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//===-- BranchProbabilityInfo.cpp - Branch Probability Analysis -----------===//
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// The LLVM Compiler Infrastructure
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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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// Loops should be simplified before this analysis.
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/ADT/PostOrderIterator.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#define DEBUG_TYPE "branch-prob"
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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(LoopInfoWrapperPass)
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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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// 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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// Using "Loop Branch Heuristics" we predict weights of edges for the
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// Probability of the edge BB2->BB1 = 124 / (124 + 4) = 0.96875
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// Probability of the edge BB2->BB3 = 4 / (124 + 4) = 0.03125
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static const uint32_t LBH_TAKEN_WEIGHT = 124;
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static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
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/// \brief Unreachable-terminating branch taken weight.
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/// This is the weight for a branch being taken to a block that terminates
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/// (eventually) in unreachable. These are predicted as unlikely as possible.
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static const uint32_t UR_TAKEN_WEIGHT = 1;
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/// \brief Unreachable-terminating branch not-taken weight.
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/// This is the weight for a branch not being taken toward a block that
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/// terminates (eventually) in unreachable. Such a branch is essentially never
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/// taken. Set the weight to an absurdly high value so that nested loops don't
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/// easily subsume it.
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static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
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/// \brief Weight for a branch taken going into a cold block.
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/// This is the weight for a branch taken toward a block marked
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/// cold. A block is marked cold if it's postdominated by a
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/// block containing a call to a cold function. Cold functions
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/// are those marked with attribute 'cold'.
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static const uint32_t CC_TAKEN_WEIGHT = 4;
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/// \brief Weight for a branch not-taken into a cold block.
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/// This is the weight for a branch not taken toward a block marked
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static const uint32_t CC_NONTAKEN_WEIGHT = 64;
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static const uint32_t PH_TAKEN_WEIGHT = 20;
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static const uint32_t PH_NONTAKEN_WEIGHT = 12;
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static const uint32_t ZH_TAKEN_WEIGHT = 20;
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static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
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static const uint32_t FPH_TAKEN_WEIGHT = 20;
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static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
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/// \brief Invoke-terminating normal branch taken weight
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/// This is the weight for branching to the normal destination of an invoke
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/// instruction. We expect this to happen most of the time. Set the weight to an
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/// absurdly high value so that nested loops subsume it.
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static const uint32_t IH_TAKEN_WEIGHT = 1024 * 1024 - 1;
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/// \brief Invoke-terminating normal branch not-taken weight.
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/// This is the weight for branching to the unwind destination of an invoke
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/// instruction. This is essentially never taken.
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static const uint32_t IH_NONTAKEN_WEIGHT = 1;
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// Standard weight value. Used when none of the heuristics set weight for
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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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/// \brief Calculate edge weights for successors lead to unreachable.
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/// Predict that a successor which leads necessarily to an
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/// unreachable-terminated block as extremely unlikely.
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bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
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TerminatorInst *TI = BB->getTerminator();
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if (TI->getNumSuccessors() == 0) {
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if (isa<UnreachableInst>(TI))
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PostDominatedByUnreachable.insert(BB);
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SmallVector<unsigned, 4> UnreachableEdges;
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SmallVector<unsigned, 4> ReachableEdges;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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if (PostDominatedByUnreachable.count(*I))
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UnreachableEdges.push_back(I.getSuccessorIndex());
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ReachableEdges.push_back(I.getSuccessorIndex());
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// If all successors are in the set of blocks post-dominated by unreachable,
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// this block is too.
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if (UnreachableEdges.size() == TI->getNumSuccessors())
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PostDominatedByUnreachable.insert(BB);
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// Skip probabilities if this block has a single successor or if all were
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if (TI->getNumSuccessors() == 1 || UnreachableEdges.empty())
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uint32_t UnreachableWeight =
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std::max(UR_TAKEN_WEIGHT / (unsigned)UnreachableEdges.size(), MIN_WEIGHT);
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for (SmallVectorImpl<unsigned>::iterator I = UnreachableEdges.begin(),
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E = UnreachableEdges.end();
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setEdgeWeight(BB, *I, UnreachableWeight);
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if (ReachableEdges.empty())
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uint32_t ReachableWeight =
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std::max(UR_NONTAKEN_WEIGHT / (unsigned)ReachableEdges.size(),
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for (SmallVectorImpl<unsigned>::iterator I = ReachableEdges.begin(),
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E = ReachableEdges.end();
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setEdgeWeight(BB, *I, ReachableWeight);
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// Propagate existing explicit probabilities from either profile data or
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// 'expect' intrinsic processing.
