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

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//===- BranchProbabilityInfo.cpp - Branch Probability Analysis ------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Loops should be simplified before this analysis.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "branch-prob"
static cl::opt<bool> PrintBranchProb(
"print-bpi", cl::init(false), cl::Hidden,
cl::desc("Print the branch probability info."));
cl::opt<std::string> PrintBranchProbFuncName(
"print-bpi-func-name", cl::Hidden,
cl::desc("The option to specify the name of the function "
"whose branch probability info is printed."));
INITIALIZE_PASS_BEGIN(BranchProbabilityInfoWrapperPass, "branch-prob",
"Branch Probability Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_END(BranchProbabilityInfoWrapperPass, "branch-prob",
"Branch Probability Analysis", false, true)
BranchProbabilityInfoWrapperPass::BranchProbabilityInfoWrapperPass()
: FunctionPass(ID) {
initializeBranchProbabilityInfoWrapperPassPass(
*PassRegistry::getPassRegistry());
}
char BranchProbabilityInfoWrapperPass::ID = 0;
// 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;
// Unlikely edges within a loop are half as likely as other edges
static const uint32_t LBH_UNLIKELY_WEIGHT = 62;
/// Unreachable-terminating branch taken probability.
///
/// This is the probability for a branch being taken to a block that terminates
/// (eventually) in unreachable. These are predicted as unlikely as possible.
/// All reachable probability will proportionally share the remaining part.
static const BranchProbability UR_TAKEN_PROB = BranchProbability::getRaw(1);
/// Weight for a branch taken going into a cold block.
///
/// This is the weight for a branch taken toward a block marked
/// cold. A block is marked cold if it's postdominated by a
/// block containing a call to a cold function. Cold functions
/// are those marked with attribute 'cold'.
static const uint32_t CC_TAKEN_WEIGHT = 4;
/// Weight for a branch not-taken into a cold block.
///
/// This is the weight for a branch not taken toward a block marked
/// cold.
static const uint32_t CC_NONTAKEN_WEIGHT = 64;
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;
/// This is the probability for an ordered floating point comparison.
static const uint32_t FPH_ORD_WEIGHT = 1024 * 1024 - 1;
/// This is the probability for an unordered floating point comparison, it means
/// one or two of the operands are NaN. Usually it is used to test for an
/// exceptional case, so the result is unlikely.
static const uint32_t FPH_UNO_WEIGHT = 1;
/// Invoke-terminating normal branch taken weight
///
/// This is the weight for branching to the normal destination of an invoke
/// instruction. We expect this to happen most of the time. Set the weight to an
/// absurdly high value so that nested loops subsume it.
static const uint32_t IH_TAKEN_WEIGHT = 1024 * 1024 - 1;
/// Invoke-terminating normal branch not-taken weight.
///
/// This is the weight for branching to the unwind destination of an invoke
/// instruction. This is essentially never taken.
static const uint32_t IH_NONTAKEN_WEIGHT = 1;
static void UpdatePDTWorklist(const BasicBlock *BB, PostDominatorTree *PDT,
SmallVectorImpl<const BasicBlock *> &WorkList,
SmallPtrSetImpl<const BasicBlock *> &TargetSet) {
SmallVector<BasicBlock *, 8> Descendants;
SmallPtrSet<const BasicBlock *, 16> NewItems;
PDT->getDescendants(const_cast<BasicBlock *>(BB), Descendants);
for (auto *BB : Descendants)
if (TargetSet.insert(BB).second)
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (!TargetSet.count(*PI))
NewItems.insert(*PI);
WorkList.insert(WorkList.end(), NewItems.begin(), NewItems.end());
}
/// Compute a set of basic blocks that are post-dominated by unreachables.
void BranchProbabilityInfo::computePostDominatedByUnreachable(
const Function &F, PostDominatorTree *PDT) {
SmallVector<const BasicBlock *, 8> WorkList;
for (auto &BB : F) {
const Instruction *TI = BB.getTerminator();
if (TI->getNumSuccessors() == 0) {
if (isa<UnreachableInst>(TI) ||
// If this block is terminated by a call to
// @llvm.experimental.deoptimize then treat it like an unreachable
// since the @llvm.experimental.deoptimize call is expected to
// practically never execute.
