| //===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass lowers the 'expect' intrinsic to LLVM metadata. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/iterator_range.h" |
| #include "llvm/IR/BasicBlock.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/Intrinsics.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/IR/MDBuilder.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Transforms/Utils/MisExpect.h" |
| |
| #include <cmath> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "lower-expect-intrinsic" |
| |
| STATISTIC(ExpectIntrinsicsHandled, |
| "Number of 'expect' intrinsic instructions handled"); |
| |
| // These default values are chosen to represent an extremely skewed outcome for |
| // a condition, but they leave some room for interpretation by later passes. |
| // |
| // If the documentation for __builtin_expect() was made explicit that it should |
| // only be used in extreme cases, we could make this ratio higher. As it stands, |
| // programmers may be using __builtin_expect() / llvm.expect to annotate that a |
| // branch is likely or unlikely to be taken. |
| |
| // WARNING: these values are internal implementation detail of the pass. |
| // They should not be exposed to the outside of the pass, front-end codegen |
| // should emit @llvm.expect intrinsics instead of using these weights directly. |
| // Transforms should use TargetTransformInfo's getPredictableBranchThreshold(). |
| static cl::opt<uint32_t> LikelyBranchWeight( |
| "likely-branch-weight", cl::Hidden, cl::init(2000), |
| cl::desc("Weight of the branch likely to be taken (default = 2000)")); |
| static cl::opt<uint32_t> UnlikelyBranchWeight( |
| "unlikely-branch-weight", cl::Hidden, cl::init(1), |
| cl::desc("Weight of the branch unlikely to be taken (default = 1)")); |
| |
| static std::tuple<uint32_t, uint32_t> |
| getBranchWeight(Intrinsic::ID IntrinsicID, CallInst *CI, int BranchCount) { |
| if (IntrinsicID == Intrinsic::expect) { |
| // __builtin_expect |
| return std::make_tuple(LikelyBranchWeight.getValue(), |
| UnlikelyBranchWeight.getValue()); |
| } else { |
| // __builtin_expect_with_probability |
| assert(CI->getNumOperands() >= 3 && |
| "expect with probability must have 3 arguments"); |
| auto *Confidence = cast<ConstantFP>(CI->getArgOperand(2)); |
| double TrueProb = Confidence->getValueAPF().convertToDouble(); |
| assert((TrueProb >= 0.0 && TrueProb <= 1.0) && |
| "probability value must be in the range [0.0, 1.0]"); |
| double FalseProb = (1.0 - TrueProb) / (BranchCount - 1); |
| uint32_t LikelyBW = ceil((TrueProb * (double)(INT32_MAX - 1)) + 1.0); |
| uint32_t UnlikelyBW = ceil((FalseProb * (double)(INT32_MAX - 1)) + 1.0); |
| return std::make_tuple(LikelyBW, UnlikelyBW); |
| } |
| } |
| |
| static bool handleSwitchExpect(SwitchInst &SI) { |
| CallInst *CI = dyn_cast<CallInst>(SI.getCondition()); |
| if (!CI) |
| return false; |
| |
| Function *Fn = CI->getCalledFunction(); |
| if (!Fn || (Fn->getIntrinsicID() != Intrinsic::expect && |
| Fn->getIntrinsicID() != Intrinsic::expect_with_probability)) |
| return false; |
| |
| Value *ArgValue = CI->getArgOperand(0); |
| ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1)); |
| if (!ExpectedValue) |
| return false; |
| |
| SwitchInst::CaseHandle Case = *SI.findCaseValue(ExpectedValue); |
| unsigned n = SI.getNumCases(); // +1 for default case. |
| uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal; |
| std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) = |
| getBranchWeight(Fn->getIntrinsicID(), CI, n + 1); |
| |
| SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeightVal); |
| |
| uint64_t Index = (Case == *SI.case_default()) ? 0 : Case.getCaseIndex() + 1; |
| Weights[Index] = LikelyBranchWeightVal; |
| |
| misexpect::checkExpectAnnotations(SI, Weights, /*IsFrontend=*/true); |
| |
