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Diffstat (limited to 'llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp | 397 |
1 files changed, 397 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp b/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp new file mode 100644 index 000000000000..d85f20b3f80c --- /dev/null +++ b/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp @@ -0,0 +1,397 @@ +//===- 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/IR/Metadata.h" +#include "llvm/Pass.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Transforms/Utils/MisExpect.h" + +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. +// +// There is a known dependency on this ratio in CodeGenPrepare when transforming +// 'select' instructions. It may be worthwhile to hoist these values to some +// shared space, so they can be used directly by other passes. + +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 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) + 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. + SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight); + + uint64_t Index = (Case == *SI.case_default()) ? 0 : Case.getCaseIndex() + 1; + Weights[Index] = LikelyBranchWeight; + + SI.setMetadata( + LLVMContext::MD_misexpect, + MDBuilder(CI->getContext()) + .createMisExpect(Index, LikelyBranchWeight, UnlikelyBranchWeight)); + + SI.setCondition(ArgValue); + misexpect::checkFrontendInstrumentation(SI); + + 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(); + + // 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 the operand value matches the expected phi + // output. + if (ExpectedPhiValue == ApplyOperations(CI->getValue())) + 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; + }; + + if (IsOpndComingFromSuccessor(BI->getSuccessor(1))) + BI->setMetadata( + LLVMContext::MD_prof, + MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight)); + else if (IsOpndComingFromSuccessor(BI->getSuccessor(0))) + BI->setMetadata( + LLVMContext::MD_prof, + MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight)); + } +} + +// 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) + 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; + MDNode *ExpNode; + + if ((ExpectedValue->getZExtValue() == ValueComparedTo) == + (Predicate == CmpInst::ICMP_EQ)) { + Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight); + ExpNode = MDB.createMisExpect(0, LikelyBranchWeight, UnlikelyBranchWeight); + } else { + Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight); + ExpNode = MDB.createMisExpect(1, LikelyBranchWeight, UnlikelyBranchWeight); + } + + BSI.setMetadata(LLVMContext::MD_misexpect, ExpNode); + + if (CmpI) + CmpI->setOperand(0, ArgValue); + else + BSI.setCondition(ArgValue); + + misexpect::checkFrontendInstrumentation(BSI); + + 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 (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) { + Instruction *Inst = &*BI++; + 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) { + // 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); } +}; +} + +char LowerExpectIntrinsic::ID = 0; +INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect", + "Lower 'expect' Intrinsics", false, false) + +FunctionPass *llvm::createLowerExpectIntrinsicPass() { + return new LowerExpectIntrinsic(); +} |