diff options
Diffstat (limited to 'llvm/lib/Transforms/Scalar/JumpThreading.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/JumpThreading.cpp | 2756 |
1 files changed, 2756 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp new file mode 100644 index 000000000000..0cf00baaa24a --- /dev/null +++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp @@ -0,0 +1,2756 @@ +//===- 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/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/TargetLibraryInfo.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/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> 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(int T = -1) : FunctionPass(ID), Impl(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>(); + } + + 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(int Threshold) { + return new JumpThreading(Threshold); +} + +JumpThreadingPass::JumpThreadingPass(int T) { + BBDupThreshold = (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; + + BranchProbability BP; + uint64_t TrueWeight, FalseWeight; + if (!CondBr->extractProfMetadata(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; + + 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 TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F); + // Get DT analysis before LVI. When LVI is initialized it conditionally adds + // DT if it's available. + 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; + bool HasProfileData = F.hasProfileData(); + if (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, LVI, AA, &DTU, HasProfileData, + std::move(BFI), std::move(BPI)); + if (PrintLVIAfterJumpThreading) { + dbgs() << "LVI for function '" << F.getName() << "':\n"; + LVI->printLVI(F, *DT, dbgs()); + } + return Changed; +} + +PreservedAnalyses JumpThreadingPass::run(Function &F, + FunctionAnalysisManager &AM) { + auto &TLI = AM.getResult<TargetLibraryAnalysis>(F); + // Get DT analysis before LVI. When LVI is initialized it conditionally adds + // DT if it's available. + 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, &LVI, &AA, &DTU, HasProfileData, + std::move(BFI), std::move(BPI)); + + if (!Changed) + return PreservedAnalyses::all(); + PreservedAnalyses PA; + PA.preserve<GlobalsAA>(); + PA.preserve<DominatorTreeAnalysis>(); + PA.preserve<LazyValueAnalysis>(); + return PA; +} + +bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_, + 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_; + 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_); + } + + // 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; + // 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() && + // The terminator must be the only non-phi instruction in BB. + BB.getFirstNonPHIOrDbg()->isTerminator() && + // Don't alter Loop headers and latches to ensure another pass can + // detect and transform nested loops later. + !LoopHeaders.count(&BB) && !LoopHeaders.count(BI->getSuccessor(0)) && + TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU)) { + // 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(); + // Flush only the Dominator Tree. + DTU->getDomTree(); + LVI->enableDT(); + 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(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; + + // Debugger intrinsics don't incur code size. + if (isa<DbgInfoIntrinsic>(I)) continue; + + // If this is a pointer->pointer bitcast, it is free. + if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) + continue; + + // 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; + + // 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 (CI->cannotDuplicate() || CI->isConvergent()) + // Blocks with NoDuplicate are modelled as having infinite cost, so they + // are never duplicated. + return ~0U; + else 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<std::pair<Value *, BasicBlock *>> &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(std::make_pair(V, BB)).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.push_back(std::make_pair(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? + + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + 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.push_back(std::make_pair(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)) { + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + Value *InVal = PN->getIncomingValue(i); + if (Constant *KC = getKnownConstant(InVal, Preference)) { + Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); + } else { + Constant *CI = LVI->getConstantOnEdge(InVal, + PN->getIncomingBlock(i), + BB, CxtI); + if (Constant *KC = getKnownConstant(CI, Preference)) + Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); + } + } + + return !Result.empty(); + } + + // Handle Cast instructions. Only see through Cast when the source operand is + // PHI or Cmp to save the compilation time. + if (CastInst *CI = dyn_cast<CastInst>(I)) { + Value *Source = CI->getOperand(0); + if (!isa<PHINode>(Source) && !isa<CmpInst>(Source)) + return false; + 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; + } + + // Handle some boolean conditions. + if (I->getType()->getPrimitiveSizeInBits() == 1) { + assert(Preference == WantInteger && "One-bit non-integer type?"); + // X | true -> true + // X & false -> false + if (I->getOpcode() == Instruction::Or || + I->getOpcode() == Instruction::And) { + PredValueInfoTy LHSVals, RHSVals; + + ComputeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals, + WantInteger, RecursionSet, CxtI); + ComputeValueKnownInPredecessorsImpl(I->getOperand(1), BB, RHSVals, + WantInteger, RecursionSet, CxtI); + + if (LHSVals.empty() && RHSVals.empty()) + return false; + + ConstantInt *InterestingVal; + if (I->getOpcode() == Instruction::Or) + 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.push_back(std::make_pair(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. + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + 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.push_back(std::make_pair(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) { + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + 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.push_back(std::make_pair(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) { + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + 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.push_back(std::make_pair(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.push_back(std::make_pair(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.push_back(std::make_pair(Val, C.second)); + } + + return !Result.empty(); + } + } + + // If all else fails, see if LVI can figure out a constant value for us. + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + Constant *CI = LVI->getConstant(V, BB, CxtI); + if (Constant *KC = getKnownConstant(CI, Preference)) { + for (BasicBlock *Pred : predecessors(BB)) + Result.push_back(std::make_pair(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 (BasicBlock *SinglePred = BB->getSinglePredecessor()) { + const Instruction *TI = SinglePred->getTerminator(); + if (!TI->isExceptionalTerminator() && TI->getNumSuccessors() == 1 && + SinglePred != BB && !hasAddressTakenAndUsed(BB)) { + // 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; + } + } + + 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. + } + + // 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; + } + } + + // If the terminator is branching on an undef, we can pick any of the + // successors to branch to. Let GetBestDestForJumpOnUndef decide. + if (isa<UndefValue>(Condition)) { + 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); + BBTerm->eraseFromParent(); + DTU->applyUpdatesPermissive(Updates); + 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); + 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 false; + } + + 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"); + + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + LazyValueInfo::Tristate Ret = + LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0), + CondConst, CondBr); + 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()); + 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}}); + 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 (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 in the current + // block, see if we can simplify. + if (PHINode *PN = dyn_cast<PHINode>(CondInst)) + if (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()); + BI->eraseFromParent(); + DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}}); + 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(AATags); + + 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. + Value *Ptr = LoadedPtr->DoPHITranslation(LoadBB, PredBB); + PredAvailable = FindAvailablePtrLoadStore( + Ptr, LoadI->getType(), 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( + Ptr, LoadI->getType(), 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.push_back(std::make_pair(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, MaybeAlign(LoadI->getAlignment()), + LoadI->getOrdering(), LoadI->getSyncScopeID(), + UnavailablePred->getTerminator()); + NewVal->setDebugLoc(LoadI->getDebugLoc()); + if (AATags) + NewVal->setAAMetadata(AATags); + + AvailablePreds.push_back(std::make_pair(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. + DenseMap<BasicBlock*, unsigned> DestPopularity; + for (const auto &PredToDest : PredToDestList) + if (PredToDest.second) + DestPopularity[PredToDest.second]++; + + if (DestPopularity.empty()) + return nullptr; + + // Find the most popular dest. + DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); + BasicBlock *MostPopularDest = DPI->first; + unsigned Popularity = DPI->second; + SmallVector<BasicBlock*, 4> SamePopularity; + + for (++DPI; DPI != DestPopularity.end(); ++DPI) { + // If the popularity of this entry isn't higher than the popularity we've + // seen so far, ignore it. + if (DPI->second < Popularity) + ; // ignore. + else if (DPI->second == Popularity) { + // If it is the same as what we've seen so far, keep track of it. + SamePopularity.push_back(DPI->first); + } else { + // If it is more popular, remember it. + SamePopularity.clear(); + MostPopularDest = DPI->first; + Popularity = DPI->second; + } + } + + // Okay, now we know the most popular destination. If there is more than one + // destination, we need to determine one. This is arbitrary, but we need + // to make a deterministic decision. Pick the first one that appears in the + // successor list. + if (!SamePopularity.empty()) { + SamePopularity.push_back(MostPopularDest); + Instruction *TI = BB->getTerminator(); + for (unsigned i = 0; ; ++i) { + assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); + + if (!is_contained(SamePopularity, TI->getSuccessor(i))) + continue; + + MostPopularDest = TI->getSuccessor(i); + break; + } + } + + // Okay, we have finally picked the most popular destination. + return MostPopularDest; +} + +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)) + return false; + + 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.push_back(std::make_pair(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); + Term->eraseFromParent(); + DTU->applyUpdatesPermissive(Updates); + + // 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.count(PredToDest.second) != 0; + }); + + 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 ThreadEdge(BB, PredsToFactor, MostPopularDest); +} + +/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on +/// a PHI node 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). + 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; + } + + // 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); + } +} + +/// 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. +bool JumpThreadingPass::ThreadEdge(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(BB, BB->getTerminator(), BBDupThreshold); + if (JumpThreadCost > BBDupThreshold) { + LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName() + << "' - Cost is too high: " << JumpThreadCost << "\n"); + return false; + } + + // 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() + << "' with cost: " << JumpThreadCost + << ", across block:\n " << *BB << "\n"); + + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + LVI->threadEdge(PredBB, BB, SuccBB); + + // 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 BB, evaluate them to + // account for entry from PredBB. + DenseMap<Instruction*, Value*> ValueMapping; + + 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()); + } + + BasicBlock::iterator BI = BB->begin(); + // Clone the phi nodes of BB 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 the non-phi instructions of BB into NewBB, keeping track of the + // mapping and using it to remap operands in the cloned instructions. + for (; !BI->isTerminator(); ++BI) { + Instruction *New = BI->clone(); + New->setName(BI->getName()); + NewBB->getInstList().push_back(New); + ValueMapping[&*BI] = New; + + // 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); + } + } + + // 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}}); + + // 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 their uses are no longer + // dominated by the previous def and if so, record them in UsesToRename. + // Also, skip phi operands from PredBB - we'll remove them anyway. + 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"); + } + + // 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; + return true; +} + +/// 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. + for (int I = 0, E = BBSuccProbs.size(); I < E; I++) + BPI->setEdgeProbability(BB, I, BBSuccProbs[I]); + + // 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(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); + + // 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(PredBB, ValueMapping[&I]); + + while (!UsesToRename.empty()) + SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); + LLVM_DEBUG(dbgs() << "\n"); + } + + // 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(); + 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->getContext(), "select.unfold", + BB->getParent(), BB); + // Move the unconditional branch to NewBB. + PredTerm->removeFromParent(); + NewBB->getInstList().insert(NewBB->end(), PredTerm); + // Create a conditional branch and update PHI nodes. + BranchInst::Create(NewBB, BB, SI->getCondition(), Pred); + SIUse->setIncomingValue(Idx, SI->getFalseValue()); + SIUse->addIncoming(SI->getTrueValue(), NewBB); + + // The select is now dead. + SI->eraseFromParent(); + DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, BB}, + {DominatorTree::Insert, Pred, NewBB}}); + + // Update any other PHI nodes in BB. + for (BasicBlock::iterator BI = BB->begin(); + PHINode *Phi = dyn_cast<PHINode>(BI); ++BI) + if (Phi != SIUse) + Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB); +} + +bool JumpThreadingPass::TryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) { + PHINode *CondPHI = dyn_cast<PHINode>(SI->getCondition()); + + if (!CondPHI || CondPHI->getParent() != BB) + return false; + + for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) { + BasicBlock *Pred = CondPHI->getIncomingBlock(I); + SelectInst *PredSI = dyn_cast<SelectInst>(CondPHI->getIncomingValue(I)); + + // The second and third condition can be potentially relaxed. Currently + // the conditions help to simplify the code and allow us to reuse existing + // code, developed for TryToUnfoldSelect(CmpInst *, BasicBlock *) + if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse()) + continue; + + BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator()); + if (!PredTerm || !PredTerm->isUnconditional()) + continue; + + UnfoldSelectInstr(Pred, BB, PredSI, CondPHI, I); + return true; + } + return false; +} + +/// TryToUnfoldSelect - Look for blocks of the form +/// bb1: +/// %a = select +/// br bb2 +/// +/// bb2: +/// %p = phi [%a, %bb1] ... +/// %c = icmp %p +/// br i1 %c +/// +/// And expand the select into a branch structure if one of its arms allows %c +/// to be folded. This later enables threading from bb1 over bb2. +bool JumpThreadingPass::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) { + BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); + PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0)); + Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1)); + + if (!CondBr || !CondBr->isConditional() || !CondLHS || + CondLHS->getParent() != BB) + return false; + + for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) { + BasicBlock *Pred = CondLHS->getIncomingBlock(I); + SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I)); + + // Look if one of the incoming values is a select in the corresponding + // predecessor. + if (!SI || SI->getParent() != Pred || !SI->hasOneUse()) + continue; + + BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator()); + if (!PredTerm || !PredTerm->isUnconditional()) + continue; + + // Now check if one of the select values would allow us to constant fold the + // terminator in BB. We don't do the transform if both sides fold, those + // cases will be threaded in any case. + if (DTU->hasPendingDomTreeUpdates()) + LVI->disableDT(); + else + LVI->enableDT(); + LazyValueInfo::Tristate LHSFolds = + LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1), + CondRHS, Pred, BB, CondCmp); + LazyValueInfo::Tristate RHSFolds = + LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2), + CondRHS, Pred, BB, CondCmp); + if ((LHSFolds != LazyValueInfo::Unknown || + RHSFolds != LazyValueInfo::Unknown) && + LHSFolds != RHSFolds) { + UnfoldSelectInstr(Pred, BB, SI, CondLHS, I); + return true; + } + } + return false; +} + +/// TryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the +/// same BB in the form +/// bb: +/// %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ... +/// %s = select %p, trueval, falseval +/// +/// or +/// +/// bb: +/// %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ... +/// %c = cmp %p, 0 +/// %s = select %c, trueval, falseval +/// +/// And expand the select into a branch structure. This later enables +/// jump-threading over bb in this pass. +/// +/// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold +/// select if the associated PHI has at least one constant. If the unfolded +/// select is not jump-threaded, it will be folded again in the later +/// optimizations. +bool JumpThreadingPass::TryToUnfoldSelectInCurrBB(BasicBlock *BB) { + // 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)) + return false; + + for (BasicBlock::iterator BI = BB->begin(); + PHINode *PN = dyn_cast<PHINode>(BI); ++BI) { + // Look for a Phi having at least one constant incoming value. + if (llvm::all_of(PN->incoming_values(), + [](Value *V) { return !isa<ConstantInt>(V); })) + continue; + + auto isUnfoldCandidate = [BB](SelectInst *SI, Value *V) { + // Check if SI is in BB and use V as condition. + if (SI->getParent() != BB) + return false; + Value *Cond = SI->getCondition(); + return (Cond && Cond == V && Cond->getType()->isIntegerTy(1)); + }; + + SelectInst *SI = nullptr; + for (Use &U : PN->uses()) { + if (ICmpInst *Cmp = dyn_cast<ICmpInst>(U.getUser())) { + // Look for a ICmp in BB that compares PN with a constant and is the + // condition of a Select. + if (Cmp->getParent() == BB && Cmp->hasOneUse() && + isa<ConstantInt>(Cmp->getOperand(1 - U.getOperandNo()))) + if (SelectInst *SelectI = dyn_cast<SelectInst>(Cmp->user_back())) + if (isUnfoldCandidate(SelectI, Cmp->use_begin()->get())) { + SI = SelectI; + break; + } + } else if (SelectInst *SelectI = dyn_cast<SelectInst>(U.getUser())) { + // Look for a Select in BB that uses PN as condition. + if (isUnfoldCandidate(SelectI, U.get())) { + SI = SelectI; + break; + } + } + } + + if (!SI) + continue; + // Expand the select. + Instruction *Term = + SplitBlockAndInsertIfThen(SI->getCondition(), SI, false); + BasicBlock *SplitBB = SI->getParent(); + BasicBlock *NewBB = Term->getParent(); + PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI); + NewPN->addIncoming(SI->getTrueValue(), Term->getParent()); + NewPN->addIncoming(SI->getFalseValue(), BB); + SI->replaceAllUsesWith(NewPN); + SI->eraseFromParent(); + // NewBB and SplitBB are newly created blocks which require insertion. + std::vector<DominatorTree::UpdateType> Updates; + Updates.reserve((2 * SplitBB->getTerminator()->getNumSuccessors()) + 