BranchProbabilityInfo.cpp [plain text]
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "branch-prob"
INITIALIZE_PASS_BEGIN(BranchProbabilityInfoWrapperPass, "branch-prob",
"Branch Probability Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(BranchProbabilityInfoWrapperPass, "branch-prob",
"Branch Probability Analysis", false, true)
char BranchProbabilityInfoWrapperPass::ID = 0;
static const uint32_t LBH_TAKEN_WEIGHT = 124;
static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
static const uint32_t UR_TAKEN_WEIGHT = 1;
static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
static const uint32_t CC_TAKEN_WEIGHT = 4;
static const uint32_t CC_NONTAKEN_WEIGHT = 64;
static const uint32_t PH_TAKEN_WEIGHT = 20;
static const uint32_t PH_NONTAKEN_WEIGHT = 12;
static const uint32_t ZH_TAKEN_WEIGHT = 20;
static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
static const uint32_t FPH_TAKEN_WEIGHT = 20;
static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
static const uint32_t IH_TAKEN_WEIGHT = 1024 * 1024 - 1;
static const uint32_t IH_NONTAKEN_WEIGHT = 1;
bool BranchProbabilityInfo::calcUnreachableHeuristics(const BasicBlock *BB) {
const TerminatorInst *TI = BB->getTerminator();
if (TI->getNumSuccessors() == 0) {
if (isa<UnreachableInst>(TI))
PostDominatedByUnreachable.insert(BB);
return false;
}
SmallVector<unsigned, 4> UnreachableEdges;
SmallVector<unsigned, 4> ReachableEdges;
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
if (PostDominatedByUnreachable.count(*I))
UnreachableEdges.push_back(I.getSuccessorIndex());
else
ReachableEdges.push_back(I.getSuccessorIndex());
}
if (UnreachableEdges.size() == TI->getNumSuccessors())
PostDominatedByUnreachable.insert(BB);
if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
return false;
if (auto *II = dyn_cast<InvokeInst>(TI))
if (PostDominatedByUnreachable.count(II->getNormalDest())) {
PostDominatedByUnreachable.insert(BB);
return false;
}
if (ReachableEdges.empty()) {
BranchProbability Prob(1, UnreachableEdges.size());
for (unsigned SuccIdx : UnreachableEdges)
setEdgeProbability(BB, SuccIdx, Prob);
return true;
}
BranchProbability UnreachableProb(UR_TAKEN_WEIGHT,
(UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
UnreachableEdges.size());
BranchProbability ReachableProb(UR_NONTAKEN_WEIGHT,
(UR_TAKEN_WEIGHT + UR_NONTAKEN_WEIGHT) *
ReachableEdges.size());
for (unsigned SuccIdx : UnreachableEdges)
setEdgeProbability(BB, SuccIdx, UnreachableProb);
for (unsigned SuccIdx : ReachableEdges)
setEdgeProbability(BB, SuccIdx, ReachableProb);
return true;
}
bool BranchProbabilityInfo::calcMetadataWeights(const BasicBlock *BB) {
const TerminatorInst *TI = BB->getTerminator();
if (TI->getNumSuccessors() == 1)
return false;
if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
return false;
MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
if (!WeightsNode)
return false;
assert(TI->getNumSuccessors() < UINT32_MAX && "Too many successors");
if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
return false;
uint64_t WeightSum = 0;
SmallVector<uint32_t, 2> Weights;
Weights.reserve(TI->getNumSuccessors());
for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
ConstantInt *Weight =
mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
if (!Weight)
return false;
assert(Weight->getValue().getActiveBits() <= 32 &&
"Too many bits for uint32_t");
Weights.push_back(Weight->getZExtValue());
WeightSum += Weights.back();
}
assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
uint64_t ScalingFactor =
(WeightSum > UINT32_MAX) ? WeightSum / UINT32_MAX + 1 : 1;
WeightSum = 0;
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
Weights[i] /= ScalingFactor;
WeightSum += Weights[i];
}
if (WeightSum == 0) {
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
setEdgeProbability(BB, i, {1, e});
} else {
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
setEdgeProbability(BB, i, {Weights[i], static_cast<uint32_t>(WeightSum)});
}
assert(WeightSum <= UINT32_MAX &&
"Expected weights to scale down to 32 bits");
return true;
}
bool BranchProbabilityInfo::calcColdCallHeuristics(const BasicBlock *BB) {
const TerminatorInst *TI = BB->getTerminator();
if (TI->getNumSuccessors() == 0)
return false;
SmallVector<unsigned, 4> ColdEdges;
SmallVector<unsigned, 4> NormalEdges;
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
if (PostDominatedByColdCall.count(*I))
ColdEdges.push_back(I.getSuccessorIndex());
else
NormalEdges.push_back(I.getSuccessorIndex());
if (ColdEdges.size() == TI->getNumSuccessors())
PostDominatedByColdCall.insert(BB);
