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llvm-project/llvm/lib/Analysis/BranchProbabilityInfo.cpp

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//===-- 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/Instructions.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<BasicBlock *, 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 = 128)
// V |
// BB2--+
// |
// | (Weight = 4)
// V
// BB3
//
// Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
// Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
static const uint32_t LBH_TAKEN_WEIGHT = 128;
static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
// 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;
}
// Multiply Edge Weight by two.
void incEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
uint32_t Weight = BP->getEdgeWeight(Src, Dst);
uint32_t MaxWeight = getMaxWeightFor(Src);
if (Weight * 2 > MaxWeight)
BP->setEdgeWeight(Src, Dst, MaxWeight);
else
BP->setEdgeWeight(Src, Dst, Weight * 2);
}
// Divide Edge Weight by two.
void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
uint32_t Weight = BP->getEdgeWeight(Src, Dst);
assert(Weight > 0);
if (Weight / 2 < MIN_WEIGHT)
BP->setEdgeWeight(Src, Dst, MIN_WEIGHT);
else
BP->setEdgeWeight(Src, Dst, Weight / 2);
}
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) {
}
// Return Heuristics
void calcReturnHeuristics(BasicBlock *BB);
// Pointer Heuristics
void calcPointerHeuristics(BasicBlock *BB);
// Loop Branch Heuristics
void calcLoopBranchHeuristics(BasicBlock *BB);
bool runOnFunction(Function &F);
};
} // end anonymous namespace
// Calculate Edge Weights using "Return Heuristics". Predict a successor which
// leads directly to Return Instruction will not be taken.
void BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
if (BB->getTerminator()->getNumSuccessors() == 1)
return;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
if (isReturningBlock(Succ)) {
decEdgeWeight(BB, Succ);
}
}
}
// Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
// between two pointer or pointer and NULL will fail.
void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return;
Value *Cond = BI->getCondition();
ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
if (!CI || !CI->isEquality())
return;
Value *LHS = CI->getOperand(0);
if (!LHS->getType()->isPointerTy())
return;
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);
incEdgeWeight(BB, Taken);
decEdgeWeight(BB, NonTaken);
}
// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
Loop *L = LI->getLoopFor(BB);
if (!L)
return;
SmallVector<BasicBlock *, 8> BackEdges;
SmallVector<BasicBlock *, 8> ExitingEdges;
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.push_back(Succ);
else if (Succ == L->getHeader())
BackEdges.push_back(Succ);
}
if (uint32_t numBackEdges = BackEdges.size()) {
uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
if (backWeight < NORMAL_WEIGHT)
backWeight = NORMAL_WEIGHT;
for (SmallVector<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
EE = BackEdges.end(); EI != EE; ++EI) {
BasicBlock *Back = *EI;
BP->setEdgeWeight(BB, Back, backWeight);
}
}
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 (SmallVector<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
EE = ExitingEdges.end(); EI != EE; ++EI) {
BasicBlock *Exiting = *EI;
BP->setEdgeWeight(BB, Exiting, exitWeight);
}
}
}
bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
BasicBlock *BB = I++;
// Only LBH uses setEdgeWeight method.
calcLoopBranchHeuristics(BB);
// PH and RH use only incEdgeWeight and decEwdgeWeight methods to
// not efface LBH results.
calcPointerHeuristics(BB);
calcReturnHeuristics(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(BasicBlock *BB) const {
uint32_t Sum = 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;
}
return Sum;
}
bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, 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(BasicBlock *Src, 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(BasicBlock *Src, 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(BasicBlock *Src, 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;
}
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