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      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 
     30 using namespace llvm;
     31 
     32 #define DEBUG_TYPE "lower-expect-intrinsic"
     33 
     34 STATISTIC(ExpectIntrinsicsHandled,
     35           "Number of 'expect' intrinsic instructions handled");
     36 
     37 // These default values are chosen to represent an extremely skewed outcome for
     38 // a condition, but they leave some room for interpretation by later passes.
     39 //
     40 // If the documentation for __builtin_expect() was made explicit that it should
     41 // only be used in extreme cases, we could make this ratio higher. As it stands,
     42 // programmers may be using __builtin_expect() / llvm.expect to annotate that a
     43 // branch is likely or unlikely to be taken.
     44 //
     45 // There is a known dependency on this ratio in CodeGenPrepare when transforming
     46 // 'select' instructions. It may be worthwhile to hoist these values to some
     47 // shared space, so they can be used directly by other passes.
     48 
     49 static cl::opt<uint32_t> LikelyBranchWeight(
     50     "likely-branch-weight", cl::Hidden, cl::init(2000),
     51     cl::desc("Weight of the branch likely to be taken (default = 2000)"));
     52 static cl::opt<uint32_t> UnlikelyBranchWeight(
     53     "unlikely-branch-weight", cl::Hidden, cl::init(1),
     54     cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
     55 
     56 static bool handleSwitchExpect(SwitchInst &SI) {
     57   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
     58   if (!CI)
     59     return false;
     60 
     61   Function *Fn = CI->getCalledFunction();
     62   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
     63     return false;
     64 
     65   Value *ArgValue = CI->getArgOperand(0);
     66   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
     67   if (!ExpectedValue)
     68     return false;
     69 
     70   SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
     71   unsigned n = SI.getNumCases(); // +1 for default case.
     72   SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
     73 
     74   if (Case == SI.case_default())
     75     Weights[0] = LikelyBranchWeight;
     76   else
     77     Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
     78 
     79   SI.setMetadata(LLVMContext::MD_prof,
     80                  MDBuilder(CI->getContext()).createBranchWeights(Weights));
     81 
     82   SI.setCondition(ArgValue);
     83   return true;
     84 }
     85 
     86 static bool handleBranchExpect(BranchInst &BI) {
     87   if (BI.isUnconditional())
     88     return false;
     89 
     90   // Handle non-optimized IR code like:
     91   //   %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
     92   //   %tobool = icmp ne i64 %expval, 0
     93   //   br i1 %tobool, label %if.then, label %if.end
     94   //
     95   // Or the following simpler case:
     96   //   %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
     97   //   br i1 %expval, label %if.then, label %if.end
     98 
     99   CallInst *CI;
    100 
    101   ICmpInst *CmpI = dyn_cast<ICmpInst>(BI.getCondition());
    102   if (!CmpI) {
    103     CI = dyn_cast<CallInst>(BI.getCondition());
    104   } else {
    105     if (CmpI->getPredicate() != CmpInst::ICMP_NE)
    106       return false;
    107     CI = dyn_cast<CallInst>(CmpI->getOperand(0));
    108   }
    109 
    110   if (!CI)
    111     return false;
    112 
    113   Function *Fn = CI->getCalledFunction();
    114   if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
    115     return false;
    116 
    117   Value *ArgValue = CI->getArgOperand(0);
    118   ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
    119   if (!ExpectedValue)
    120     return false;
    121 
    122   MDBuilder MDB(CI->getContext());
    123   MDNode *Node;
    124 
    125   // If expect value is equal to 1 it means that we are more likely to take
    126   // branch 0, in other case more likely is branch 1.
    127   if (ExpectedValue->isOne())
    128     Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
    129   else
    130     Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
    131 
    132   BI.setMetadata(LLVMContext::MD_prof, Node);
    133 
    134   if (CmpI)
    135     CmpI->setOperand(0, ArgValue);
    136   else
    137     BI.setCondition(ArgValue);
    138   return true;
    139 }
    140 
    141 static bool lowerExpectIntrinsic(Function &F) {
    142   bool Changed = false;
    143 
    144   for (BasicBlock &BB : F) {
    145     // Create "block_weights" metadata.
    146     if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
    147       if (handleBranchExpect(*BI))
    148         ExpectIntrinsicsHandled++;
    149     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
    150       if (handleSwitchExpect(*SI))
    151         ExpectIntrinsicsHandled++;
    152     }
    153 
    154     // Remove llvm.expect intrinsics.
    155     for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); 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         Changed = true;
    166       }
    167     }
    168   }
    169 
    170   return Changed;
    171 }
    172 
    173 PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
    174                                                 FunctionAnalysisManager &) {
    175   if (lowerExpectIntrinsic(F))
    176     return PreservedAnalyses::none();
    177 
    178   return PreservedAnalyses::all();
    179 }
    180 
    181 namespace {
    182 /// \brief Legacy pass for lowering expect intrinsics out of the IR.
    183 ///
    184 /// When this pass is run over a function it uses expect intrinsics which feed
    185 /// branches and switches to provide branch weight metadata for those
    186 /// terminators. It then removes the expect intrinsics from the IR so the rest
    187 /// of the optimizer can ignore them.
    188 class LowerExpectIntrinsic : public FunctionPass {
    189 public:
    190   static char ID;
    191   LowerExpectIntrinsic() : FunctionPass(ID) {
    192     initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
    193   }
    194 
    195   bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
    196 };
    197 }
    198 
    199 char LowerExpectIntrinsic::ID = 0;
    200 INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
    201                 "Lower 'expect' Intrinsics", false, false)
    202 
    203 FunctionPass *llvm::createLowerExpectIntrinsicPass() {
    204   return new LowerExpectIntrinsic();
    205 }
    206