BranchProbabilityInfo.cpp [plain text]
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/LLVMContext.h"
#include "llvm/Metadata.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
"Branch Probability Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
"Branch Probability Analysis", false, true)
char BranchProbabilityInfo::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 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 NORMAL_WEIGHT = 16;
static const uint32_t MIN_WEIGHT = 1;
static uint32_t getMaxWeightFor(BasicBlock *BB) {
return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
}
bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
TerminatorInst *TI = BB->getTerminator();
if (TI->getNumSuccessors() == 0) {
if (isa<UnreachableInst>(TI))
PostDominatedByUnreachable.insert(BB);
return false;
}
SmallPtrSet<BasicBlock *, 4> UnreachableEdges;
SmallPtrSet<BasicBlock *, 4> ReachableEdges;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
if (PostDominatedByUnreachable.count(*I))
UnreachableEdges.insert(*I);
else
ReachableEdges.insert(*I);
}
if (UnreachableEdges.size() == TI->getNumSuccessors())
PostDominatedByUnreachable.insert(BB);
if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
return false;
uint32_t UnreachableWeight =
std::max(UR_TAKEN_WEIGHT / UnreachableEdges.size(), MIN_WEIGHT);
for (SmallPtrSet<BasicBlock *, 4>::iterator I = UnreachableEdges.begin(),
E = UnreachableEdges.end();
I != E; ++I)
setEdgeWeight(BB, *I, UnreachableWeight);
if (ReachableEdges.empty())
return true;
uint32_t ReachableWeight =
std::max(UR_NONTAKEN_WEIGHT / ReachableEdges.size(), NORMAL_WEIGHT);
for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReachableEdges.begin(),
E = ReachableEdges.end();
I != E; ++I)
setEdgeWeight(BB, *I, ReachableWeight);
return true;
}
bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
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;
if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
return false;
uint32_t WeightLimit = getMaxWeightFor(BB);
SmallVector<uint32_t, 2> Weights;
Weights.reserve(TI->getNumSuccessors());
for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
ConstantInt *Weight = dyn_cast<ConstantInt>(WeightsNode->getOperand(i));
if (!Weight)
return false;
Weights.push_back(
std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
}
assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
return true;
}
bool BranchProbabilityInfo::calcPointerHeuristics(BasicBlock *BB) {
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());
BasicBlock *Taken = BI->getSuccessor(0);
BasicBlock *NonTaken = BI->getSuccessor(1);
bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
if (!isProb)
std::swap(Taken, NonTaken);
setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
return true;
}
bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
Loop *L = LI->getLoopFor(BB);
if (!L)
return false;
SmallPtrSet<BasicBlock *, 8> BackEdges;
SmallPtrSet<BasicBlock *, 8> ExitingEdges;
SmallPtrSet<BasicBlock *, 8> InEdges;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
if (!L->contains(*I))
ExitingEdges.insert(*I);
else if (L->getHeader() == *I)
BackEdges.insert(*I);
else
InEdges.insert(*I);
}
if (uint32_t numBackEdges = BackEdges.size()) {
uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
if (backWeight < NORMAL_WEIGHT)
backWeight = NORMAL_WEIGHT;
for (SmallPtrSet<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
EE = BackEdges.end(); EI != EE; ++EI) {
BasicBlock *Back = *EI;
setEdgeWeight(BB, Back, backWeight);
}
}
if (uint32_t numInEdges = InEdges.size()) {
uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
if (inWeight < NORMAL_WEIGHT)
inWeight = NORMAL_WEIGHT;
for (SmallPtrSet<BasicBlock *, 8>::iterator EI = InEdges.begin(),
EE = InEdges.end(); EI != EE; ++EI) {
BasicBlock *Back = *EI;
setEdgeWeight(BB, Back, inWeight);
}
}
if (uint32_t numExitingEdges = ExitingEdges.size()) {
uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numExitingEdges;
if (exitWeight < MIN_WEIGHT)
exitWeight = MIN_WEIGHT;
for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
EE = ExitingEdges.end(); EI != EE; ++EI) {
BasicBlock *Exiting = *EI;
setEdgeWeight(BB, Exiting, exitWeight);
}
}
return true;
}
bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
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;
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() && CI->getPredicate() == CmpInst::ICMP_SGT) {
isProb = true;
} else {
return false;
}
BasicBlock *Taken = BI->getSuccessor(0);
BasicBlock *NonTaken = BI->getSuccessor(1);
if (!isProb)
std::swap(Taken, NonTaken);
setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
return true;
}
bool BranchProbabilityInfo::calcFloatingPointHeuristics(BasicBlock *BB) {
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;
}
BasicBlock *Taken = BI->getSuccessor(0);
BasicBlock *NonTaken = BI->getSuccessor(1);
if (!isProb)
std::swap(Taken, NonTaken);
setEdgeWeight(BB, Taken, FPH_TAKEN_WEIGHT);
setEdgeWeight(BB, NonTaken, FPH_NONTAKEN_WEIGHT);
return true;
}
void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
AU.setPreservesAll();
}
bool BranchProbabilityInfo::runOnFunction(Function &F) {
LastF = &F; LI = &getAnalysis<LoopInfo>();
assert(PostDominatedByUnreachable.empty());
for (po_iterator<BasicBlock *> I = po_begin(&F.getEntryBlock()),
E = po_end(&F.getEntryBlock());
I != E; ++I) {
DEBUG(dbgs() << "Computing probabilities for " << I->getName() << "\n");
if (calcUnreachableHeuristics(*I))
continue;
if (calcMetadataWeights(*I))
continue;
if (calcLoopBranchHeuristics(*I))
continue;
if (calcPointerHeuristics(*I))
continue;
if (calcZeroHeuristics(*I))
continue;
calcFloatingPointHeuristics(*I);
}
PostDominatedByUnreachable.clear();
return false;
}
void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
OS << "---- Branch Probabilities ----\n";
assert(LastF && "Cannot print prior to running over a function");
for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
BI != BE; ++BI) {
for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
SI != SE; ++SI) {
printEdgeProbability(OS << " ", BI, *SI);
}
}
}
uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
uint32_t Sum = 0;
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
const BasicBlock *Succ = *I;
uint32_t Weight = getEdgeWeight(BB, Succ);
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
}
return Sum;
}
bool BranchProbabilityInfo::
isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
}
BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
uint32_t Sum = 0;
uint32_t MaxWeight = 0;
BasicBlock *MaxSucc = 0;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
uint32_t Weight = getEdgeWeight(BB, Succ);
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
if (Weight > MaxWeight) {
MaxWeight = Weight;
MaxSucc = Succ;
}
}
if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
return MaxSucc;
return 0;
}
uint32_t BranchProbabilityInfo::
getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
Edge E(Src, Dst);
DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
if (I != Weights.end())
return I->second;
return DEFAULT_WEIGHT;
}
void BranchProbabilityInfo::
setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst, uint32_t Weight) {
Weights[std::make_pair(Src, Dst)] = Weight;
DEBUG(dbgs() << "set edge " << Src->getName() << " -> "
<< Dst->getName() << " weight to " << Weight
<< (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
}
BranchProbability BranchProbabilityInfo::
getEdgeProbability(const BasicBlock *Src, const BasicBlock *Dst) const {
uint32_t N = getEdgeWeight(Src, Dst);
uint32_t D = getSumForBlock(Src);
return BranchProbability(N, D);
}
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;
}