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| //===--- MisExpect.cpp - Check the use of llvm.expect with PGO data -------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This contains code to emit warnings for potentially incorrect usage of the
// llvm.expect intrinsic. This utility extracts the threshold values from
// metadata associated with the instrumented Branch or Switch instruction. The
// threshold values are then used to determine if a warning should be emmited.
//
// MisExpect metadata is generated when llvm.expect intrinsics are lowered see
// LowerExpectIntrinsic.cpp
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/MisExpect.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormatVariadic.h"
#include <cstdint>
#include <functional>
#include <numeric>
#define DEBUG_TYPE "misexpect"
using namespace llvm;
using namespace misexpect;
namespace llvm {
// Command line option to enable/disable the warning when profile data suggests
// a mismatch with the use of the llvm.expect intrinsic
static cl::opt<bool> PGOWarnMisExpect(
"pgo-warn-misexpect", cl::init(false), cl::Hidden,
cl::desc("Use this option to turn on/off "
"warnings about incorrect usage of llvm.expect intrinsics."));
} // namespace llvm
namespace {
Instruction *getOprndOrInst(Instruction *I) {
assert(I != nullptr && "MisExpect target Instruction cannot be nullptr");
Instruction *Ret = nullptr;
if (auto *B = dyn_cast<BranchInst>(I)) {
Ret = dyn_cast<Instruction>(B->getCondition());
}
// TODO: Find a way to resolve condition location for switches
// Using the condition of the switch seems to often resolve to an earlier
// point in the program, i.e. the calculation of the switch condition, rather
// than the switches location in the source code. Thus, we should use the
// instruction to get source code locations rather than the condition to
// improve diagnostic output, such as the caret. If the same problem exists
// for branch instructions, then we should remove this function and directly
// use the instruction
//
// else if (auto S = dyn_cast<SwitchInst>(I)) {
// Ret = I;
//}
return Ret ? Ret : I;
}
void emitMisexpectDiagnostic(Instruction *I, LLVMContext &Ctx,
uint64_t ProfCount, uint64_t TotalCount) {
double PercentageCorrect = (double)ProfCount / TotalCount;
auto PerString =
formatv("{0:P} ({1} / {2})", PercentageCorrect, ProfCount, TotalCount);
auto RemStr = formatv(
"Potential performance regression from use of the llvm.expect intrinsic: "
"Annotation was correct on {0} of profiled executions.",
PerString);
Twine Msg(PerString);
Instruction *Cond = getOprndOrInst(I);
if (PGOWarnMisExpect)
Ctx.diagnose(DiagnosticInfoMisExpect(Cond, Msg));
OptimizationRemarkEmitter ORE(I->getParent()->getParent());
ORE.emit(OptimizationRemark(DEBUG_TYPE, "misexpect", Cond) << RemStr.str());
}
} // namespace
namespace llvm {
namespace misexpect {
void verifyMisExpect(Instruction *I, const SmallVector<uint32_t, 4> &Weights,
LLVMContext &Ctx) {
if (auto *MisExpectData = I->getMetadata(LLVMContext::MD_misexpect)) {
auto *MisExpectDataName = dyn_cast<MDString>(MisExpectData->getOperand(0));
if (MisExpectDataName &&
MisExpectDataName->getString().equals("misexpect")) {
LLVM_DEBUG(llvm::dbgs() << "------------------\n");
LLVM_DEBUG(llvm::dbgs()
<< "Function: " << I->getFunction()->getName() << "\n");
LLVM_DEBUG(llvm::dbgs() << "Instruction: " << *I << ":\n");
LLVM_DEBUG(for (int Idx = 0, Size = Weights.size(); Idx < Size; ++Idx) {
llvm::dbgs() << "Weights[" << Idx << "] = " << Weights[Idx] << "\n";
});
// extract values from misexpect metadata
const auto *IndexCint =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(1));
const auto *LikelyCInt =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(2));
const auto *UnlikelyCInt =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(3));
if (!IndexCint || !LikelyCInt || !UnlikelyCInt)
return;
const uint64_t Index = IndexCint->getZExtValue();
const uint64_t LikelyBranchWeight = LikelyCInt->getZExtValue();
const uint64_t UnlikelyBranchWeight = UnlikelyCInt->getZExtValue();
const uint64_t ProfileCount = Weights[Index];
const uint64_t CaseTotal = std::accumulate(
Weights.begin(), Weights.end(), (uint64_t)0, std::plus<uint64_t>());
const uint64_t NumUnlikelyTargets = Weights.size() - 1;
const uint64_t TotalBranchWeight =
LikelyBranchWeight + (UnlikelyBranchWeight * NumUnlikelyTargets);
const llvm::BranchProbability LikelyThreshold(LikelyBranchWeight,
TotalBranchWeight);
uint64_t ScaledThreshold = LikelyThreshold.scale(CaseTotal);
LLVM_DEBUG(llvm::dbgs()
<< "Unlikely Targets: " << NumUnlikelyTargets << ":\n");
LLVM_DEBUG(llvm::dbgs() << "Profile Count: " << ProfileCount << ":\n");
LLVM_DEBUG(llvm::dbgs()
<< "Scaled Threshold: " << ScaledThreshold << ":\n");
LLVM_DEBUG(llvm::dbgs() << "------------------\n");
if (ProfileCount < ScaledThreshold)
emitMisexpectDiagnostic(I, Ctx, ProfileCount, CaseTotal);
}
}
}
void checkFrontendInstrumentation(Instruction &I) {
if (auto *MD = I.getMetadata(LLVMContext::MD_prof)) {
unsigned NOps = MD->getNumOperands();
// Only emit misexpect diagnostics if at least 2 branch weights are present.
// Less than 2 branch weights means that the profiling metadata is:
// 1) incorrect/corrupted
// 2) not branch weight metadata
// 3) completely deterministic
// In these cases we should not emit any diagnostic related to misexpect.
if (NOps < 3)
return;
// Operand 0 is a string tag "branch_weights"
if (MDString *Tag = cast<MDString>(MD->getOperand(0))) {
if (Tag->getString().equals("branch_weights")) {
SmallVector<uint32_t, 4> RealWeights(NOps - 1);
for (unsigned i = 1; i < NOps; i++) {
ConstantInt *Value =
mdconst::dyn_extract<ConstantInt>(MD->getOperand(i));
RealWeights[i - 1] = Value->getZExtValue();
}
verifyMisExpect(&I, RealWeights, I.getContext());
}
}
}
}
} // namespace misexpect
} // namespace llvm
#undef DEBUG_TYPE
|