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359 lines
10 KiB
C++
359 lines
10 KiB
C++
//===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// Loops should be simplified before this analysis.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Support/Debug.h"
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using namespace llvm;
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INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
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"Branch Probability Analysis", false, true)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_END(BranchProbabilityInfo, "branch-prob",
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"Branch Probability Analysis", false, true)
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char BranchProbabilityInfo::ID = 0;
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namespace {
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// Please note that BranchProbabilityAnalysis is not a FunctionPass.
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// It is created by BranchProbabilityInfo (which is a FunctionPass), which
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// provides a clear interface. Thanks to that, all heuristics and other
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// private methods are hidden in the .cpp file.
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class BranchProbabilityAnalysis {
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typedef std::pair<BasicBlock *, BasicBlock *> Edge;
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DenseMap<Edge, uint32_t> *Weights;
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BranchProbabilityInfo *BP;
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LoopInfo *LI;
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// Weights are for internal use only. They are used by heuristics to help to
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// estimate edges' probability. Example:
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//
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// Using "Loop Branch Heuristics" we predict weights of edges for the
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// block BB2.
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// ...
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// |
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// V
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// BB1<-+
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// | |
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// | | (Weight = 128)
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// V |
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// BB2--+
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// |
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// | (Weight = 4)
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// V
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// BB3
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//
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// Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
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// Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
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static const uint32_t LBH_TAKEN_WEIGHT = 128;
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static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
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// Standard weight value. Used when none of the heuristics set weight for
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// the edge.
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static const uint32_t NORMAL_WEIGHT = 16;
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// Minimum weight of an edge. Please note, that weight is NEVER 0.
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static const uint32_t MIN_WEIGHT = 1;
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// Return TRUE if BB leads directly to a Return Instruction.
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static bool isReturningBlock(BasicBlock *BB) {
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SmallPtrSet<BasicBlock *, 8> Visited;
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while (true) {
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TerminatorInst *TI = BB->getTerminator();
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if (isa<ReturnInst>(TI))
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return true;
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if (TI->getNumSuccessors() > 1)
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break;
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// It is unreachable block which we can consider as a return instruction.
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if (TI->getNumSuccessors() == 0)
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return true;
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Visited.insert(BB);
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BB = TI->getSuccessor(0);
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// Stop if cycle is detected.
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if (Visited.count(BB))
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return false;
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}
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return false;
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}
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// Multiply Edge Weight by two.
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void incEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
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uint32_t Weight = BP->getEdgeWeight(Src, Dst);
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uint32_t MaxWeight = getMaxWeightFor(Src);
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if (Weight * 2 > MaxWeight)
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BP->setEdgeWeight(Src, Dst, MaxWeight);
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else
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BP->setEdgeWeight(Src, Dst, Weight * 2);
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}
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// Divide Edge Weight by two.
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void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
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uint32_t Weight = BP->getEdgeWeight(Src, Dst);
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assert(Weight > 0);
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if (Weight / 2 < MIN_WEIGHT)
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BP->setEdgeWeight(Src, Dst, MIN_WEIGHT);
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else
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BP->setEdgeWeight(Src, Dst, Weight / 2);
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}
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uint32_t getMaxWeightFor(BasicBlock *BB) const {
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return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
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}
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public:
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BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
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BranchProbabilityInfo *BP, LoopInfo *LI)
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: Weights(W), BP(BP), LI(LI) {
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}
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// Return Heuristics
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void calcReturnHeuristics(BasicBlock *BB);
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// Pointer Heuristics
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void calcPointerHeuristics(BasicBlock *BB);
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// Loop Branch Heuristics
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void calcLoopBranchHeuristics(BasicBlock *BB);
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bool runOnFunction(Function &F);
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};
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} // end anonymous namespace
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// Calculate Edge Weights using "Return Heuristics". Predict a successor which
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// leads directly to Return Instruction will not be taken.
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void BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
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if (BB->getTerminator()->getNumSuccessors() == 1)
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return;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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BasicBlock *Succ = *I;
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if (isReturningBlock(Succ)) {
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decEdgeWeight(BB, Succ);
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}
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}
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}
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// Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
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// between two pointer or pointer and NULL will fail.
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void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
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BranchInst * BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return;
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Value *Cond = BI->getCondition();
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ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
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if (!CI)
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return;
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Value *LHS = CI->getOperand(0);
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if (!LHS->getType()->isPointerTy())
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return;
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assert(CI->getOperand(1)->getType()->isPointerTy());
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BasicBlock *Taken = BI->getSuccessor(0);
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BasicBlock *NonTaken = BI->getSuccessor(1);
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// p != 0 -> isProb = true
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// p == 0 -> isProb = false
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// p != q -> isProb = true
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// p == q -> isProb = false;
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bool isProb = !CI->isEquality();
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if (!isProb)
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std::swap(Taken, NonTaken);
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incEdgeWeight(BB, Taken);
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decEdgeWeight(BB, NonTaken);
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}
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// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
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// as taken, exiting edges as not-taken.
