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//===- HotColdSplitting.cpp -- Outline Cold Regions -------------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// Outline cold regions to a separate function.
// TODO: Update BFI and BPI
// TODO: Add all the outlined functions to a separate section.
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/HotColdSplitting.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/CodeExtractor.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#define DEBUG_TYPE "hotcoldsplit"
STATISTIC(NumColdRegionsFound, "Number of cold regions found.");
STATISTIC(NumColdRegionsOutlined, "Number of cold regions outlined.");
using namespace llvm;
static cl::opt<bool> EnableStaticAnalyis("hot-cold-static-analysis",
cl::init(true), cl::Hidden);
static cl::opt<int>
MinOutliningThreshold("min-outlining-thresh", cl::init(3), cl::Hidden,
cl::desc("Code size threshold for outlining within a "
"single BB (as a multiple of TCC_Basic)"));
namespace {
struct PostDomTree : PostDomTreeBase<BasicBlock> {
PostDomTree(Function &F) { recalculate(F); }
/// A sequence of basic blocks.
/// A 0-sized SmallVector is slightly cheaper to move than a std::vector.
using BlockSequence = SmallVector<BasicBlock *, 0>;
// Same as blockEndsInUnreachable in CodeGen/BranchFolding.cpp. Do not modify
// this function unless you modify the MBB version as well.
/// A no successor, non-return block probably ends in unreachable and is cold.
/// Also consider a block that ends in an indirect branch to be a return block,
/// since many targets use plain indirect branches to return.
bool blockEndsInUnreachable(const BasicBlock &BB) {
if (!succ_empty(&BB))
return false;
if (BB.empty())
return true;
const Instruction *I = BB.getTerminator();
return !(isa<ReturnInst>(I) || isa<IndirectBrInst>(I));
bool unlikelyExecuted(BasicBlock &BB) {
// Exception handling blocks are unlikely executed.
if (BB.isEHPad())
return true;
// The block is cold if it calls/invokes a cold function.
for (Instruction &I : BB)
if (auto CS = CallSite(&I))
if (CS.hasFnAttr(Attribute::Cold))
return true;
// The block is cold if it has an unreachable terminator, unless it's
// preceded by a call to a (possibly warm) noreturn call (e.g. longjmp).
if (blockEndsInUnreachable(BB)) {
if (auto *CI =
if (CI->hasFnAttr(Attribute::NoReturn))
return false;
return true;
return false;
/// Check whether it's safe to outline \p BB.
static bool mayExtractBlock(const BasicBlock &BB) {
return !BB.hasAddressTaken() && !BB.isEHPad();
/// Check whether \p Region is profitable to outline.
static bool isProfitableToOutline(const BlockSequence &Region,
TargetTransformInfo &TTI) {
if (Region.size() > 1)
return true;
int Cost = 0;
const BasicBlock &BB = *Region[0];
for (const Instruction &I : BB) {
if (isa<DbgInfoIntrinsic>(&I) || &I == BB.getTerminator())
Cost += TTI.getInstructionCost(&I, TargetTransformInfo::TCK_CodeSize);
if (Cost >= (MinOutliningThreshold * TargetTransformInfo::TCC_Basic))
return true;
return false;
/// Mark \p F cold. Return true if it's changed.
static bool markEntireFunctionCold(Function &F) {
assert(!F.hasFnAttribute(Attribute::OptimizeNone) && "Can't mark this cold");
bool Changed = false;
if (!F.hasFnAttribute(Attribute::MinSize)) {
Changed = true;
// TODO: Move this function into a cold section.