BB.getTerminatingDeoptimizeCall())
UpdatePDTWorklist(&BB, PDT, WorkList, PostDominatedByUnreachable);
}
}
while (!WorkList.empty()) {
const BasicBlock *BB = WorkList.pop_back_val();
if (PostDominatedByUnreachable.count(BB))
continue;
// If the terminator is an InvokeInst, check only the normal destination
// block as the unwind edge of InvokeInst is also very unlikely taken.
if (auto *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
if (PostDominatedByUnreachable.count(II->getNormalDest()))
UpdatePDTWorklist(BB, PDT, WorkList, PostDominatedByUnreachable);
}
// If all the successors are unreachable, BB is unreachable as well.
else if (!successors(BB).empty() &&
llvm::all_of(successors(BB), [this](const BasicBlock *Succ) {
return PostDominatedByUnreachable.count(Succ);
}))
UpdatePDTWorklist(BB, PDT, WorkList, PostDominatedByUnreachable);
}
}
/// compute a set of basic blocks that are post-dominated by ColdCalls.
void BranchProbabilityInfo::computePostDominatedByColdCall(
const Function &F, PostDominatorTree *PDT) {
SmallVector<const BasicBlock *, 8> WorkList;
for (auto &BB : F)
for (auto &I : BB)
if (const CallInst *CI = dyn_cast<CallInst>(&I))
if (CI->hasFnAttr(Attribute::Cold))
UpdatePDTWorklist(&BB, PDT, WorkList, PostDominatedByColdCall);
while (!WorkList.empty()) {
const BasicBlock *BB = WorkList.pop_back_val();
// If the terminator is an InvokeInst, check only the normal destination
// block as the unwind edge of InvokeInst is also very unlikely taken.
if (auto *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
if (PostDominatedByColdCall.count(II->getNormalDest()))
UpdatePDTWorklist(BB, PDT, WorkList, PostDominatedByColdCall);
}
// If all of successor are post dominated then BB is also done.
else if (!successors(BB).empty() &&
llvm::all_of(successors(BB), [this](const BasicBlock *Succ) {
return PostDominatedByColdCall.count(Succ);
}))
UpdatePDTWorklist(BB, PDT, WorkList, PostDominatedByColdCall);
}
}
/// Calculate edge weights for successors lead to unreachable.
///
/// Predict that a successor which leads necessarily to an
/// unreachable-terminated block as extremely unlikely.
bool BranchProbabilityInfo::calcUnreachableHeuristics(const BasicBlock *BB) {
const Instruction *TI = BB->getTerminator();
(void) TI;
assert(TI->getNumSuccessors() > 1 && "expected more than one successor!");
assert(!isa<InvokeInst>(TI) &&
"Invokes should have already been handled by calcInvokeHeuristics");
SmallVector<unsigned, 4> UnreachableEdges;
SmallVector<unsigned, 4> ReachableEdges;
for (const_succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
if (PostDominatedByUnreachable.count(*I))
UnreachableEdges.push_back(I.getSuccessorIndex());
else
ReachableEdges.push_back(I.getSuccessorIndex());
// Skip probabilities if all were reachable.
if (UnreachableEdges.empty())
return false;
SmallVector<BranchProbability, 4> EdgeProbabilities(
BB->getTerminator()->getNumSuccessors(), BranchProbability::getUnknown());
if (ReachableEdges.empty()) {
BranchProbability Prob(1, UnreachableEdges.size());
for (unsigned SuccIdx : UnreachableEdges)
EdgeProbabilities[SuccIdx] = Prob;
setEdgeProbability(BB, EdgeProbabilities);
return true;
}
auto UnreachableProb = UR_TAKEN_PROB;
auto ReachableProb =
(BranchProbability::getOne() - UR_TAKEN_PROB * UnreachableEdges.size()) /
ReachableEdges.size();
for (unsigned SuccIdx : UnreachableEdges)
EdgeProbabilities[SuccIdx] = UnreachableProb;
for (unsigned SuccIdx : ReachableEdges)
EdgeProbabilities[SuccIdx] = ReachableProb;
setEdgeProbability(BB, EdgeProbabilities);
return true;
}
// Propagate existing explicit probabilities from either profile data or
// 'expect' intrinsic processing. Examine metadata against unreachable
// heuristic. The probability of the edge coming to unreachable block is
// set to min of metadata and unreachable heuristic.
bool BranchProbabilityInfo::calcMetadataWeights(const BasicBlock *BB) {
const Instruction *TI = BB->getTerminator();
assert(TI->getNumSuccessors() > 1 && "expected more than one successor!");
if (!(isa<BranchInst>(TI) || isa<SwitchInst>(TI) || isa<IndirectBrInst>(TI) ||
isa<InvokeInst>(TI)))
return false;
MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
if (!WeightsNode)
return false;
// Check that the number of successors is manageable.
assert(TI->getNumSuccessors() < UINT32_MAX && "Too many successors");
// 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.