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bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
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TerminatorInst *TI = BB->getTerminator();
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if (TI->getNumSuccessors() == 1)
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if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
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MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
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// Check that the number of successors is manageable.
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assert(TI->getNumSuccessors() < UINT32_MAX && "Too many successors");
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// Ensure there are weights for all of the successors. Note that the first
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// operand to the metadata node is a name, not a weight.
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if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
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// Build up the final weights that will be used in a temporary buffer.
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// Compute the sum of all weights to later decide whether they need to
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// be scaled to fit in 32 bits.
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uint64_t WeightSum = 0;
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SmallVector<uint32_t, 2> Weights;
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Weights.reserve(TI->getNumSuccessors());
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for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
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ConstantInt *Weight =
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mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
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assert(Weight->getValue().getActiveBits() <= 32 &&
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"Too many bits for uint32_t");
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Weights.push_back(Weight->getZExtValue());
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WeightSum += Weights.back();
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assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
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// If the sum of weights does not fit in 32 bits, scale every weight down
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uint64_t ScalingFactor =
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(WeightSum > UINT32_MAX) ? WeightSum / UINT32_MAX + 1 : 1;
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
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uint32_t W = Weights[i] / ScalingFactor;
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setEdgeWeight(BB, i, W);
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assert(WeightSum <= UINT32_MAX &&
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"Expected weights to scale down to 32 bits");
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/// \brief Calculate edge weights for edges leading to cold blocks.
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/// A cold block is one post-dominated by a block with a call to a
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/// cold function. Those edges are unlikely to be taken, so we give
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/// them relatively low weight.
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/// Return true if we could compute the weights for cold edges.
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/// Return false, otherwise.
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bool BranchProbabilityInfo::calcColdCallHeuristics(BasicBlock *BB) {
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TerminatorInst *TI = BB->getTerminator();
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if (TI->getNumSuccessors() == 0)
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// Determine which successors are post-dominated by a cold block.
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SmallVector<unsigned, 4> ColdEdges;
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SmallVector<unsigned, 4> NormalEdges;
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
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if (PostDominatedByColdCall.count(*I))
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ColdEdges.push_back(I.getSuccessorIndex());
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NormalEdges.push_back(I.getSuccessorIndex());
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// If all successors are in the set of blocks post-dominated by cold calls,
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// this block is in the set post-dominated by cold calls.
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if (ColdEdges.size() == TI->getNumSuccessors())
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PostDominatedByColdCall.insert(BB);
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// Otherwise, if the block itself contains a cold function, add it to the
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// set of blocks postdominated by a cold call.
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assert(!PostDominatedByColdCall.count(BB));
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (CallInst *CI = dyn_cast<CallInst>(I))
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if (CI->hasFnAttr(Attribute::Cold)) {
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PostDominatedByColdCall.insert(BB);
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// Skip probabilities if this block has a single successor.
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if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
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uint32_t ColdWeight =
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std::max(CC_TAKEN_WEIGHT / (unsigned) ColdEdges.size(), MIN_WEIGHT);
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for (SmallVectorImpl<unsigned>::iterator I = ColdEdges.begin(),
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setEdgeWeight(BB, *I, ColdWeight);
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if (NormalEdges.empty())
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uint32_t NormalWeight = std::max(
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CC_NONTAKEN_WEIGHT / (unsigned) NormalEdges.size(), NORMAL_WEIGHT);
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for (SmallVectorImpl<unsigned>::iterator I = NormalEdges.begin(),
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E = NormalEdges.end();
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setEdgeWeight(BB, *I, NormalWeight);
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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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bool BranchProbabilityInfo::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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Value *Cond = BI->getCondition();
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ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
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if (!CI || !CI->isEquality())
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Value *LHS = CI->getOperand(0);
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if (!LHS->getType()->isPointerTy())
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assert(CI->getOperand(1)->getType()->isPointerTy());
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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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unsigned TakenIdx = 0, NonTakenIdx = 1;
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bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
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std::swap(TakenIdx, NonTakenIdx);
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setEdgeWeight(BB, TakenIdx, PH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTakenIdx, PH_NONTAKEN_WEIGHT);
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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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bool BranchProbabilityInfo::calcLoopBranchHeuristics(BasicBlock *BB) {
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Loop *L = LI->getLoopFor(BB);
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SmallVector<unsigned, 8> BackEdges;
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SmallVector<unsigned, 8> ExitingEdges;
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SmallVector<unsigned, 8> InEdges; // Edges from header to the loop.
