llvm/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp

//===- LowerExpectIntrinsic.cpp - Lower expect intrinsic ------------------===//
//
// 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 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/ProfDataUtils.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Utils/MisExpect.h"

#include <cmath>

usingnamespacellvm;

#define DEBUG_TYPE …

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.

// WARNING: these values are internal implementation detail of the pass.
// They should not be exposed to the outside of the pass, front-end codegen
// should emit @llvm.expect intrinsics instead of using these weights directly.
// Transforms should use TargetTransformInfo's getPredictableBranchThreshold().
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 std::tuple<uint32_t, uint32_t>
getBranchWeight(Intrinsic::ID IntrinsicID, CallInst *CI, int BranchCount) { … }

static bool handleSwitchExpect(SwitchInst &SI) { … }

/// Handler for PHINodes that define the value argument to an
/// @llvm.expect call.
///
/// If the operand of the phi has a constant value and it 'contradicts'
/// with the expected value of phi def, then the corresponding incoming
/// edge of the phi is unlikely to be taken. Using that information,
/// the branch probability info for the originating branch can be inferred.
static void handlePhiDef(CallInst *Expect) { … }

// Handle both BranchInst and SelectInst.
template <class BrSelInst> static bool handleBrSelExpect(BrSelInst &BSI) { … }

static bool handleBranchExpect(BranchInst &BI) { … }

static bool lowerExpectIntrinsic(Function &F) { … }

PreservedAnalyses LowerExpectIntrinsicPass::run(Function &F,
                                                FunctionAnalysisManager &) { … }