// Compute the sum of all weights to later decide whether they need to
// be scaled to fit in 32 bits.
uint64_t WeightSum = 0;
SmallVector<uint32_t, 2> Weights;
SmallVector<unsigned, 2> UnreachableIdxs;
SmallVector<unsigned, 2> ReachableIdxs;
Weights.reserve(TI->getNumSuccessors());
for (unsigned I = 1, E = WeightsNode->getNumOperands(); I != E; ++I) {
ConstantInt *Weight =
mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(I));
if (!Weight)
return false;
assert(Weight->getValue().getActiveBits() <= 32 &&
"Too many bits for uint32_t");
Weights.push_back(Weight->getZExtValue());
WeightSum += Weights.back();
if (PostDominatedByUnreachable.count(TI->getSuccessor(I - 1)))
UnreachableIdxs.push_back(I - 1);
else
ReachableIdxs.push_back(I - 1);
}
assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
// If the sum of weights does not fit in 32 bits, scale every weight down
// accordingly.
uint64_t ScalingFactor =
(WeightSum > UINT32_MAX) ? WeightSum / UINT32_MAX + 1 : 1;
if (ScalingFactor > 1) {
WeightSum = 0;
for (unsigned I = 0, E = TI->getNumSuccessors(); I != E; ++I) {
Weights[I] /= ScalingFactor;
WeightSum += Weights[I];
}
}
assert(WeightSum <= UINT32_MAX &&
"Expected weights to scale down to 32 bits");
if (WeightSum == 0 || ReachableIdxs.size() == 0) {
for (unsigned I = 0, E = TI->getNumSuccessors(); I != E; ++I)
Weights[I] = 1;
WeightSum = TI->getNumSuccessors();
}
// Set the probability.
SmallVector<BranchProbability, 2> BP;
for (unsigned I = 0, E = TI->getNumSuccessors(); I != E; ++I)
BP.push_back({ Weights[I], static_cast<uint32_t>(WeightSum) });
// Examine the metadata against unreachable heuristic.
// If the unreachable heuristic is more strong then we use it for this edge.
if (UnreachableIdxs.size() == 0 || ReachableIdxs.size() == 0) {
setEdgeProbability(BB, BP);
return true;
}
auto UnreachableProb = UR_TAKEN_PROB;
for (auto I : UnreachableIdxs)
if (UnreachableProb < BP[I]) {
BP[I] = UnreachableProb;
}
// Sum of all edge probabilities must be 1.0. If we modified the probability
// of some edges then we must distribute the introduced difference over the
// reachable blocks.
//
// Proportional distribution: the relation between probabilities of the
// reachable edges is kept unchanged. That is for any reachable edges i and j:
// newBP[i] / newBP[j] == oldBP[i] / oldBP[j] =>
// newBP[i] / oldBP[i] == newBP[j] / oldBP[j] == K
// Where K is independent of i,j.
// newBP[i] == oldBP[i] * K
// We need to find K.
// Make sum of all reachables of the left and right parts:
// sum_of_reachable(newBP) == K * sum_of_reachable(oldBP)
// Sum of newBP must be equal to 1.0:
// sum_of_reachable(newBP) + sum_of_unreachable(newBP) == 1.0 =>
// sum_of_reachable(newBP) = 1.0 - sum_of_unreachable(newBP)
// Where sum_of_unreachable(newBP) is what has been just changed.
// Finally:
// K == sum_of_reachable(newBP) / sum_of_reachable(oldBP) =>
// K == (1.0 - sum_of_unreachable(newBP)) / sum_of_reachable(oldBP)
BranchProbability NewUnreachableSum = BranchProbability::getZero();
for (auto I : UnreachableIdxs)
NewUnreachableSum += BP[I];
BranchProbability NewReachableSum =
BranchProbability::getOne() - NewUnreachableSum;
BranchProbability OldReachableSum = BranchProbability::getZero();
for (auto I : ReachableIdxs)
OldReachableSum += BP[I];
if (OldReachableSum != NewReachableSum) { // Anything to dsitribute?
if (OldReachableSum.isZero()) {
// If all oldBP[i] are zeroes then the proportional distribution results
// in all zero probabilities and the error stays big. In this case we
// evenly spread NewReachableSum over the reachable edges.