| SI.setCondition(ArgValue); |
| |
| SI.setMetadata(LLVMContext::MD_prof, |
| MDBuilder(CI->getContext()).createBranchWeights(Weights)); |
| |
| return true; |
| } |
| |
| /// Handler for PHINodes that define the value argument to an |
| /// @llvm.expect call. |
| /// |
| /// If the operand of the phi has a constant value and it 'contradicts' |
| /// with the expected value of phi def, then the corresponding incoming |
| /// edge of the phi is unlikely to be taken. Using that information, |
| /// the branch probability info for the originating branch can be inferred. |
| static void handlePhiDef(CallInst *Expect) { |
| Value &Arg = *Expect->getArgOperand(0); |
| ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(Expect->getArgOperand(1)); |
| if (!ExpectedValue) |
| return; |
| const APInt &ExpectedPhiValue = ExpectedValue->getValue(); |
| bool ExpectedValueIsLikely = true; |
| Function *Fn = Expect->getCalledFunction(); |
| // If the function is expect_with_probability, then we need to take the |
| // probability into consideration. For example, in |
| // expect.with.probability.i64(i64 %a, i64 1, double 0.0), the |
| // "ExpectedValue" 1 is unlikely. This affects probability propagation later. |
| if (Fn->getIntrinsicID() == Intrinsic::expect_with_probability) { |
| auto *Confidence = cast<ConstantFP>(Expect->getArgOperand(2)); |
| double TrueProb = Confidence->getValueAPF().convertToDouble(); |
| ExpectedValueIsLikely = (TrueProb > 0.5); |
| } |
| |
| // Walk up in backward a list of instructions that |
| // have 'copy' semantics by 'stripping' the copies |
| // until a PHI node or an instruction of unknown kind |
| // is reached. Negation via xor is also handled. |
| // |
| // C = PHI(...); |
| // B = C; |
| // A = B; |
| // D = __builtin_expect(A, 0); |
| // |
| Value *V = &Arg; |
| SmallVector<Instruction *, 4> Operations; |
| while (!isa<PHINode>(V)) { |
| if (ZExtInst *ZExt = dyn_cast<ZExtInst>(V)) { |
| V = ZExt->getOperand(0); |
| Operations.push_back(ZExt); |
| continue; |
| } |
| |
| if (SExtInst *SExt = dyn_cast<SExtInst>(V)) { |
| V = SExt->getOperand(0); |
| Operations.push_back(SExt); |
| continue; |
| } |
| |
| BinaryOperator *BinOp = dyn_cast<BinaryOperator>(V); |
| if (!BinOp || BinOp->getOpcode() != Instruction::Xor) |
| return; |
| |
| ConstantInt *CInt = dyn_cast<ConstantInt>(BinOp->getOperand(1)); |
| if (!CInt) |
| return; |
| |
| V = BinOp->getOperand(0); |
| Operations.push_back(BinOp); |
| } |
| |
| // Executes the recorded operations on input 'Value'. |
| auto ApplyOperations = [&](const APInt &Value) { |
| APInt Result = Value; |
| for (auto *Op : llvm::reverse(Operations)) { |
| switch (Op->getOpcode()) { |
| case Instruction::Xor: |
| Result ^= cast<ConstantInt>(Op->getOperand(1))->getValue(); |
| break; |
| case Instruction::ZExt: |
| Result = Result.zext(Op->getType()->getIntegerBitWidth()); |
| break; |
| case Instruction::SExt: |
| Result = Result.sext(Op->getType()->getIntegerBitWidth()); |
| break; |
| default: |
| llvm_unreachable("Unexpected operation"); |
| } |
| } |
| return Result; |
| }; |
| |
| auto *PhiDef = cast<PHINode>(V); |
| |
| // Get the first dominating conditional branch of the operand |
| // i's incoming block. |
| auto GetDomConditional = [&](unsigned i) -> BranchInst * { |
| BasicBlock *BB = PhiDef->getIncomingBlock(i); |
| BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); |
| if (BI && BI->isConditional()) |
| return BI; |
| BB = BB->getSinglePredecessor(); |
| if (!BB) |
| return nullptr; |
| BI = dyn_cast<BranchInst>(BB->getTerminator()); |
| if (!BI || BI->isUnconditional()) |
| return nullptr; |
| return BI; |
| }; |
| |
| // Now walk through all Phi operands to find phi oprerands with values |
| // conflicting with the expected phi output value. Any such operand |
| // indicates the incoming edge to that operand is unlikely. |
