1 //===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass lowers the 'expect' intrinsic to LLVM metadata.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
15 #include "llvm/ADT/SmallVector.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/IR/BasicBlock.h"
18 #include "llvm/IR/Constants.h"
19 #include "llvm/IR/Function.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Intrinsics.h"
22 #include "llvm/IR/LLVMContext.h"
23 #include "llvm/IR/MDBuilder.h"
24 #include "llvm/IR/Metadata.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Transforms/Scalar.h"
32 #define DEBUG_TYPE "lower-expect-intrinsic"
34 STATISTIC(ExpectIntrinsicsHandled,
35 "Number of 'expect' intrinsic instructions handled");
37 static cl::opt<uint32_t>
38 LikelyBranchWeight("likely-branch-weight", cl::Hidden, cl::init(64),
39 cl::desc("Weight of the branch likely to be taken (default = 64)"));
40 static cl::opt<uint32_t>
41 UnlikelyBranchWeight("unlikely-branch-weight", cl::Hidden, cl::init(4),
42 cl::desc("Weight of the branch unlikely to be taken (default = 4)"));
44 static bool handleSwitchExpect(SwitchInst &SI) {
45 CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
49 Function *Fn = CI->getCalledFunction();
50 if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
53 Value *ArgValue = CI->getArgOperand(0);
54 ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
58 SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
59 unsigned n = SI.getNumCases(); // +1 for default case.
60 SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
62 if (Case == SI.case_default())
63 Weights[0] = LikelyBranchWeight;
65 Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
67 SI.setMetadata(LLVMContext::MD_prof,
68 MDBuilder(CI->getContext()).createBranchWeights(Weights));
70 SI.setCondition(ArgValue);
74 static bool handleBranchExpect(BranchInst &BI) {
75 if (BI.isUnconditional())
78 // Handle non-optimized IR code like:
79 // %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
80 // %tobool = icmp ne i64 %expval, 0
81 // br i1 %tobool, label %if.then, label %if.end
83 // Or the following simpler case:
84 // %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
85 // br i1 %expval, label %if.then, label %if.end
89 ICmpInst *CmpI = dyn_cast<ICmpInst>(BI.getCondition());
91 CI = dyn_cast<CallInst>(BI.getCondition());
93 if (CmpI->getPredicate() != CmpInst::ICMP_NE)
95 CI = dyn_cast<CallInst>(CmpI->getOperand(0));
101 Function *Fn = CI->getCalledFunction();
102 if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
105 Value *ArgValue = CI->getArgOperand(0);
106 ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
110 MDBuilder MDB(CI->getContext());
113 // If expect value is equal to 1 it means that we are more likely to take
114 // branch 0, in other case more likely is branch 1.
115 if (ExpectedValue->isOne())
116 Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
118 Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
120 BI.setMetadata(LLVMContext::MD_prof, Node);
123 CmpI->setOperand(0, ArgValue);
125 BI.setCondition(ArgValue);
129 static bool lowerExpectIntrinsic(Function &F) {
130 bool Changed = false;
132 for (BasicBlock &BB : F) {
133 // Create "block_weights" metadata.
134 if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
135 if (handleBranchExpect(*BI))
136 ExpectIntrinsicsHandled++;
137 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
138 if (handleSwitchExpect(*SI))
139 ExpectIntrinsicsHandled++;
142 // remove llvm.expect intrinsics.
143 for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
144 CallInst *CI = dyn_cast<CallInst>(BI++);
148 Function *Fn = CI->getCalledFunction();
149 if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
150 Value *Exp = CI->getArgOperand(0);
151 CI->replaceAllUsesWith(Exp);
152 CI->eraseFromParent();
161 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F) {
162 if (lowerExpectIntrinsic(F))
163 return PreservedAnalyses::none();
165 return PreservedAnalyses::all();
169 /// \brief Legacy pass for lowering expect intrinsics out of the IR.
171 /// When this pass is run over a function it uses expect intrinsics which feed
172 /// branches and switches to provide branch weight metadata for those
173 /// terminators. It then removes the expect intrinsics from the IR so the rest
174 /// of the optimizer can ignore them.
175 class LowerExpectIntrinsic : public FunctionPass {
178 LowerExpectIntrinsic() : FunctionPass(ID) {
179 initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
182 bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
186 char LowerExpectIntrinsic::ID = 0;
187 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
188 "Lower 'expect' Intrinsics", false, false)
190 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
191 return new LowerExpectIntrinsic();