return Changed;
class HotColdSplitting {
HotColdSplitting(ProfileSummaryInfo *ProfSI,
function_ref<BlockFrequencyInfo *(Function &)> GBFI,
function_ref<TargetTransformInfo &(Function &)> GTTI,
std::function<OptimizationRemarkEmitter &(Function &)> *GORE)
bool run(Module &M);
bool shouldOutlineFrom(const Function &F) const;
bool outlineColdRegions(Function &F, ProfileSummaryInfo &PSI,
BlockFrequencyInfo *BFI, TargetTransformInfo &TTI,
DominatorTree &DT, PostDomTree &PDT,
OptimizationRemarkEmitter &ORE);
Function *extractColdRegion(const BlockSequence &Region, DominatorTree &DT,
BlockFrequencyInfo *BFI, TargetTransformInfo &TTI,
OptimizationRemarkEmitter &ORE, unsigned Count);
SmallPtrSet<const Function *, 2> OutlinedFunctions;
ProfileSummaryInfo *PSI;
function_ref<BlockFrequencyInfo *(Function &)> GetBFI;
function_ref<TargetTransformInfo &(Function &)> GetTTI;
std::function<OptimizationRemarkEmitter &(Function &)> *GetORE;
class HotColdSplittingLegacyPass : public ModulePass {
static char ID;
HotColdSplittingLegacyPass() : ModulePass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnModule(Module &M) override;
} // end anonymous namespace
// Returns false if the function should not be considered for hot-cold split
// optimization.
bool HotColdSplitting::shouldOutlineFrom(const Function &F) const {
// Do not try to outline again from an already outlined cold function.
if (OutlinedFunctions.count(&F))
return false;
if (F.size() <= 2)
return false;
// TODO: Consider only skipping functions marked `optnone` or `cold`.
if (F.hasAddressTaken())
return false;
if (F.hasFnAttribute(Attribute::AlwaysInline))
return false;
if (F.hasFnAttribute(Attribute::NoInline))
return false;
if (F.getCallingConv() == CallingConv::Cold)
return false;
if (PSI->isFunctionEntryCold(&F))
return false;
return true;
Function *HotColdSplitting::extractColdRegion(const BlockSequence &Region,
DominatorTree &DT,
BlockFrequencyInfo *BFI,
TargetTransformInfo &TTI,
OptimizationRemarkEmitter &ORE,
unsigned Count) {
// TODO: Pass BFI and BPI to update profile information.
CodeExtractor CE(Region, &DT, /* AggregateArgs */ false, /* BFI */ nullptr,
/* BPI */ nullptr, /* AllowVarArgs */ false,
/* AllowAlloca */ false,
/* Suffix */ "cold." + std::to_string(Count));
SetVector<Value *> Inputs, Outputs, Sinks;
CE.findInputsOutputs(Inputs, Outputs, Sinks);
// Do not extract regions that have live exit variables.
if (Outputs.size() > 0) {
LLVM_DEBUG(llvm::dbgs() << "Not outlining; live outputs\n");
return nullptr;
// TODO: Run MergeBasicBlockIntoOnlyPred on the outlined function.
Function *OrigF = Region[0]->getParent();
if (Function *OutF = CE.extractCodeRegion()) {
User *U = *OutF->user_begin();
CallInst *CI = cast<CallInst>(U);
CallSite CS(CI);
if (TTI.useColdCCForColdCall(*OutF)) {
// Try to make the outlined code as small as possible on the assumption
// that it's cold.
LLVM_DEBUG(llvm::dbgs() << "Outlined Region: " << *OutF);
ORE.emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "HotColdSplit",
<< ore::NV("Original", OrigF) << " split cold code into "
<< ore::NV("Split", OutF);
return OutF;
ORE.emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "ExtractFailed",
<< "Failed to extract region at block "
<< ore::NV("Block", Region.front());
return nullptr;
/// A pair of (basic block, score).
using BlockTy = std::pair<BasicBlock *, unsigned>;
/// A maximal outlining region. This contains all blocks post-dominated by a
/// sink block, the sink block itself, and all blocks dominated by the sink.
class OutliningRegion {
/// A list of (block, score) pairs. A block's score is non-zero iff it's a
/// viable sub-region entry point. Blocks with higher scores are better entry
/// points (i.e. they are more distant ancestors of the sink block).
