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//===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass lowers the 'expect' intrinsic to LLVM metadata.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
#define DEBUG_TYPE "lower-expect-intrinsic"
STATISTIC(ExpectIntrinsicsHandled,
"Number of 'expect' intrinsic instructions handled");
// These default values are chosen to represent an extremely skewed outcome for
// a condition, but they leave some room for interpretation by later passes.
//
// If the documentation for __builtin_expect() was made explicit that it should
// only be used in extreme cases, we could make this ratio higher. As it stands,
// programmers may be using __builtin_expect() / llvm.expect to annotate that a
// branch is likely or unlikely to be taken.
//
// There is a known dependency on this ratio in CodeGenPrepare when transforming
// 'select' instructions. It may be worthwhile to hoist these values to some
// shared space, so they can be used directly by other passes.
static cl::opt<uint32_t> LikelyBranchWeight(
"likely-branch-weight", cl::Hidden, cl::init(2000),
cl::desc("Weight of the branch likely to be taken (default = 2000)"));
static cl::opt<uint32_t> UnlikelyBranchWeight(
"unlikely-branch-weight", cl::Hidden, cl::init(1),
cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
static bool handleSwitchExpect(SwitchInst &SI) {
CallInst *CI = dyn_cast<CallInst>(SI.getCondition());
if (!CI)
return false;
Function *Fn = CI->getCalledFunction();
if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
return false;
Value *ArgValue = CI->getArgOperand(0);
ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
if (!ExpectedValue)
return false;
SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
unsigned n = SI.getNumCases(); // +1 for default case.
SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
if (Case == SI.case_default())
Weights[0] = LikelyBranchWeight;
else
Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
SI.setMetadata(LLVMContext::MD_prof,
MDBuilder(CI->getContext()).createBranchWeights(Weights));
SI.setCondition(ArgValue);
return true;
}
// Handle both BranchInst and SelectInst.
template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) {
// Handle non-optimized IR code like:
// %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
// %tobool = icmp ne i64 %expval, 0
// br i1 %tobool, label %if.then, label %if.end
//
// Or the following simpler case:
// %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
// br i1 %expval, label %if.then, label %if.end
CallInst *CI;
ICmpInst *CmpI = dyn_cast<ICmpInst>(BSI.getCondition());
if (!CmpI) {
CI = dyn_cast<CallInst>(BSI.getCondition());
} else {
if (CmpI->getPredicate() != CmpInst::ICMP_NE)
return false;
CI = dyn_cast<CallInst>(CmpI->getOperand(0));
}
if (!CI)
return false;
Function *Fn = CI->getCalledFunction();
if (!Fn || Fn->getIntrinsicID() != Intrinsic::expect)
return false;
Value *ArgValue = CI->getArgOperand(0);
ConstantInt *ExpectedValue = dyn_cast<ConstantInt>(CI->getArgOperand(1));
if (!ExpectedValue)
return false;
MDBuilder MDB(CI->getContext());
MDNode *Node;
// If expect value is equal to 1 it means that we are more likely to take
// branch 0, in other case more likely is branch 1.
if (ExpectedValue->isOne())
Node = MDB.createBranchWeights(LikelyBranchWeight, UnlikelyBranchWeight);
else
Node = MDB.createBranchWeights(UnlikelyBranchWeight, LikelyBranchWeight);
BSI.setMetadata(LLVMContext::MD_prof, Node);
if (CmpI)
CmpI->setOperand(0, ArgValue);
else
BSI.setCondition(ArgValue);
return true;
}
static bool handleBranchExpect(BranchInst &BI) {
if (BI.isUnconditional())
return false;
return handleBrSelExpect<BranchInst>(BI);
}
static bool lowerExpectIntrinsic(Function &F) {
bool Changed = false;
for (BasicBlock &BB : F) {
// Create "block_weights" metadata.
if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator())) {
if (handleBranchExpect(*BI))
ExpectIntrinsicsHandled++;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator())) {
if (handleSwitchExpect(*SI))
ExpectIntrinsicsHandled++;
}
// Remove llvm.expect intrinsics. Iterate backwards in order
// to process select instructions before the intrinsic gets
// removed.
for (auto BI = BB.rbegin(), BE = BB.rend(); BI != BE;) {
Instruction *Inst = &*BI++;
CallInst *CI = dyn_cast<CallInst>(Inst);
if (!CI) {
if (SelectInst *SI = dyn_cast<SelectInst>(Inst)) {
if (handleBrSelExpect(*SI))
ExpectIntrinsicsHandled++;
}
continue;
}
Function *Fn = CI->getCalledFunction();
if (Fn && Fn->getIntrinsicID() == Intrinsic::expect) {
Value *Exp = CI->getArgOperand(0);
CI->replaceAllUsesWith(Exp);
CI->eraseFromParent();
Changed = true;
}
}
}
return Changed;
}
PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
FunctionAnalysisManager &) {
if (lowerExpectIntrinsic(F))
return PreservedAnalyses::none();
return PreservedAnalyses::all();
}
namespace {
/// \brief Legacy pass for lowering expect intrinsics out of the IR.
///
/// When this pass is run over a function it uses expect intrinsics which feed
/// branches and switches to provide branch weight metadata for those
/// terminators. It then removes the expect intrinsics from the IR so the rest
/// of the optimizer can ignore them.
class LowerExpectIntrinsic : public FunctionPass {
public:
static char ID;
LowerExpectIntrinsic() : FunctionPass(ID) {
initializeLowerExpectIntrinsicPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override { return lowerExpectIntrinsic(F); }
};
}
char LowerExpectIntrinsic::ID = 0;
INITIALIZE_PASS(LowerExpectIntrinsic, "lower-expect",
"Lower 'expect' Intrinsics", false, false)
FunctionPass *llvm::createLowerExpectIntrinsicPass() {
return new LowerExpectIntrinsic();
}
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