else {
assert(!PostDominatedByColdCall.count(BB));
for (BasicBlock::const_iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (const CallInst *CI = dyn_cast<CallInst>(I))
if (CI->hasFnAttr(Attribute::Cold)) {
PostDominatedByColdCall.insert(BB);
break;
}
}
if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
return false;
if (NormalEdges.empty()) {
BranchProbability Prob(1, ColdEdges.size());
for (unsigned SuccIdx : ColdEdges)
setEdgeProbability(BB, SuccIdx, Prob);
return true;
}
BranchProbability ColdProb(CC_TAKEN_WEIGHT,
(CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
ColdEdges.size());
BranchProbability NormalProb(CC_NONTAKEN_WEIGHT,
(CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) *
NormalEdges.size());
for (unsigned SuccIdx : ColdEdges)
setEdgeProbability(BB, SuccIdx, ColdProb);
for (unsigned SuccIdx : NormalEdges)
setEdgeProbability(BB, SuccIdx, NormalProb);
return true;
}
bool BranchProbabilityInfo::calcPointerHeuristics(const BasicBlock *BB) {
const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
Value *Cond = BI->getCondition();
ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
if (!CI || !CI->isEquality())
return false;
Value *LHS = CI->getOperand(0);
if (!LHS->getType()->isPointerTy())
return false;
assert(CI->getOperand(1)->getType()->isPointerTy());
unsigned TakenIdx = 0, NonTakenIdx = 1;
bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(PH_TAKEN_WEIGHT,
PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
return true;
}
bool BranchProbabilityInfo::calcLoopBranchHeuristics(const BasicBlock *BB,
const LoopInfo &LI) {
Loop *L = LI.getLoopFor(BB);
if (!L)
return false;
SmallVector<unsigned, 8> BackEdges;
SmallVector<unsigned, 8> ExitingEdges;
SmallVector<unsigned, 8> InEdges;
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
if (!L->contains(*I))
ExitingEdges.push_back(I.getSuccessorIndex());
else if (L->getHeader() == *I)
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
}
if (BackEdges.empty() && ExitingEdges.empty())
return false;
SmallVector<BranchProbability, 4> Probs(3, BranchProbability::getZero());
unsigned Denom = (BackEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
(InEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
(ExitingEdges.empty() ? 0 : LBH_NONTAKEN_WEIGHT);
if (!BackEdges.empty())
Probs[0] = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
if (!InEdges.empty())
Probs[1] = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
if (!ExitingEdges.empty())
Probs[2] = BranchProbability(LBH_NONTAKEN_WEIGHT, Denom);
if (uint32_t numBackEdges = BackEdges.size()) {
auto Prob = Probs[0] / numBackEdges;
for (unsigned SuccIdx : BackEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
if (uint32_t numInEdges = InEdges.size()) {
auto Prob = Probs[1] / numInEdges;
for (unsigned SuccIdx : InEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
if (uint32_t numExitingEdges = ExitingEdges.size()) {
auto Prob = Probs[2] / numExitingEdges;
for (unsigned SuccIdx : ExitingEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
return true;
}
bool BranchProbabilityInfo::calcZeroHeuristics(const BasicBlock *BB) {
const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
Value *Cond = BI->getCondition();
ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
if (!CI)
return false;
Value *RHS = CI->getOperand(1);
ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
if (!CV)
return false;
if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
if (LHS->getOpcode() == Instruction::And)
if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
if (AndRHS->getUniqueInteger().isPowerOf2())
return false;
bool isProb;
if (CV->isZero()) {
switch (CI->getPredicate()) {
case CmpInst::ICMP_EQ:
isProb = false;
break;
case CmpInst::ICMP_NE:
isProb = true;
break;
case CmpInst::ICMP_SLT:
isProb = false;
break;
case CmpInst::ICMP_SGT:
isProb = true;
break;
default:
return false;
}
} else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
isProb = false;
} else if (CV->isAllOnesValue()) {
switch (CI->getPredicate()) {
case CmpInst::ICMP_EQ:
isProb = false;
break;
case CmpInst::ICMP_NE:
isProb = true;
break;
case CmpInst::ICMP_SGT:
isProb = true;
break;
default:
return false;
}
} else {
return false;
}
unsigned TakenIdx = 0, NonTakenIdx = 1;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(ZH_TAKEN_WEIGHT,
ZH_TAKEN_WEIGHT + ZH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
return true;
}
bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
Value *Cond = BI->getCondition();
FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
if (!FCmp)