SmallVector<BlockTy, 0> Blocks = {};
/// The suggested entry point into the region. If the region has multiple
/// entry points, all blocks within the region may not be reachable from this
/// entry point.
BasicBlock *SuggestedEntryPoint = nullptr;
/// Whether the entire function is cold.
bool EntireFunctionCold = false;
/// Whether or not \p BB could be the entry point of an extracted region.
static bool isViableEntryPoint(BasicBlock &BB) { return !BB.isEHPad(); }
/// If \p BB is a viable entry point, return \p Score. Return 0 otherwise.
static unsigned getEntryPointScore(BasicBlock &BB, unsigned Score) {
return isViableEntryPoint(BB) ? Score : 0;
/// These scores should be lower than the score for predecessor blocks,
/// because regions starting at predecessor blocks are typically larger.
static constexpr unsigned ScoreForSuccBlock = 1;
static constexpr unsigned ScoreForSinkBlock = 1;
OutliningRegion(const OutliningRegion &) = delete;
OutliningRegion &operator=(const OutliningRegion &) = delete;
OutliningRegion() = default;
OutliningRegion(OutliningRegion &&) = default;
OutliningRegion &operator=(OutliningRegion &&) = default;
static OutliningRegion create(BasicBlock &SinkBB, const DominatorTree &DT,
const PostDomTree &PDT) {
OutliningRegion ColdRegion;
SmallPtrSet<BasicBlock *, 4> RegionBlocks;
auto addBlockToRegion = [&](BasicBlock *BB, unsigned Score) {
ColdRegion.Blocks.emplace_back(BB, Score);
assert(RegionBlocks.size() == ColdRegion.Blocks.size() && "Duplicate BB");
// The ancestor farthest-away from SinkBB, and also post-dominated by it.
unsigned SinkScore = getEntryPointScore(SinkBB, ScoreForSinkBlock);
ColdRegion.SuggestedEntryPoint = (SinkScore > 0) ? &SinkBB : nullptr;
unsigned BestScore = SinkScore;
// Visit SinkBB's ancestors using inverse DFS.
auto PredIt = ++idf_begin(&SinkBB);
auto PredEnd = idf_end(&SinkBB);
while (PredIt != PredEnd) {
BasicBlock &PredBB = **PredIt;
bool SinkPostDom = PDT.dominates(&SinkBB, &PredBB);
// If the predecessor is cold and has no predecessors, the entire
// function must be cold.
if (SinkPostDom && pred_empty(&PredBB)) {
ColdRegion.EntireFunctionCold = true;
return ColdRegion;
// If SinkBB does not post-dominate a predecessor, do not mark the
// predecessor (or any of its predecessors) cold.
if (!SinkPostDom || !mayExtractBlock(PredBB)) {
// Keep track of the post-dominated ancestor farthest away from the sink.
// The path length is always >= 2, ensuring that predecessor blocks are
// considered as entry points before the sink block.
unsigned PredScore = getEntryPointScore(PredBB, PredIt.getPathLength());
if (PredScore > BestScore) {
ColdRegion.SuggestedEntryPoint = &PredBB;
BestScore = PredScore;
addBlockToRegion(&PredBB, PredScore);
// Add SinkBB to the cold region. It's considered as an entry point before
// any sink-successor blocks.
addBlockToRegion(&SinkBB, SinkScore);
// Find all successors of SinkBB dominated by SinkBB using DFS.
auto SuccIt = ++df_begin(&SinkBB);
auto SuccEnd = df_end(&SinkBB);
while (SuccIt != SuccEnd) {
BasicBlock &SuccBB = **SuccIt;
bool SinkDom = DT.dominates(&SinkBB, &SuccBB);
// Don't allow the backwards & forwards DFSes to mark the same block.