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void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
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uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
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Loop *L = LI->getLoopFor(BB);
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if (!L)
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return;
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SmallVector<BasicBlock *, 8> BackEdges;
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SmallVector<BasicBlock *, 8> ExitingEdges;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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BasicBlock *Succ = *I;
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Loop *SuccL = LI->getLoopFor(Succ);
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if (SuccL != L)
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ExitingEdges.push_back(Succ);
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else if (Succ == L->getHeader())
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BackEdges.push_back(Succ);
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}
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if (uint32_t numBackEdges = BackEdges.size()) {
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uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
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if (backWeight < NORMAL_WEIGHT)
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backWeight = NORMAL_WEIGHT;
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for (SmallVector<BasicBlock *, 8>::iterator EI = BackEdges.begin(),
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EE = BackEdges.end(); EI != EE; ++EI) {
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BasicBlock *Back = *EI;
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BP->setEdgeWeight(BB, Back, backWeight);
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}
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}
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uint32_t numExitingEdges = ExitingEdges.size();
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if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
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uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
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if (exitWeight < MIN_WEIGHT)
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exitWeight = MIN_WEIGHT;
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for (SmallVector<BasicBlock *, 8>::iterator EI = ExitingEdges.begin(),
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EE = ExitingEdges.end(); EI != EE; ++EI) {
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BasicBlock *Exiting = *EI;
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BP->setEdgeWeight(BB, Exiting, exitWeight);
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}
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}
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}
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bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
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BasicBlock *BB = I++;
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// Only LBH uses setEdgeWeight method.
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calcLoopBranchHeuristics(BB);
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// PH and RH use only incEdgeWeight and decEwdgeWeight methods to
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// not efface LBH results.
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calcPointerHeuristics(BB);
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calcReturnHeuristics(BB);
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}
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return false;
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}
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bool BranchProbabilityInfo::runOnFunction(Function &F) {
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LoopInfo &LI = getAnalysis<LoopInfo>();
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BranchProbabilityAnalysis BPA(&Weights, this, &LI);
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return BPA.runOnFunction(F);
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}
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uint32_t BranchProbabilityInfo::getSumForBlock(BasicBlock *BB) const {
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uint32_t Sum = 0;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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BasicBlock *Succ = *I;
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uint32_t Weight = getEdgeWeight(BB, Succ);
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uint32_t PrevSum = Sum;
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Sum += Weight;
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assert(Sum > PrevSum); (void) PrevSum;
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}
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return Sum;
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}
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bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
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// Hot probability is at least 4/5 = 80%
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uint32_t Weight = getEdgeWeight(Src, Dst);
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uint32_t Sum = getSumForBlock(Src);
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// FIXME: Implement BranchProbability::compare then change this code to
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// compare this BranchProbability against a static "hot" BranchProbability.
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return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
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}
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BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
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uint32_t Sum = 0;
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uint32_t MaxWeight = 0;
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BasicBlock *MaxSucc = 0;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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BasicBlock *Succ = *I;
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uint32_t Weight = getEdgeWeight(BB, Succ);
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uint32_t PrevSum = Sum;
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Sum += Weight;
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assert(Sum > PrevSum); (void) PrevSum;
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if (Weight > MaxWeight) {
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MaxWeight = Weight;
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MaxSucc = Succ;
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}
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}
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// FIXME: Use BranchProbability::compare.
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if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
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return MaxSucc;
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return 0;
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}
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// Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
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uint32_t
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BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
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Edge E(Src, Dst);
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DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
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if (I != Weights.end())
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return I->second;
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return DEFAULT_WEIGHT;
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}
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void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
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uint32_t Weight) {
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Weights[std::make_pair(Src, Dst)] = Weight;
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DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
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<< Dst->getNameStr() << " weight to " << Weight
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<< (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
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}
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BranchProbability BranchProbabilityInfo::
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getEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const {
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uint32_t N = getEdgeWeight(Src, Dst);
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uint32_t D = getSumForBlock(Src);
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return BranchProbability(N, D);
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}
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raw_ostream &
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BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
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BasicBlock *Dst) const {
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const BranchProbability Prob = getEdgeProbability(Src, Dst);
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OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
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<< " probability is " << Prob
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<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
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return OS;
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}
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