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//===- BranchProbabilityInfo.h - Branch Probability Analysis ----*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass is used to evaluate branch probabilties.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
namespace llvm {
class Function;
class LoopInfo;
class raw_ostream;
class TargetLibraryInfo;
class Value;
/// Analysis providing branch probability information.
/// This is a function analysis which provides information on the relative
/// probabilities of each "edge" in the function's CFG where such an edge is
/// defined by a pair (PredBlock and an index in the successors). The
/// probability of an edge from one block is always relative to the
/// probabilities of other edges from the block. The probabilites of all edges
/// from a block sum to exactly one (100%).
/// We use a pair (PredBlock and an index in the successors) to uniquely
/// identify an edge, since we can have multiple edges from Src to Dst.
/// As an example, we can have a switch which jumps to Dst with value 0 and
/// value 10.
class BranchProbabilityInfo {
BranchProbabilityInfo() = default;
BranchProbabilityInfo(const Function &F, const LoopInfo &LI,
const TargetLibraryInfo *TLI = nullptr) {
calculate(F, LI, TLI);
BranchProbabilityInfo(BranchProbabilityInfo &&Arg)
: Probs(std::move(Arg.Probs)), LastF(Arg.LastF),
PostDominatedByColdCall(std::move(Arg.PostDominatedByColdCall)) {}
BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;
BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;
BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {
Probs = std::move(RHS.Probs);
PostDominatedByColdCall = std::move(RHS.PostDominatedByColdCall);
PostDominatedByUnreachable = std::move(RHS.PostDominatedByUnreachable);
return *this;
void releaseMemory();
void print(raw_ostream &OS) const;
/// Get an edge's probability, relative to other out-edges of the Src.
/// This routine provides access to the fractional probability between zero
/// (0%) and one (100%) of this edge executing, relative to other edges
/// leaving the 'Src' block. The returned probability is never zero, and can
/// only be one if the source block has only one successor.
BranchProbability getEdgeProbability(const BasicBlock *Src,
unsigned IndexInSuccessors) const;
/// Get the probability of going from Src to Dst.
/// It returns the sum of all probabilities for edges from Src to Dst.
BranchProbability getEdgeProbability(const BasicBlock *Src,
const BasicBlock *Dst) const;
BranchProbability getEdgeProbability(const BasicBlock *Src,
succ_const_iterator Dst) const;
/// Test if an edge is hot relative to other out-edges of the Src.
/// Check whether this edge out of the source block is 'hot'. We define hot
/// as having a relative probability >= 80%.
bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
/// Retrieve the hot successor of a block if one exists.
/// Given a basic block, look through its successors and if one exists for
/// which \see isEdgeHot would return true, return that successor block.
const BasicBlock *getHotSucc(const BasicBlock *BB) const;
/// Print an edge's probability.
/// Retrieves an edge's probability similarly to \see getEdgeProbability, but
/// then prints that probability to the provided stream. That stream is then
/// returned.
raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
const BasicBlock *Dst) const;
/// Set the raw edge probability for the given edge.
/// This allows a pass to explicitly set the edge probability for an edge. It
/// can be used when updating the CFG to update and preserve the branch
/// probability information. Read the implementation of how these edge
/// probabilities are calculated carefully before using!
void setEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors,
BranchProbability Prob);
static BranchProbability getBranchProbStackProtector(bool IsLikely) {
static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);
return IsLikely ? LikelyProb : LikelyProb.getCompl();
void calculate(const Function &F, const LoopInfo &LI,
const TargetLibraryInfo *TLI = nullptr);
/// Forget analysis results for the given basic block.
void eraseBlock(const BasicBlock *BB);
// Use to track SCCs for handling irreducible loops.
using SccMap = DenseMap<const BasicBlock *, int>;
using SccHeaderMap = DenseMap<const BasicBlock *, bool>;
using SccHeaderMaps = std::vector<SccHeaderMap>;
struct SccInfo {
SccMap SccNums;
SccHeaderMaps SccHeaders;
// We need to store CallbackVH's in order to correctly handle basic block
// removal.
class BasicBlockCallbackVH final : public CallbackVH {
BranchProbabilityInfo *BPI;
void deleted() override {
assert(BPI != nullptr);
BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)
: CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}
DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;
// Since we allow duplicate edges from one basic block to another, we use
// a pair (PredBlock and an index in the successors) to specify an edge.
using Edge = std::pair<const BasicBlock *, unsigned>;
// Default weight value. Used when we don't have information about the edge.
// TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
// the successors have a weight yet. But it doesn't make sense when providing
// weight to an edge that may have siblings with non-zero weights. This can
// be handled various ways, but it's probably fine for an edge with unknown
// weight to just "inherit" the non-zero weight of an adjacent successor.
static const uint32_t DEFAULT_WEIGHT = 16;
DenseMap<Edge, BranchProbability> Probs;
/// Track the last function we run over for printing.
const Function *LastF;
/// Track the set of blocks directly succeeded by a returning block.
SmallPtrSet<const BasicBlock *, 16> PostDominatedByUnreachable;
/// Track the set of blocks that always lead to a cold call.
SmallPtrSet<const BasicBlock *, 16> PostDominatedByColdCall;
void updatePostDominatedByUnreachable(const BasicBlock *BB);
void updatePostDominatedByColdCall(const BasicBlock *BB);
bool calcUnreachableHeuristics(const BasicBlock *BB);
bool calcMetadataWeights(const BasicBlock *BB);
bool calcColdCallHeuristics(const BasicBlock *BB);
bool calcPointerHeuristics(const BasicBlock *BB);
bool calcLoopBranchHeuristics(const BasicBlock *BB, const LoopInfo &LI,
SccInfo &SccI);
bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);
bool calcFloatingPointHeuristics(const BasicBlock *BB);
bool calcInvokeHeuristics(const BasicBlock *BB);
/// Analysis pass which computes \c BranchProbabilityInfo.
class BranchProbabilityAnalysis
: public AnalysisInfoMixin<BranchProbabilityAnalysis> {
friend AnalysisInfoMixin<BranchProbabilityAnalysis>;
static AnalysisKey Key;
/// Provide the result type for this analysis pass.
using Result = BranchProbabilityInfo;
/// Run the analysis pass over a function and produce BPI.
BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);
/// Printer pass for the \c BranchProbabilityAnalysis results.
class BranchProbabilityPrinterPass
: public PassInfoMixin<BranchProbabilityPrinterPass> {
raw_ostream &OS;
explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
/// Legacy analysis pass which computes \c BranchProbabilityInfo.
class BranchProbabilityInfoWrapperPass : public FunctionPass {
BranchProbabilityInfo BPI;
static char ID;
BranchProbabilityInfoWrapperPass() : FunctionPass(ID) {
BranchProbabilityInfo &getBPI() { return BPI; }
const BranchProbabilityInfo &getBPI() const { return BPI; }
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnFunction(Function &F) override;
void releaseMemory() override;
void print(raw_ostream &OS, const Module *M = nullptr) const override;
} // end namespace llvm