bool DuplicateBlock = RegionBlocks.count(&SuccBB);
// If SinkBB does not dominate a successor, do not mark the successor (or
// any of its successors) cold.
if (DuplicateBlock || !SinkDom || !mayExtractBlock(SuccBB)) {
unsigned SuccScore = getEntryPointScore(SuccBB, ScoreForSuccBlock);
if (SuccScore > BestScore) {
ColdRegion.SuggestedEntryPoint = &SuccBB;
BestScore = SuccScore;
addBlockToRegion(&SuccBB, SuccScore);
return ColdRegion;
/// Whether this region has nothing to extract.
bool empty() const { return !SuggestedEntryPoint; }
/// The blocks in this region.
ArrayRef<std::pair<BasicBlock *, unsigned>> blocks() const { return Blocks; }
/// Whether the entire function containing this region is cold.
bool isEntireFunctionCold() const { return EntireFunctionCold; }
/// Remove a sub-region from this region and return it as a block sequence.
BlockSequence takeSingleEntrySubRegion(DominatorTree &DT) {
assert(!empty() && !isEntireFunctionCold() && "Nothing to extract");
// Remove blocks dominated by the suggested entry point from this region.
// During the removal, identify the next best entry point into the region.
// Ensure that the first extracted block is the suggested entry point.
BlockSequence SubRegion = {SuggestedEntryPoint};
BasicBlock *NextEntryPoint = nullptr;
unsigned NextScore = 0;
auto RegionEndIt = Blocks.end();
auto RegionStartIt = remove_if(Blocks, [&](const BlockTy &Block) {
BasicBlock *BB = Block.first;
unsigned Score = Block.second;
bool InSubRegion =
BB == SuggestedEntryPoint || DT.dominates(SuggestedEntryPoint, BB);
if (!InSubRegion && Score > NextScore) {
NextEntryPoint = BB;
NextScore = Score;
if (InSubRegion && BB != SuggestedEntryPoint)
return InSubRegion;
Blocks.erase(RegionStartIt, RegionEndIt);
// Update the suggested entry point.
SuggestedEntryPoint = NextEntryPoint;
return SubRegion;
bool HotColdSplitting::outlineColdRegions(Function &F, ProfileSummaryInfo &PSI,
BlockFrequencyInfo *BFI,
TargetTransformInfo &TTI,
DominatorTree &DT, PostDomTree &PDT,
OptimizationRemarkEmitter &ORE) {
bool Changed = false;
// The set of cold blocks.
SmallPtrSet<BasicBlock *, 4> ColdBlocks;
// The worklist of non-intersecting regions left to outline.
SmallVector<OutliningRegion, 2> OutliningWorklist;
// Set up an RPO traversal. Experimentally, this performs better (outlines
// more) than a PO traversal, because we prevent region overlap by keeping
// the first region to contain a block.
ReversePostOrderTraversal<Function *> RPOT(&F);
// Find all cold regions.
for (BasicBlock *BB : RPOT) {
// Skip blocks which can't be outlined.
if (!mayExtractBlock(*BB))
// This block is already part of some outlining region.
if (ColdBlocks.count(BB))
bool Cold = PSI.isColdBlock(BB, BFI) ||
(EnableStaticAnalyis && unlikelyExecuted(*BB));
if (!Cold)
dbgs() << "Found a cold block:\n";
auto Region = OutliningRegion::create(*BB, DT, PDT);
if (Region.empty())
if (Region.isEntireFunctionCold()) {
LLVM_DEBUG(dbgs() << "Entire function is cold\n");
return markEntireFunctionCold(F);
// If this outlining region intersects with another, drop the new region.
