//===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Loops should be simplified before this analysis.
//
//===----------------------------------------------------------------------===//
#include "llvm/Constants.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/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;
namespace {
// Please note that BranchProbabilityAnalysis is not a FunctionPass.
// It is created by BranchProbabilityInfo (which is a FunctionPass), which
// provides a clear interface. Thanks to that, all heuristics and other
// private methods are hidden in the .cpp file.
class BranchProbabilityAnalysis {
typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;
DenseMap<Edge, uint32_t> *Weights;
BranchProbabilityInfo *BP;
LoopInfo *LI;
// Weights are for internal use only. They are used by heuristics to help to
// estimate edges' probability. Example:
//
// Using "Loop Branch Heuristics" we predict weights of edges for the
// block BB2.
// ...
// |
// V
// BB1<-+
// | |
// | | (Weight = 124)
// V |
// BB2--+
// |
// | (Weight = 4)
// V
// BB3
//
// Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
// Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
static const uint32_t LBH_TAKEN_WEIGHT = 124;
static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
static const uint32_t RH_TAKEN_WEIGHT = 24;
static const uint32_t RH_NONTAKEN_WEIGHT = 8;
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;
// Standard weight value. Used when none of the heuristics set weight for
// the edge.
static const uint32_t NORMAL_WEIGHT = 16;
// Minimum weight of an edge. Please note, that weight is NEVER 0.
static const uint32_t MIN_WEIGHT = 1;
// Return TRUE if BB leads directly to a Return Instruction.
static bool isReturningBlock(BasicBlock *BB) {
SmallPtrSet<BasicBlock *, 8> Visited;
while (true) {
TerminatorInst *TI = BB->getTerminator();
if (isa<ReturnInst>(TI))
return true;
if (TI->getNumSuccessors() > 1)
break;
// It is unreachable block which we can consider as a return instruction.
if (TI->getNumSuccessors() == 0)
return true;
Visited.insert(BB);
BB = TI->getSuccessor(0);
// Stop if cycle is detected.
if (Visited.count(BB))
return false;
}
return false;
}
uint32_t getMaxWeightFor(BasicBlock *BB) const {
return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
}
public:
BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
BranchProbabilityInfo *BP, LoopInfo *LI)
: Weights(W), BP(BP), LI(LI) {
}
// Metadata Weights
bool calcMetadataWeights(BasicBlock *BB);
// Return Heuristics
bool calcReturnHeuristics(BasicBlock *BB);
// Pointer Heuristics
bool calcPointerHeuristics(BasicBlock *BB);
// Loop Branch Heuristics
bool calcLoopBranchHeuristics(BasicBlock *BB);
// Zero Heurestics
bool calcZeroHeuristics(BasicBlock *BB);
bool runOnFunction(Function &F);
};
} // end anonymous namespace
// Propagate existing explicit probabilities from either profile data or
// 'expect' intrinsic processing.
bool BranchProbabilityAnalysis::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;
// Ensure there are weights for all of the successors. Note that the first
// operand to the metadata node is a name, not a weight.
if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
return false;
// Build up the final weights that will be used in a temporary buffer, but
// don't add them until all weihts are present. Each weight value is clamped
// to [1, getMaxWeightFor(BB)].
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)
BP->setEdgeWeight(BB, TI->getSuccessor(i), Weights[i]);
return true;
}
// Calculate Edge Weights using "Return Heuristics". Predict a successor which
// leads directly to Return Instruction will not be taken.
bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
if (BB->getTerminator()->getNumSuccessors() == 1)
return false;
SmallPtrSet<BasicBlock *, 4> ReturningEdges;
SmallPtrSet<BasicBlock *, 4> StayEdges;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
if (isReturningBlock(Succ))
ReturningEdges.insert(Succ);
else
StayEdges.insert(Succ);
}
if (uint32_t numStayEdges = StayEdges.size()) {
uint32_t stayWeight = RH_TAKEN_WEIGHT / numStayEdges;
if (stayWeight < NORMAL_WEIGHT)
stayWeight = NORMAL_WEIGHT;
for (SmallPtrSet<BasicBlock *, 4>::iterator I = StayEdges.begin(),
E = StayEdges.end(); I != E; ++I)
BP->setEdgeWeight(BB, *I, stayWeight);
}
if (uint32_t numRetEdges = ReturningEdges.size()) {
uint32_t retWeight = RH_NONTAKEN_WEIGHT / numRetEdges;
if (retWeight < MIN_WEIGHT)
retWeight = MIN_WEIGHT;
for (SmallPtrSet<BasicBlock *, 4>::iterator I = ReturningEdges.begin(),
E = ReturningEdges.end(); I != E; ++I) {
BP->setEdgeWeight(BB, *I, retWeight);
}
}
return ReturningEdges.size() > 0;
}
// Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
// between two pointer or pointer and NULL will fail.
