LowerExpectIntrinsic.cpp revision 263508 
1//===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass lowers the 'expect' intrinsic to LLVM metadata.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "lower-expect-intrinsic"
15#include "llvm/Transforms/Scalar.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 <vector>
29
30using namespace llvm;
31
32STATISTIC(IfHandled, "Number of 'expect' intrinsic instructions handled");
33
34static cl::opt<uint32_t>
35LikelyBranchWeight("likely-branch-weight", cl::Hidden, cl::init(64),
36                   cl::desc("Weight of the branch likely to be taken (default = 64)"));
37static cl::opt<uint32_t>
38UnlikelyBranchWeight("unlikely-branch-weight", cl::Hidden, cl::init(4),
39                   cl::desc("Weight of the branch unlikely to be taken (default = 4)"));
40
41namespace {
42
43  class LowerExpectIntrinsic : public FunctionPass {
44
45    bool HandleSwitchExpect(SwitchInst *SI);
46
47    bool HandleIfExpect(BranchInst *BI);
48
49  public:
50    static char ID;
51    LowerExpectIntrinsic() : FunctionPass(ID) {
52      initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
53    }
54
55    bool runOnFunction(Function &F);
56  };
57}
58
59
60bool LowerExpectIntrinsic::HandleSwitchExpect(SwitchInst *SI) {
61  CallInst *CI = dyn_cast<CallInst>(SI->getCondition());
62  if (!CI)
63    return false;
64
65  Function *Fn = CI->getCalledFunction();
66  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
67    return false;
68
69  Value *ArgValue = CI->getArgOperand(0);
70  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
71  if (!ExpectedValue)
72    return false;
73
74  SwitchInst::CaseIt Case = SI->findCaseValue(ExpectedValue);
75  unsigned n = SI->getNumCases(); // +1 for default case.
76  std::vector<uint32_t> Weights(n + 1);
77
78  Weights[0] = Case == SI->case_default() ? LikelyBranchWeight
79                                          : UnlikelyBranchWeight;
80  for (unsigned i = 0; i != n; ++i)
81    Weights[i + 1] = i == Case.getCaseIndex() ? LikelyBranchWeight
82                                              : UnlikelyBranchWeight;
83
84  SI->setMetadata(LLVMContext::MD_prof,
85                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
86
87  SI->setCondition(ArgValue);
88  return true;
89}
90
91
92bool LowerExpectIntrinsic::HandleIfExpect(BranchInst *BI) {
93  if (BI->isUnconditional())
94    return false;
95
96  // Handle non-optimized IR code like:
97  //   %expval = call i64 @llvm.expect.i64.i64(i64 %conv1, i64 1)
98  //   %tobool = icmp ne i64 %expval, 0
99  //   br i1 %tobool, label %if.then, label %if.end
100
101  ICmpInst *CmpI = dyn_cast<ICmpInst>(BI->getCondition());
102  if (!CmpI || CmpI->getPredicate() != CmpInst::ICMP_NE)
103    return false;
104
105  CallInst *CI = dyn_cast<CallInst>(CmpI->getOperand(0));
106  if (!CI)
107    return false;
108
109  Function *Fn = CI->getCalledFunction();
110  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
111    return false;
112
113  Value *ArgValue = CI->getArgOperand(0);
114  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
115  if (!ExpectedValue)
116    return false;
117
118  MDBuilder MDB(CI->getContext());
119  MDNode *Node;
120
121  // If expect value is equal to 1 it means that we are more likely to take
122  // branch 0, in other case more likely is branch 1.
123  if (ExpectedValue->isOne())
124    Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
125  else
126    Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
127
128  BI->setMetadata(LLVMContext::MD_prof, Node);
129
130  CmpI->setOperand(0, ArgValue);
131  return true;
132}
133
134
135bool LowerExpectIntrinsic::runOnFunction(Function &F) {
136  for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
137    BasicBlock *BB = I++;
138
139    // Create "block_weights" metadata.
140    if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
141      if (HandleIfExpect(BI))
142        IfHandled++;
143    } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
144      if (HandleSwitchExpect(SI))
145        IfHandled++;
146    }
147
148    // remove llvm.expect intrinsics.
149    for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
150         BI != BE; ) {
151      CallInst *CI = dyn_cast<CallInst>(BI++);
152      if (!CI)
153        continue;
154
155      Function *Fn = CI->getCalledFunction();
156      if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
157        Value *Exp = CI->getArgOperand(0);
158        CI->replaceAllUsesWith(Exp);
159        CI->eraseFromParent();
160      }
161    }
162  }
163
164  return false;
165}
166
167
168char LowerExpectIntrinsic::ID = 0;
169INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect", "Lower 'expect' "
170                "Intrinsics", false, false)
171
172FunctionPass *llvm::createLowerExpectIntrinsicPass() {
173  return new LowerExpectIntrinsic();
174}
175