| for (unsigned i = 0, e = PhiDef->getNumIncomingValues(); i != e; ++i) { |
| |
| Value *PhiOpnd = PhiDef->getIncomingValue(i); |
| ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd); |
| if (!CI) |
| continue; |
| |
| // Not an interesting case when IsUnlikely is false -- we can not infer |
| // anything useful when: |
| // (1) We expect some phi output and the operand value matches it, or |
| // (2) We don't expect some phi output (i.e. the "ExpectedValue" has low |
| // probability) and the operand value doesn't match that. |
| const APInt &CurrentPhiValue = ApplyOperations(CI->getValue()); |
| if (ExpectedValueIsLikely == (ExpectedPhiValue == CurrentPhiValue)) |
| continue; |
| |
| BranchInst *BI = GetDomConditional(i); |
| if (!BI) |
| continue; |
| |
| MDBuilder MDB(PhiDef->getContext()); |
| |
| // There are two situations in which an operand of the PhiDef comes |
| // from a given successor of a branch instruction BI. |
| // 1) When the incoming block of the operand is the successor block; |
| // 2) When the incoming block is BI's enclosing block and the |
| // successor is the PhiDef's enclosing block. |
| // |
| // Returns true if the operand which comes from OpndIncomingBB |
| // comes from outgoing edge of BI that leads to Succ block. |
| auto *OpndIncomingBB = PhiDef->getIncomingBlock(i); |
| auto IsOpndComingFromSuccessor = [&](BasicBlock *Succ) { |
| if (OpndIncomingBB == Succ) |
| // If this successor is the incoming block for this |
| // Phi operand, then this successor does lead to the Phi. |
| return true; |
| if (OpndIncomingBB == BI->getParent() && Succ == PhiDef->getParent()) |
| // Otherwise, if the edge is directly from the branch |
| // to the Phi, this successor is the one feeding this |
| // Phi operand. |
| return true; |
| return false; |
| }; |
| uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal; |
| std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) = getBranchWeight( |
| Expect->getCalledFunction()->getIntrinsicID(), Expect, 2); |
| if (!ExpectedValueIsLikely) |
| std::swap(LikelyBranchWeightVal, UnlikelyBranchWeightVal); |
| |
| if (IsOpndComingFromSuccessor(BI->getSuccessor(1))) |
| BI->setMetadata(LLVMContext::MD_prof, |
| MDB.createBranchWeights(LikelyBranchWeightVal, |
| UnlikelyBranchWeightVal)); |
| else if (IsOpndComingFromSuccessor(BI->getSuccessor(0))) |
| BI->setMetadata(LLVMContext::MD_prof, |
| MDB.createBranchWeights(UnlikelyBranchWeightVal, |
| LikelyBranchWeightVal)); |
| } |
| } |
| |
| // Handle both BranchInst and SelectInst. |
| template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) { |
| |
| // Handle non-optimized IR code like: |
| // %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1) |
| // %tobool = icmp ne i64 %expval, 0 |
| // br i1 %tobool, label %if.then, label %if.end |
| // |
| // Or the following simpler case: |
| // %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1) |
| // br i1 %expval, label %if.then, label %if.end |
| |
| CallInst *CI; |
| |
| ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition()); |
| CmpInst::Predicate Predicate; |
| ConstantInt *CmpConstOperand = nullptr; |
| if (!CmpI) { |
| CI = dyn_cast<CallInst>(BSI.getCondition()); |
| Predicate = CmpInst::ICMP_NE; |
| } else { |
| Predicate = CmpI->getPredicate(); |
| if (Predicate != CmpInst::ICMP_NE && Predicate != CmpInst::ICMP_EQ) |
| return false; |
| |
| CmpConstOperand = dyn_cast<ConstantInt>(CmpI->getOperand(1)); |
| if (!CmpConstOperand) |
| return false; |
| CI = dyn_cast<CallInst>(CmpI->getOperand(0)); |
| } |
| |
| if (!CI) |
| return false; |
| |
| uint64_t ValueComparedTo = 0; |
| if (CmpConstOperand) { |
| if (CmpConstOperand->getBitWidth() > 64) |
| return false; |
| ValueComparedTo = CmpConstOperand->getZExtValue(); |
| } |
| |
| Function *Fn = CI->getCalledFunction(); |