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for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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if (!L->contains(*I))
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ExitingEdges.push_back(I.getSuccessorIndex());
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else if (L->getHeader() == *I)
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BackEdges.push_back(I.getSuccessorIndex());
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InEdges.push_back(I.getSuccessorIndex());
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if (BackEdges.empty() && ExitingEdges.empty())
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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 (SmallVectorImpl<unsigned>::iterator EI = BackEdges.begin(),
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EE = BackEdges.end(); EI != EE; ++EI) {
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setEdgeWeight(BB, *EI, backWeight);
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if (uint32_t numInEdges = InEdges.size()) {
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uint32_t inWeight = LBH_TAKEN_WEIGHT / numInEdges;
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if (inWeight < NORMAL_WEIGHT)
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inWeight = NORMAL_WEIGHT;
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for (SmallVectorImpl<unsigned>::iterator EI = InEdges.begin(),
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EE = InEdges.end(); EI != EE; ++EI) {
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setEdgeWeight(BB, *EI, inWeight);
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if (uint32_t numExitingEdges = ExitingEdges.size()) {
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uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numExitingEdges;
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if (exitWeight < MIN_WEIGHT)
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exitWeight = MIN_WEIGHT;
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for (SmallVectorImpl<unsigned>::iterator EI = ExitingEdges.begin(),
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EE = ExitingEdges.end(); EI != EE; ++EI) {
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setEdgeWeight(BB, *EI, exitWeight);
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bool BranchProbabilityInfo::calcZeroHeuristics(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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Value *Cond = BI->getCondition();
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ICmpInst *CI = dyn_cast<ICmpInst>(Cond);
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Value *RHS = CI->getOperand(1);
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ConstantInt *CV = dyn_cast<ConstantInt>(RHS);
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// If the LHS is the result of AND'ing a value with a single bit bitmask,
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// we don't have information about probabilities.
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if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
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if (LHS->getOpcode() == Instruction::And)
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if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
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if (AndRHS->getUniqueInteger().isPowerOf2())
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switch (CI->getPredicate()) {
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case CmpInst::ICMP_EQ:
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// X == 0 -> Unlikely
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case CmpInst::ICMP_NE:
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case CmpInst::ICMP_SLT:
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case CmpInst::ICMP_SGT:
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} else if (CV->isOne() && CI->getPredicate() == CmpInst::ICMP_SLT) {
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// InstCombine canonicalizes X <= 0 into X < 1.
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// X <= 0 -> Unlikely
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} else if (CV->isAllOnesValue()) {
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switch (CI->getPredicate()) {
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case CmpInst::ICMP_EQ:
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// X == -1 -> Unlikely
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case CmpInst::ICMP_NE:
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case CmpInst::ICMP_SGT:
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// InstCombine canonicalizes X >= 0 into X > -1.
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unsigned TakenIdx = 0, NonTakenIdx = 1;
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std::swap(TakenIdx, NonTakenIdx);
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setEdgeWeight(BB, TakenIdx, ZH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTakenIdx, ZH_NONTAKEN_WEIGHT);
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bool BranchProbabilityInfo::calcFloatingPointHeuristics(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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Value *Cond = BI->getCondition();
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FCmpInst *FCmp = dyn_cast<FCmpInst>(Cond);
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if (FCmp->isEquality()) {
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// f1 == f2 -> Unlikely
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// f1 != f2 -> Likely
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isProb = !FCmp->isTrueWhenEqual();
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} else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
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} else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
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unsigned TakenIdx = 0, NonTakenIdx = 1;
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std::swap(TakenIdx, NonTakenIdx);
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setEdgeWeight(BB, TakenIdx, FPH_TAKEN_WEIGHT);
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setEdgeWeight(BB, NonTakenIdx, FPH_NONTAKEN_WEIGHT);
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bool BranchProbabilityInfo::calcInvokeHeuristics(BasicBlock *BB) {
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InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator());
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setEdgeWeight(BB, 0/*Index for Normal*/, IH_TAKEN_WEIGHT);
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setEdgeWeight(BB, 1/*Index for Unwind*/, IH_NONTAKEN_WEIGHT);
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void BranchProbabilityInfo::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<LoopInfoWrapperPass>();
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AU.setPreservesAll();
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bool BranchProbabilityInfo::runOnFunction(Function &F) {
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DEBUG(dbgs() << "---- Branch Probability Info : " << F.getName()
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LastF = &F; // Store the last function we ran on for printing.
