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#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"
29
30using namespace llvm;
31
32#define DEBUG_TYPE "lower-expect-intrinsic"
33
34STATISTIC(ExpectIntrinsicsHandled,
35          "Number of 'expect' intrinsic instructions handled");
36
37static cl::opt<uint32_t>
38LikelyBranchWeight("likely-branch-weight", cl::Hidden, cl::init(64),
39                   cl::desc("Weight of the branch likely to be taken (default = 64)"));
40static cl::opt<uint32_t>
41UnlikelyBranchWeight("unlikely-branch-weight", cl::Hidden, cl::init(4),
42                   cl::desc("Weight of the branch unlikely to be taken (default = 4)"));
43
44static bool handleSwitchExpect(SwitchInst &SI) {
45  CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
46  if (!CI)
47    return false;
48
49  Function *Fn = CI->getCalledFunction();
50  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
51    return false;
52
53  Value *ArgValue = CI->getArgOperand(0);
54  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
55  if (!ExpectedValue)
56    return false;
57
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);
61
62  if (Case == SI.case_default())
63    Weights[0] = LikelyBranchWeight;
64  else
65    Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
66
67  SI.setMetadata(LLVMContext::MD_prof,
68                 MDBuilder(CI->getContext()).createBranchWeights(Weights));
69
70  SI.setCondition(ArgValue);
71  return true;
72}
73
74static bool handleBranchExpect(BranchInst &BI) {
75  if (BI.isUnconditional())
76    return false;
77
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
82  //
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
86
87  CallInst *CI;
88
89  ICmpInst *CmpI = dyn_cast<ICmpInst>(BI.getCondition());
90  if (!CmpI) {
91    CI = dyn_cast<CallInst>(BI.getCondition());
92  } else {
93    if (CmpI->getPredicate() != CmpInst::ICMP_NE)
94      return false;
95    CI = dyn_cast<CallInst>(CmpI->getOperand(0));
96  }
97
98  if (!CI)
99    return false;
100
101  Function *Fn = CI->getCalledFunction();
102  if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
103    return false;
104
105  Value *ArgValue = CI->getArgOperand(0);
106  ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
107  if (!ExpectedValue)
108    return false;
109
110  MDBuilder MDB(CI->getContext());
111  MDNode *Node;
112
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);
117  else
118    Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
119
120  BI.setMetadata(LLVMContext::MD_prof, Node);
121
122  if (CmpI)
123    CmpI->setOperand(0, ArgValue);
124  else
125    BI.setCondition(ArgValue);
126  return true;
127}
128
129static bool lowerExpectIntrinsic(Function &F) {
130  bool Changed = false;
131
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++;
140    }
141
142    // remove llvm.expect intrinsics.
143    for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
144      CallInst *CI = dyn_cast<CallInst>(BI++);
145      if (!CI)
146        continue;
147
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();
153        Changed = true;
154      }
155    }
156  }
157
158  return Changed;
159}
160
161PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F) {
162  if (lowerExpectIntrinsic(F))
163    return PreservedAnalyses::none();
164
165  return PreservedAnalyses::all();
166}
167
168namespace {
169/// \brief Legacy pass for lowering expect intrinsics out of the IR.
170///
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.
175class LowerExpectIntrinsic : public FunctionPass {
176public:
177  static char ID;
178  LowerExpectIntrinsic() : FunctionPass(ID) {
179    initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
180  }
181
182  bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
183};
184}
185
186char LowerExpectIntrinsic::ID = 0;
187INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
188                "Lower 'expect' Intrinsics", false, false)
189
190FunctionPass *llvm::createLowerExpectIntrinsicPass() {
191  return new LowerExpectIntrinsic();
192}
193