| if (!Fn || (Fn->getIntrinsicID() != Intrinsic::expect && |
| Fn->getIntrinsicID() != Intrinsic::expect_with_probability)) |
| return false; |
| |
| Value *ArgValue = CI->getArgOperand(0); |
| ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1)); |
| if (!ExpectedValue) |
| return false; |
| |
| MDBuilder MDB(CI->getContext()); |
| MDNode *Node; |
| |
| uint32_t LikelyBranchWeightVal, UnlikelyBranchWeightVal; |
| std::tie(LikelyBranchWeightVal, UnlikelyBranchWeightVal) = |
| getBranchWeight(Fn->getIntrinsicID(), CI, 2); |
| |
| SmallVector<uint32_t, 4> ExpectedWeights; |
| if ((ExpectedValue->getZExtValue() == ValueComparedTo) == |
| (Predicate == CmpInst::ICMP_EQ)) { |
| Node = |
| MDB.createBranchWeights(LikelyBranchWeightVal, UnlikelyBranchWeightVal); |
| ExpectedWeights = {LikelyBranchWeightVal, UnlikelyBranchWeightVal}; |
| } else { |
| Node = |
| MDB.createBranchWeights(UnlikelyBranchWeightVal, LikelyBranchWeightVal); |
| ExpectedWeights = {UnlikelyBranchWeightVal, LikelyBranchWeightVal}; |
| } |
| |
| if (CmpI) |
| CmpI->setOperand(0, ArgValue); |
| else |
| BSI.setCondition(ArgValue); |
| |
| misexpect::checkFrontendInstrumentation(BSI, ExpectedWeights); |
| |
| BSI.setMetadata(LLVMContext::MD_prof, Node); |
| |
| return true; |
| } |
| |
| static bool handleBranchExpect(BranchInst &BI) { |
| if (BI.isUnconditional()) |
| return false; |
| |
| return handleBrSelExpect<BranchInst>(BI); |
| } |
| |
| static bool lowerExpectIntrinsic(Function &F) { |
| bool Changed = false; |
| |
| for (BasicBlock &BB : F) { |
| // Create "block_weights" metadata. |
| if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) { |
| if (handleBranchExpect(*BI)) |
| ExpectIntrinsicsHandled++; |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) { |
| if (handleSwitchExpect(*SI)) |
| ExpectIntrinsicsHandled++; |
| } |
| |
| // Remove llvm.expect intrinsics. Iterate backwards in order |
| // to process select instructions before the intrinsic gets |
| // removed. |
| for (Instruction &Inst : llvm::make_early_inc_range(llvm::reverse(BB))) { |
| CallInst *CI = dyn_cast<CallInst>(&Inst); |
| if (!CI) { |
| if (SelectInst *SI = dyn_cast<SelectInst>(&Inst)) { |
| if (handleBrSelExpect(*SI)) |
| ExpectIntrinsicsHandled++; |
| } |
| continue; |
| } |
| |
| Function *Fn = CI->getCalledFunction(); |
| if (Fn && (Fn->getIntrinsicID() == Intrinsic::expect || |
| Fn->getIntrinsicID() == Intrinsic::expect_with_probability)) { |
| // Before erasing the llvm.expect, walk backward to find |
| // phi that define llvm.expect's first arg, and |
| // infer branch probability: |
| handlePhiDef(CI); |
| Value *Exp = CI->getArgOperand(0); |
| CI->replaceAllUsesWith(Exp); |
| CI->eraseFromParent(); |
| Changed = true; |
| } |
| } |
| } |
| |
| return Changed; |
| } |
| |
| PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F, |
| FunctionAnalysisManager &) { |
| if (lowerExpectIntrinsic(F)) |
| return PreservedAnalyses::none(); |
| |
| return PreservedAnalyses::all(); |
| } |
| |
| namespace { |
| /// Legacy pass for lowering expect intrinsics out of the IR. |
| /// |
| /// When this pass is run over a function it uses expect intrinsics which feed |
| /// branches and switches to provide branch weight metadata for those |
| /// terminators. It then removes the expect intrinsics from the IR so the rest |
| /// of the optimizer can ignore them. |
| class LowerExpectIntrinsic : public FunctionPass { |
| public: |
| static char ID; |
| LowerExpectIntrinsic() : FunctionPass(ID) { |
| initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); } |
| }; |
| } // namespace |
| |
| char LowerExpectIntrinsic::ID = 0; |
| INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect", |
| "Lower 'expect' Intrinsics", false, false) |
| |
| FunctionPass *llvm::createLowerExpectIntrinsicPass() { |
| return new LowerExpectIntrinsic(); |
| } |