LowerExpectIntrinsic.cpp revision 5820d6280375716df3f18f39966eb2c595befabe 
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/Constants.h"
16#include "llvm/Function.h"
17#include "llvm/BasicBlock.h"
18#include "llvm/LLVMContext.h"
19#include "llvm/Instructions.h"
20#include "llvm/Intrinsics.h"
21#include "llvm/Metadata.h"
22#include "llvm/Pass.h"
23#include "llvm/Transforms/Scalar.h"
24#include "llvm/Support/CommandLine.h"
25#include "llvm/Support/Debug.h"
26#include "llvm/ADT/Statistic.h"
27#include <vector>
28
29using namespace llvm;
30
31STATISTIC(IfHandled, "Number of 'expect' intrinsic intructions handled");
32
33static cl::opt<uint32_t>
34LikelyBranchWeight("likely-branch-weight", cl::Hidden, cl::init(64),
35                   cl::desc("Weight of the branch likely to be taken (default = 64)"));
36static cl::opt<uint32_t>
37UnlikelyBranchWeight("unlikely-branch-weight", cl::Hidden, cl::init(4),
38                   cl::desc("Weight of the branch unlikely to be taken (default = 4)"));
39
40namespace {
41
42  class LowerExpectIntrinsic : public FunctionPass {
43
44    bool HandleSwitchExpect(SwitchInst *SI);
45
46    bool HandleIfExpect(BranchInst *BI);
47
48  public:
49    static char ID;
50    LowerExpectIntrinsic() : FunctionPass(ID) {
51      initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
52    }
53
54    bool runOnFunction(Function &F);
55  };
56}
57
58
59bool LowerExpectIntrinsic::HandleSwitchExpect(SwitchInst *SI) {
60  CallInst *CI = dyn_cast<CallInst>(SI->getCondition());
61  if (!CI)
62    return false;
63
64  Function *Fn = CI->getCalledFunction();
65  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
66    return false;
67
68  Value *ArgValue = CI->getArgOperand(0);
69  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
70  if (!ExpectedValue)
71    return false;
72
73  LLVMContext &Context = CI->getContext();
74  Type *Int32Ty = Type::getInt32Ty(Context);
75
76  unsigned caseNo = SI->findCaseValue(ExpectedValue);
77  std::vector<Value *> Vec;
78  unsigned n = SI->getNumCases();
79  Vec.resize(n + 1); // +1 for MDString
80
81  Vec[0] = MDString::get(Context, "branch_weights");
82  for (unsigned i = 0; i < n; ++i) {
83    Vec[i + 1] = ConstantInt::get(Int32Ty, i == caseNo ? LikelyBranchWeight : UnlikelyBranchWeight);
84  }
85
86  MDNode *WeightsNode = llvm::MDNode::get(Context, Vec);
87  SI->setMetadata(LLVMContext::MD_prof, WeightsNode);
88
89  SI->setCondition(ArgValue);
90  return true;
91}
92
93
94bool LowerExpectIntrinsic::HandleIfExpect(BranchInst *BI) {
95  if (BI->isUnconditional())
96    return false;
97
98  // Handle non-optimized IR code like:
99  //   %expval = call i64 @llvm.expect.i64.i64(i64 %conv1, i64 1)
100  //   %tobool = icmp ne i64 %expval, 0
101  //   br i1 %tobool, label %if.then, label %if.end
102
103  ICmpInst *CmpI = dyn_cast<ICmpInst>(BI->getCondition());
104  if (!CmpI || CmpI->getPredicate() != CmpInst::ICMP_NE)
105    return false;
106
107  CallInst *CI = dyn_cast<CallInst>(CmpI->getOperand(0));
108  if (!CI)
109    return false;
110
111  Function *Fn = CI->getCalledFunction();
112  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
113    return false;
114
115  Value *ArgValue = CI->getArgOperand(0);
116  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
117  if (!ExpectedValue)
118    return false;
119
120  LLVMContext &Context = CI->getContext();
121  Type *Int32Ty = Type::getInt32Ty(Context);
122  bool Likely = ExpectedValue->isOne();
123
124  // If expect value is equal to 1 it means that we are more likely to take
125  // branch 0, in other case more likely is branch 1.
126  Value *Ops[] = {
127    MDString::get(Context, "branch_weights"),
128    ConstantInt::get(Int32Ty, Likely ? LikelyBranchWeight : UnlikelyBranchWeight),
129    ConstantInt::get(Int32Ty, Likely ? UnlikelyBranchWeight : LikelyBranchWeight)
130  };
131
132  MDNode *WeightsNode = MDNode::get(Context, Ops);
133  BI->setMetadata(LLVMContext::MD_prof, WeightsNode);
134
135  CmpI->setOperand(0, ArgValue);
136  return true;
137}
138
139
140bool LowerExpectIntrinsic::runOnFunction(Function &F) {
141  for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
142    BasicBlock *BB = I++;
143
144    // Create "block_weights" metadata.
145    if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
146      if (HandleIfExpect(BI))
147        IfHandled++;
148    } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
149      if (HandleSwitchExpect(SI))
150        IfHandled++;
151    }
152
153    // remove llvm.expect intrinsics.
154    for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
155         BI != BE; ) {
156      CallInst *CI = dyn_cast<CallInst>(BI++);
157      if (!CI)
158        continue;
159
160      Function *Fn = CI->getCalledFunction();
161      if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
162        Value *Exp = CI->getArgOperand(0);
163        CI->replaceAllUsesWith(Exp);
164        CI->eraseFromParent();
165      }
166    }
167  }
168
169  return false;
170}
171
172
173char LowerExpectIntrinsic::ID = 0;
174INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect", "Lower 'expect' "
175                "Intrinsics", false, false)
176
177FunctionPass *llvm::createLowerExpectIntrinsicPass() {
178  return new LowerExpectIntrinsic();
179}
180