// TODO: It's theoretically possible to outline more by only keeping the
// largest region which contains a block, but the extra bookkeeping to do
// this is tricky/expensive.
bool RegionsOverlap = any_of(Region.blocks(), [&](const BlockTy &Block) {
return !ColdBlocks.insert(Block.first).second;
if (RegionsOverlap)
// Outline single-entry cold regions, splitting up larger regions as needed.
unsigned OutlinedFunctionID = 1;
while (!OutliningWorklist.empty()) {
OutliningRegion Region = OutliningWorklist.pop_back_val();
assert(!Region.empty() && "Empty outlining region in worklist");
do {
BlockSequence SubRegion = Region.takeSingleEntrySubRegion(DT);
if (!isProfitableToOutline(SubRegion, TTI)) {
dbgs() << "Skipping outlining; not profitable to outline\n";
dbgs() << "Hot/cold splitting attempting to outline these blocks:\n";
for (BasicBlock *BB : SubRegion)
Function *Outlined =
extractColdRegion(SubRegion, DT, BFI, TTI, ORE, OutlinedFunctionID);
if (Outlined) {
Changed = true;
} while (!Region.empty());
return Changed;
bool HotColdSplitting::run(Module &M) {
bool Changed = false;
for (auto &F : M) {
if (!shouldOutlineFrom(F)) {
LLVM_DEBUG(llvm::dbgs() << "Skipping " << F.getName() << "\n");
LLVM_DEBUG(llvm::dbgs() << "Outlining in " << F.getName() << "\n");
DominatorTree DT(F);
PostDomTree PDT(F);
BlockFrequencyInfo *BFI = GetBFI(F);
TargetTransformInfo &TTI = GetTTI(F);
OptimizationRemarkEmitter &ORE = (*GetORE)(F);
Changed |= outlineColdRegions(F, *PSI, BFI, TTI, DT, PDT, ORE);
return Changed;
bool HotColdSplittingLegacyPass::runOnModule(Module &M) {
if (skipModule(M))
return false;
ProfileSummaryInfo *PSI =
auto GTTI = [this](Function &F) -> TargetTransformInfo & {
return this->getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
auto GBFI = [this](Function &F) {
return &this->getAnalysis<BlockFrequencyInfoWrapperPass>(F).getBFI();
std::unique_ptr<OptimizationRemarkEmitter> ORE;
std::function<OptimizationRemarkEmitter &(Function &)> GetORE =
[&ORE](Function &F) -> OptimizationRemarkEmitter & {
ORE.reset(new OptimizationRemarkEmitter(&F));
return *ORE.get();
return HotColdSplitting(PSI, GBFI, GTTI, &GetORE).run(M);
HotColdSplittingPass::run(Module &M, ModuleAnalysisManager &AM) {
auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
std::function<AssumptionCache &(Function &)> GetAssumptionCache =
[&FAM](Function &F) -> AssumptionCache & {
return FAM.getResult<AssumptionAnalysis>(F);
auto GBFI = [&FAM](Function &F) {
return &FAM.getResult<BlockFrequencyAnalysis>(F);
std::function<TargetTransformInfo &(Function &)> GTTI =
[&FAM](Function &F) -> TargetTransformInfo & {
return FAM.getResult<TargetIRAnalysis>(F);
std::unique_ptr<OptimizationRemarkEmitter> ORE;
std::function<OptimizationRemarkEmitter &(Function &)> GetORE =
[&ORE](Function &F) -> OptimizationRemarkEmitter & {
ORE.reset(new OptimizationRemarkEmitter(&F));
return *ORE.get();
ProfileSummaryInfo *PSI = &AM.getResult<ProfileSummaryAnalysis>(M);
if (HotColdSplitting(PSI, GBFI, GTTI, &GetORE).run(M))
return PreservedAnalyses::none();
return PreservedAnalyses::all();
char HotColdSplittingLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(HotColdSplittingLegacyPass, "hotcoldsplit",
"Hot Cold Splitting", false, false)
INITIALIZE_PASS_END(HotColdSplittingLegacyPass, "hotcoldsplit",
"Hot Cold Splitting", false, false)
ModulePass *llvm::createHotColdSplittingPass() {
return new HotColdSplittingLegacyPass();