BranchProbability PerEdge = NewReachableSum / ReachableIdxs.size();
for (auto I : ReachableIdxs)
BP[I] = PerEdge;
} else {
for (auto I : ReachableIdxs) {
// We use uint64_t to avoid double rounding error of the following
// calculation: BP[i] = BP[i] * NewReachableSum / OldReachableSum
// The formula is taken from the private constructor
// BranchProbability(uint32_t Numerator, uint32_t Denominator)
uint64_t Mul = static_cast<uint64_t>(NewReachableSum.getNumerator()) *
BP[I].getNumerator();
uint32_t Div = static_cast<uint32_t>(
divideNearest(Mul, OldReachableSum.getNumerator()));
BP[I] = BranchProbability::getRaw(Div);
}
}
}
setEdgeProbability(BB, BP);
return true;
}
/// Calculate edge weights for edges leading to cold blocks.
///
/// A cold block is one post-dominated by a block with a call to a
/// cold function. Those edges are unlikely to be taken, so we give
/// them relatively low weight.
///
/// Return true if we could compute the weights for cold edges.
/// Return false, otherwise.
bool BranchProbabilityInfo::calcColdCallHeuristics(const BasicBlock *BB) {
const Instruction *TI = BB->getTerminator();
(void) TI;
assert(TI->getNumSuccessors() > 1 && "expected more than one successor!");
assert(!isa<InvokeInst>(TI) &&
"Invokes should have already been handled by calcInvokeHeuristics");
// Determine which successors are post-dominated by a cold block.
SmallVector<unsigned, 4> ColdEdges;
SmallVector<unsigned, 4> NormalEdges;
for (const_succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
if (PostDominatedByColdCall.count(*I))
ColdEdges.push_back(I.getSuccessorIndex());
else
NormalEdges.push_back(I.getSuccessorIndex());
// Skip probabilities if no cold edges.
if (ColdEdges.empty())
return false;
SmallVector<BranchProbability, 4> EdgeProbabilities(
BB->getTerminator()->getNumSuccessors(), BranchProbability::getUnknown());
if (NormalEdges.empty()) {
BranchProbability Prob(1, ColdEdges.size());
for (unsigned SuccIdx : ColdEdges)
EdgeProbabilities[SuccIdx] = Prob;
setEdgeProbability(BB, EdgeProbabilities);
return true;
}
auto ColdProb = BranchProbability::getBranchProbability(
CC_TAKEN_WEIGHT,
(CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) * uint64_t(ColdEdges.size()));
auto NormalProb = BranchProbability::getBranchProbability(
CC_NONTAKEN_WEIGHT,
(CC_TAKEN_WEIGHT + CC_NONTAKEN_WEIGHT) * uint64_t(NormalEdges.size()));
for (unsigned SuccIdx : ColdEdges)
EdgeProbabilities[SuccIdx] = ColdProb;
for (unsigned SuccIdx : NormalEdges)
EdgeProbabilities[SuccIdx] = NormalProb;
setEdgeProbability(BB, EdgeProbabilities);
return true;
}
// Calculate Edge Weights using "Pointer Heuristics". Predict a comparison
// between two pointer or pointer and NULL will fail.
bool BranchProbabilityInfo::calcPointerHeuristics(const BasicBlock *BB) {
const 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());
BranchProbability TakenProb(PH_TAKEN_WEIGHT,
PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT);
BranchProbability UntakenProb(PH_NONTAKEN_WEIGHT,
PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT);
// 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(TakenProb, UntakenProb);
setEdgeProbability(
BB, SmallVector<BranchProbability, 2>({TakenProb, UntakenProb}));
return true;
}
static int getSCCNum(const BasicBlock *BB,
const BranchProbabilityInfo::SccInfo &SccI) {
auto SccIt = SccI.SccNums.find(BB);
if (SccIt == SccI.SccNums.end())
return -1;
return SccIt->second;
}
// Consider any block that is an entry point to the SCC as a header.
static bool isSCCHeader(const BasicBlock *BB, int SccNum,
BranchProbabilityInfo::SccInfo &SccI) {
assert(getSCCNum(BB, SccI) == SccNum);
// Lazily compute the set of headers for a given SCC and cache the results
// in the SccHeaderMap.