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LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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assert(PostDominatedByUnreachable.empty());
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assert(PostDominatedByColdCall.empty());
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// Walk the basic blocks in post-order so that we can build up state about
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// the successors of a block iteratively.
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for (auto BB : post_order(&F.getEntryBlock())) {
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DEBUG(dbgs() << "Computing probabilities for " << BB->getName() << "\n");
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if (calcUnreachableHeuristics(BB))
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if (calcMetadataWeights(BB))
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if (calcColdCallHeuristics(BB))
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if (calcLoopBranchHeuristics(BB))
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if (calcPointerHeuristics(BB))
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if (calcZeroHeuristics(BB))
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if (calcFloatingPointHeuristics(BB))
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calcInvokeHeuristics(BB);
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PostDominatedByUnreachable.clear();
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PostDominatedByColdCall.clear();
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void BranchProbabilityInfo::releaseMemory() {
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void BranchProbabilityInfo::print(raw_ostream &OS, const Module *) const {
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OS << "---- Branch Probabilities ----\n";
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// We print the probabilities from the last function the analysis ran over,
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// or the function it is currently running over.
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assert(LastF && "Cannot print prior to running over a function");
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for (Function::const_iterator BI = LastF->begin(), BE = LastF->end();
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for (succ_const_iterator SI = succ_begin(BI), SE = succ_end(BI);
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printEdgeProbability(OS << " ", BI, *SI);
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uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
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for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
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uint32_t Weight = getEdgeWeight(BB, I.getSuccessorIndex());
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uint32_t PrevSum = Sum;
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assert(Sum >= PrevSum); (void) PrevSum;
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bool BranchProbabilityInfo::
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isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
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// Hot probability is at least 4/5 = 80%
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// FIXME: Compare against a static "hot" BranchProbability.
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return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
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BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
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uint32_t MaxWeight = 0;
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BasicBlock *MaxSucc = nullptr;
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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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assert(Sum > PrevSum); (void) PrevSum;
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if (Weight > MaxWeight) {
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// Hot probability is at least 4/5 = 80%
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if (BranchProbability(MaxWeight, Sum) > BranchProbability(4, 5))
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/// Get the raw edge weight for the edge. If can't find it, return
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/// DEFAULT_WEIGHT value. Here an edge is specified using PredBlock and an index
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/// to the successors.
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uint32_t BranchProbabilityInfo::
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getEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors) const {
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DenseMap<Edge, uint32_t>::const_iterator I =
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Weights.find(std::make_pair(Src, IndexInSuccessors));
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if (I != Weights.end())
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return DEFAULT_WEIGHT;
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uint32_t BranchProbabilityInfo::getEdgeWeight(const BasicBlock *Src,
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succ_const_iterator Dst) const {
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return getEdgeWeight(Src, Dst.getSuccessorIndex());
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/// Get the raw edge weight calculated for the block pair. This returns the sum
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/// of all raw edge weights from Src to Dst.
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uint32_t BranchProbabilityInfo::
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getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
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bool FoundWeight = false;
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DenseMap<Edge, uint32_t>::const_iterator MapI;
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for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
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MapI = Weights.find(std::make_pair(Src, I.getSuccessorIndex()));
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if (MapI != Weights.end()) {
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Weight += MapI->second;
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return (!FoundWeight) ? DEFAULT_WEIGHT : Weight;
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/// Set the edge weight for a given edge specified by PredBlock and an index
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/// to the successors.
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void BranchProbabilityInfo::
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setEdgeWeight(const BasicBlock *Src, unsigned IndexInSuccessors,
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Weights[std::make_pair(Src, IndexInSuccessors)] = Weight;
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DEBUG(dbgs() << "set edge " << Src->getName() << " -> "
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<< IndexInSuccessors << " successor weight to "
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/// Get an edge's probability, relative to other out-edges from Src.
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BranchProbability BranchProbabilityInfo::
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getEdgeProbability(const BasicBlock *Src, unsigned IndexInSuccessors) const {
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uint32_t N = getEdgeWeight(Src, IndexInSuccessors);
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uint32_t D = getSumForBlock(Src);
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return BranchProbability(N, D);
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/// Get the probability of going from Src to Dst. It returns the sum of all
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/// probabilities for edges from Src to Dst.
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BranchProbability BranchProbabilityInfo::
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getEdgeProbability(const BasicBlock *Src, const 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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BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS,
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const BasicBlock *Src,
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const BasicBlock *Dst) const {
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const BranchProbability Prob = getEdgeProbability(Src, Dst);
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OS << "edge " << Src->getName() << " -> " << Dst->getName()
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<< " probability is " << Prob
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<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");