, ValueMapping); |
| |
| // PredBB no longer jumps to BB, remove entries in the PHI node for the edge |
| // that we nuked. |
| BB->removePredecessor(PredBB, true); |
| |
| // Remove the unconditional branch at the end of the PredBB block. |
| OldPredBranch->eraseFromParent(); |
| if (HasProfileData) |
| BPI->copyEdgeProbabilities(BB, PredBB); |
| DTU->applyUpdatesPermissive(Updates); |
| |
| ++NumDupes; |
| return true; |
| } |
| |
| // Pred is a predecessor of BB with an unconditional branch to BB. SI is |
| // a Select instruction in Pred. BB has other predecessors and SI is used in |
| // a PHI node in BB. SI has no other use. |
| // A new basic block, NewBB, is created and SI is converted to compare and |
| // conditional branch. SI is erased from parent. |
| void JumpThreadingPass::unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB, |
| SelectInst *SI, PHINode *SIUse, |
| unsigned Idx) { |
| // Expand the select. |
| // |
| // Pred -- |
| // | v |
| // | NewBB |
| // | | |
| // |----- |
| // v |
| // BB |
| BranchInst *PredTerm = cast<BranchInst>(Pred->getTerminator()); |
| BasicBlock *NewBB = BasicBlock::Create(BB-> |