return false;
bool isProb;
if (FCmp->isEquality()) {
isProb = !FCmp->isTrueWhenEqual();
} else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
isProb = true;
} else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
isProb = false;
} else {
return false;
}
unsigned TakenIdx = 0, NonTakenIdx = 1;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(FPH_TAKEN_WEIGHT,
FPH_TAKEN_WEIGHT + FPH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
return true;
}
bool BranchProbabilityInfo::calcInvokeHeuristics(const BasicBlock *BB) {
const InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator());
if (!II)
return false;
BranchProbability TakenProb(IH_TAKEN_WEIGHT,
IH_TAKEN_WEIGHT + IH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, 0 , TakenProb);
setEdgeProbability(BB, 1 , TakenProb.getCompl());
return true;
}
void BranchProbabilityInfo::releaseMemory() {
Probs.clear();
}
void BranchProbabilityInfo::print(raw_ostream &OS) const {
OS << "---- Branch Probabilities ----\n";
assert(LastF && "Cannot print prior to running over a function");
for (const auto &BI : *LastF) {
for (succ_const_iterator SI = succ_begin(&BI), SE = succ_end(&BI); SI != SE;
++SI) {
printEdgeProbability(OS << " ", &BI, *SI);
}
}
}
bool BranchProbabilityInfo::
isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
}
const BasicBlock *
BranchProbabilityInfo::getHotSucc(const BasicBlock *BB) const {
auto MaxProb = BranchProbability::getZero();
const BasicBlock *MaxSucc = nullptr;
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
const BasicBlock *Succ = *I;
auto Prob = getEdgeProbability(BB, Succ);
if (Prob > MaxProb) {
MaxProb = Prob;
MaxSucc = Succ;
}
}
if (MaxProb > BranchProbability(4, 5))
return MaxSucc;
return nullptr;
}
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
unsigned IndexInSuccessors) const {
auto I = Probs.find(std::make_pair(Src, IndexInSuccessors));
if (I != Probs.end())
return I->second;
return {1,
static_cast<uint32_t>(std::distance(succ_begin(Src), succ_end(Src)))};
}
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
succ_const_iterator Dst) const {
return getEdgeProbability(Src, Dst.getSuccessorIndex());
}
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
const BasicBlock *Dst) const {
auto Prob = BranchProbability::getZero();
bool FoundProb = false;
for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
if (*I == Dst) {
auto MapI = Probs.find(std::make_pair(Src, I.getSuccessorIndex()));
if (MapI != Probs.end()) {
FoundProb = true;
Prob += MapI->second;
}
}
uint32_t succ_num = std::distance(succ_begin(Src), succ_end(Src));
return FoundProb ? Prob : BranchProbability(1, succ_num);
}
void BranchProbabilityInfo::setEdgeProbability(const BasicBlock *Src,
unsigned IndexInSuccessors,
BranchProbability Prob) {
Probs[std::make_pair(Src, IndexInSuccessors)] = Prob;
DEBUG(dbgs() << "set edge " << Src->getName() << " -> " << IndexInSuccessors
<< " successor probability to " << Prob << "\n");
}
raw_ostream &
BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
const BasicBlock *Src,
const BasicBlock *Dst) const {
const BranchProbability Prob = getEdgeProbability(Src, Dst);
OS << "edge " << Src->getName() << " -> " << Dst->getName()
<< " probability is " << Prob
<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
return OS;
}
void BranchProbabilityInfo::calculate(const Function &F, const LoopInfo &LI) {
DEBUG(dbgs() << "---- Branch Probability Info : " << F.getName()
<< " ----\n\n");
LastF = &F; assert(PostDominatedByUnreachable.empty());
assert(PostDominatedByColdCall.empty());
for (auto BB : post_order(&F.getEntryBlock())) {
DEBUG(dbgs() << "Computing probabilities for " << BB->getName() << "\n");
if (calcUnreachableHeuristics(BB))
continue;
if (calcMetadataWeights(BB))
continue;
if (calcColdCallHeuristics(BB))
continue;
if (calcLoopBranchHeuristics(BB, LI))
continue;
if (calcPointerHeuristics(BB))
continue;
if (calcZeroHeuristics(BB))
continue;
if (calcFloatingPointHeuristics(BB))
continue;
calcInvokeHeuristics(BB);
}
PostDominatedByUnreachable.clear();
PostDominatedByColdCall.clear();
}
void BranchProbabilityInfoWrapperPass::getAnalysisUsage(
AnalysisUsage &AU) const {
AU.addRequired<LoopInfoWrapperPass>();
AU.setPreservesAll();
}
bool BranchProbabilityInfoWrapperPass::runOnFunction(Function &F) {
const LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
BPI.calculate(F, LI);
return false;
}
void BranchProbabilityInfoWrapperPass::releaseMemory() { BPI.releaseMemory(); }
void BranchProbabilityInfoWrapperPass::print(raw_ostream &OS,
const Module *) const {
BPI.print(OS);
}