3); + Updates.push_back({DominatorTree::Insert, BB, SplitBB}); + Updates.push_back({DominatorTree::Insert, BB, NewBB}); + Updates.push_back({DominatorTree::Insert, NewBB, SplitBB}); + // BB's successors were moved to SplitBB, update DTU accordingly. + for (auto *Succ : successors(SplitBB)) { + Updates.push_back({DominatorTree::Delete, BB, Succ}); + Updates.push_back({DominatorTree::Insert, SplitBB, Succ}); + } + DTU->applyUpdatesPermissive(Updates); + return true; + } + return false; +} + +/// Try to propagate a guard from the current BB into one of its predecessors +/// in case if another branch of execution implies that the condition of this +/// guard is always true. Currently we only process the simplest case that +/// looks like: +/// +/// Start: +/// %cond = ... +/// br i1 %cond, label %T1, label %F1 +/// T1: +/// br label %Merge +/// F1: +/// br label %Merge +/// Merge: +/// %condGuard = ... +/// call void(i1, ...) @llvm.experimental.guard( i1 %condGuard )[ "deopt"() ] +/// +/// And cond either implies condGuard or !condGuard. In this case all the +/// instructions before the guard can be duplicated in both branches, and the +/// guard is then threaded to one of them. +bool JumpThreadingPass::ProcessGuards(BasicBlock *BB) { + using namespace PatternMatch; + + // We only want to deal with two predecessors. + BasicBlock *Pred1, *Pred2; + auto PI = pred_begin(BB), PE = pred_end(BB); + if (PI == PE) + return false; + Pred1 = *PI++; + if (PI == PE) + return false; + Pred2 = *PI++; + if (PI != PE) + return false; + if (Pred1 == Pred2) + return false; + + // Try to thread one of the guards of the block. + // TODO: Look up deeper than to immediate predecessor? + auto *Parent = Pred1->getSinglePredecessor(); + if (!Parent || Parent != Pred2->getSinglePredecessor()) + return false; + + if (auto *BI = dyn_cast<BranchInst>(Parent->getTerminator())) + for (auto &I : *BB) + if (isGuard(&I) && ThreadGuard(BB, cast<IntrinsicInst>(&I), BI)) + return true; + + return false; +} + +/// Try to propagate the guard from BB which is the lower block of a diamond +/// to one of its branches, in case if diamond's condition implies guard's +/// condition. +bool JumpThreadingPass::ThreadGuard(BasicBlock *BB, IntrinsicInst *Guard, + BranchInst *BI) { + assert(BI->getNumSuccessors() == 2 && "Wrong number of successors?"); + assert(BI->isConditional() && "Unconditional branch has 2 successors?"); + Value *GuardCond = Guard->getArgOperand(0); + Value *BranchCond = BI->getCondition(); + BasicBlock *TrueDest = BI->getSuccessor(0); + BasicBlock *FalseDest = BI->getSuccessor(1); + + auto &DL = BB->getModule()->getDataLayout(); + bool TrueDestIsSafe = false; + bool FalseDestIsSafe = false; + + // True dest is safe if BranchCond => GuardCond. + auto Impl = isImpliedCondition(BranchCond, GuardCond, DL); + if (Impl && *Impl) + TrueDestIsSafe = true; + else { + // False dest is safe if !BranchCond => GuardCond. + Impl = isImpliedCondition(BranchCond, GuardCond, DL, /* LHSIsTrue */ false); + if (Impl && *Impl) + FalseDestIsSafe = true; + } + + if (!TrueDestIsSafe && !FalseDestIsSafe) + return false; + + BasicBlock *PredUnguardedBlock = TrueDestIsSafe ? TrueDest : FalseDest; + BasicBlock *PredGuardedBlock = FalseDestIsSafe ? TrueDest : FalseDest; + + ValueToValueMapTy UnguardedMapping, GuardedMapping; + Instruction *AfterGuard = Guard->getNextNode(); + unsigned Cost = getJumpThreadDuplicationCost(BB, AfterGuard, BBDupThreshold); + if (Cost > BBDupThreshold) + return false; + // Duplicate all instructions before the guard and the guard itself to the + // branch where implication is not proved. + BasicBlock *GuardedBlock = DuplicateInstructionsInSplitBetween( + BB, PredGuardedBlock, AfterGuard, GuardedMapping, *DTU); + assert(GuardedBlock && "Could not create the guarded block?"); + // Duplicate all instructions before the guard in the unguarded branch. + // Since we have successfully duplicated the guarded block and this block + // has fewer instructions, we expect it to succeed. + BasicBlock *UnguardedBlock = DuplicateInstructionsInSplitBetween( + BB, PredUnguardedBlock, Guard, UnguardedMapping, *DTU); + assert(UnguardedBlock && "Could not create the unguarded block?"); + LLVM_DEBUG(dbgs() << "Moved guard " << *Guard << " to block " + << GuardedBlock->getName() << "\n"); + // Some instructions before the guard may still have uses. For them, we need + // to create Phi nodes merging their copies in both guarded and unguarded + // branches. Those instructions that have no uses can be just removed. + SmallVector<Instruction *, 4> ToRemove; + for (auto BI = BB->begin(); &*BI != AfterGuard; ++BI) + if (!isa<PHINode>(&*BI)) + ToRemove.push_back(&*BI); + + Instruction *InsertionPoint = &*BB->getFirstInsertionPt(); + assert(InsertionPoint && "Empty block?"); + // Substitute with Phis & remove. + for (auto *Inst : reverse(ToRemove)) { + if (!Inst->use_empty()) { + PHINode *NewPN = PHINode::Create(Inst->getType(), 2); + NewPN->addIncoming(UnguardedMapping[Inst], UnguardedBlock); + NewPN->addIncoming(GuardedMapping[Inst], GuardedBlock); + NewPN->insertBefore(InsertionPoint); + Inst->replaceAllUsesWith(NewPN); + } + Inst->eraseFromParent(); + } + return true; +} |