bool BranchProbabilityAnalysis::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);
// p != 0 -> isProb = true
// p == 0 -> isProb = false
// p != q -> isProb = true
// p == q -> isProb = false;
bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
if (!isProb)
std::swap(Taken, NonTaken);
BP->setEdgeWeight(BB, Taken, PH_TAKEN_WEIGHT);
BP->setEdgeWeight(BB, NonTaken, PH_NONTAKEN_WEIGHT);
return true;
}
// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
bool BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
Loop *L = LI->getLoopFor(BB);
if (!L)
return false;
SmallPtrSet<BasicBlock *, 8> BackEdges;
SmallPtrSet<BasicBlock *, 8> ExitingEdges;
SmallPtrSet<BasicBlock *, 8> InEdges; // Edges from header to the loop.
bool isHeader = BB == L->getHeader();
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
Loop *SuccL = LI->getLoopFor(Succ);
if (SuccL != L)
ExitingEdges.insert(Succ);
else if (Succ == L->getHeader())
BackEdges.insert(Succ);
else if (isHeader)
InEdges.insert(Succ);
}
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;
BP->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;
BP->setEdgeWeight(BB, Back, inWeight);
}
}
uint32_t numExitingEdges = ExitingEdges.size();
if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
if (exitWeight < MIN_WEIGHT)
exitWeight = MIN_WEIGHT;
for (SmallPtrSet<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
EE = ExitingEdges.end(); EI != EE; ++EI) {
BasicBlock *Exiting = *EI;
BP->setEdgeWeight(BB, Exiting, exitWeight);
}
}
return true;
}
bool BranchProbabilityAnalysis::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:
// X == 0 -> Unlikely
isProb = false;
break;
case CmpInst::ICMP_NE:
// X != 0 -> Likely
isProb = true;
break;
case CmpInst::ICMP_SLT:
// X < 0 -> Unlikely
isProb = false;
break;
case CmpInst::ICMP_SGT:
// X > 0 -> Likely
isProb = true;
break;
default:
return false;
}
} else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
// InstCombine canonicalizes X <= 0 into X < 1.
// X <= 0 -> Unlikely
isProb = false;
} else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
// InstCombine canonicalizes X >= 0 into X > -1.
// X >= 0 -> Likely
isProb = true;
} else {
return false;
}
BasicBlock *Taken = BI->getSuccessor(0);
BasicBlock *NonTaken = BI->getSuccessor(1);
if (!isProb)
std::swap(Taken, NonTaken);
BP->setEdgeWeight(BB, Taken, ZH_TAKEN_WEIGHT);
BP->setEdgeWeight(BB, NonTaken, ZH_NONTAKEN_WEIGHT);
return true;
}
bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
BasicBlock *BB = I++;
if (calcMetadataWeights(BB))
continue;
if (calcLoopBranchHeuristics(BB))
continue;
if (calcReturnHeuristics(BB))
continue;
if (calcPointerHeuristics(BB))
continue;
calcZeroHeuristics(BB);
}
return false;
}
void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<LoopInfo>();
AU.setPreservesAll();
}
bool BranchProbabilityInfo::runOnFunction(Function &F) {
LoopInfo &LI = getAnalysis<LoopInfo>();
BranchProbabilityAnalysis BPA(&Weights, this, &LI);
return BPA.runOnFunction(F);
}
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 {
// Hot probability is at least 4/5 = 80%
uint32_t Weight = getEdgeWeight(Src, Dst);
uint32_t Sum = getSumForBlock(Src);
// FIXME: Implement BranchProbability::compare then change this code to
// compare this BranchProbability against a static "hot" BranchProbability.
return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
}
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;
}
}
// FIXME: Use BranchProbability::compare.
if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
return MaxSucc;
return 0;
}
// Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
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->getNameStr() << " -> "
<< Dst->getNameStr() << " 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, BasicBlock *Src,
BasicBlock *Dst) const {
const BranchProbability Prob = getEdgeProbability(Src, Dst);
OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
<< " probability is " << Prob
<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
return OS;
}