llvm/llvm/lib/Transforms/Utils/MisExpect.cpp

//===--- 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's implementation relies on two assumptions about how branch weights
// are managed in LLVM.
//
// 1) Frontend profiling weights are always in place before llvm.expect is
// lowered in LowerExpectIntrinsic.cpp. Frontend based instrumentation therefore
// needs to extract the branch weights and then compare them to the weights
// being added by the llvm.expect intrinsic lowering.
//
// 2) Sampling and IR based profiles will *only* have branch weight metadata
// before profiling data is consulted if they are from a lowered llvm.expect
// intrinsic. These profiles thus always extract the expected weights and then
// compare them to the weights collected during profiling to determine if a
// diagnostic message is warranted.
//
//===----------------------------------------------------------------------===//

#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/IR/ProfDataUtils.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormatVariadic.h"
#include <algorithm>
#include <cstdint>
#include <functional>
#include <numeric>

#define DEBUG_TYPE …

usingnamespacellvm;
usingnamespacemisexpect;

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."));

// Command line option for setting the diagnostic tolerance threshold
static cl::opt<uint32_t> MisExpectTolerance(
    "misexpect-tolerance", cl::init(0),
    cl::desc("Prevents emitting diagnostics when profile counts are "
             "within N% of the threshold.."));

} // namespace llvm

namespace {

bool isMisExpectDiagEnabled(LLVMContext &Ctx) { … }

uint32_t getMisExpectTolerance(LLVMContext &Ctx) { … }

Instruction *getInstCondition(Instruction *I) { … }

void emitMisexpectDiagnostic(Instruction *I, LLVMContext &Ctx,
                             uint64_t ProfCount, uint64_t TotalCount) { … }

} // namespace

namespace llvm {
namespace misexpect {

void verifyMisExpect(Instruction &I, ArrayRef<uint32_t> RealWeights,
                     ArrayRef<uint32_t> ExpectedWeights) { … }

void checkBackendInstrumentation(Instruction &I,
                                 const ArrayRef<uint32_t> RealWeights) { … }

void checkFrontendInstrumentation(Instruction &I,
                                  const ArrayRef<uint32_t> ExpectedWeights) { … }

void checkExpectAnnotations(Instruction &I,
                            const ArrayRef<uint32_t> ExistingWeights,
                            bool IsFrontend) { … }

} // namespace misexpect
} // namespace llvm
#undef DEBUG_TYPE