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Diffstat (limited to 'gnu/llvm/lib/Transforms/Scalar/JumpThreading.cpp')
| -rw-r--r-- | gnu/llvm/lib/Transforms/Scalar/JumpThreading.cpp | 2731 |
1 files changed, 0 insertions, 2731 deletions
diff --git a/gnu/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/gnu/llvm/lib/Transforms/Scalar/JumpThreading.cpp deleted file mode 100644 index 48de56a0283..00000000000 --- a/gnu/llvm/lib/Transforms/Scalar/JumpThreading.cpp +++ /dev/null @@ -1,2731 +0,0 @@ -//===- JumpThreading.cpp - Thread control through conditional blocks ------===// -// -// The LLVM Compiler Infrastructure -// -// This file is distributed under the University of Illinois Open Source -// License. See LICENSE.TXT for details. -// -//===----------------------------------------------------------------------===// -// -// 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/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/DomTreeUpdater.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); - -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; - while (true) { - BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()); - if (PredBr && PredBr->isConditional()) - return {PredBB, SuccBB}; - auto *SinglePredBB = PredBB->getSinglePredecessor(); - if (!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 = cast<BranchInst>(PredBB->getTerminator()); - - 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(); - // 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); - - 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. - } - - // 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->applyUpdates(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::Create(CondBr->getSuccessor(ToKeep), CondBr); - 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->deleteEdgeRelaxed(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())) - TryToUnfoldSelect(SI, BB); - - // 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::Create(KeepSucc, BI); - BI->eraseFromParent(); - DTU->deleteEdgeRelaxed(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. - if (isa<IndirectBrInst>(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(LoadedPtr->DoPHITranslation(LoadBB, UnavailablePred), - LoadI->getName() + ".pr", false, 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 = - std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), - 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; - - unsigned PredWithKnownDest = 0; - 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; - } - - // We know where this predecessor is going. - ++PredWithKnownDest; - - // If the predecessor ends with an indirect goto, we can't change its - // destination. - if (isa<IndirectBrInst>(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 (PredWithKnownDest == (size_t)pred_size(BB)) { - 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->applyUpdates(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(); - for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) - ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); - - // 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->applyUpdates({{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->applyUpdates(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->applyUpdates(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 = dyn_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->applyUpdates({{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->applyUpdates(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; -} |