if (SccI.SccHeaders.size() <= static_cast<unsigned>(SccNum))
SccI.SccHeaders.resize(SccNum + 1);
auto &HeaderMap = SccI.SccHeaders[SccNum];
bool Inserted;
BranchProbabilityInfo::SccHeaderMap::iterator HeaderMapIt;
std::tie(HeaderMapIt, Inserted) = HeaderMap.insert(std::make_pair(BB, false));
if (Inserted) {
bool IsHeader = llvm::any_of(make_range(pred_begin(BB), pred_end(BB)),
[&](const BasicBlock *Pred) {
return getSCCNum(Pred, SccI) != SccNum;
});
HeaderMapIt->second = IsHeader;
return IsHeader;
} else
return HeaderMapIt->second;
}
// Compute the unlikely successors to the block BB in the loop L, specifically
// those that are unlikely because this is a loop, and add them to the
// UnlikelyBlocks set.
static void
computeUnlikelySuccessors(const BasicBlock *BB, Loop *L,
SmallPtrSetImpl<const BasicBlock*> &UnlikelyBlocks) {
// Sometimes in a loop we have a branch whose condition is made false by
// taking it. This is typically something like
// int n = 0;
// while (...) {
// if (++n >= MAX) {
// n = 0;
// }
// }
// In this sort of situation taking the branch means that at the very least it
// won't be taken again in the next iteration of the loop, so we should
// consider it less likely than a typical branch.
//
// We detect this by looking back through the graph of PHI nodes that sets the
// value that the condition depends on, and seeing if we can reach a successor
// block which can be determined to make the condition false.
//
// FIXME: We currently consider unlikely blocks to be half as likely as other
// blocks, but if we consider the example above the likelyhood is actually
// 1/MAX. We could therefore be more precise in how unlikely we consider
// blocks to be, but it would require more careful examination of the form
// of the comparison expression.
const BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return;
// Check if the branch is based on an instruction compared with a constant
CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
if (!CI || !isa<Instruction>(CI->getOperand(0)) ||
!isa<Constant>(CI->getOperand(1)))
return;
// Either the instruction must be a PHI, or a chain of operations involving
// constants that ends in a PHI which we can then collapse into a single value
// if the PHI value is known.
Instruction *CmpLHS = dyn_cast<Instruction>(CI->getOperand(0));
PHINode *CmpPHI = dyn_cast<PHINode>(CmpLHS);
Constant *CmpConst = dyn_cast<Constant>(CI->getOperand(1));
// Collect the instructions until we hit a PHI
SmallVector<BinaryOperator *, 1> InstChain;
while (!CmpPHI && CmpLHS && isa<BinaryOperator>(CmpLHS) &&
isa<Constant>(CmpLHS->getOperand(1))) {
// Stop if the chain extends outside of the loop
if (!L->contains(CmpLHS))
return;
InstChain.push_back(cast<BinaryOperator>(CmpLHS));
CmpLHS = dyn_cast<Instruction>(CmpLHS->getOperand(0));
if (CmpLHS)
CmpPHI = dyn_cast<PHINode>(CmpLHS);
}
if (!CmpPHI || !L->contains(CmpPHI))
return;
// Trace the phi node to find all values that come from successors of BB
SmallPtrSet<PHINode*, 8> VisitedInsts;
SmallVector<PHINode*, 8> WorkList;
WorkList.push_back(CmpPHI);
VisitedInsts.insert(CmpPHI);
while (!WorkList.empty()) {
PHINode *P = WorkList.back();
WorkList.pop_back();
for (BasicBlock *B : P->blocks()) {
// Skip blocks that aren't part of the loop
if (!L->contains(B))
continue;
Value *V = P->getIncomingValueForBlock(B);
// If the source is a PHI add it to the work list if we haven't
// already visited it.
if (PHINode *PN = dyn_cast<PHINode>(V)) {
if (VisitedInsts.insert(PN).second)
WorkList.push_back(PN);
continue;
}
// If this incoming value is a constant and B is a successor of BB, then
// we can constant-evaluate the compare to see if it makes the branch be
// taken or not.
Constant *CmpLHSConst = dyn_cast<Constant>(V);
if (!CmpLHSConst ||
std::find(succ_begin(BB), succ_end(BB), B) == succ_end(BB))
continue;
// First collapse InstChain
for (Instruction *I : llvm::reverse(InstChain)) {
CmpLHSConst = ConstantExpr::get(I->getOpcode(), CmpLHSConst,
cast<Constant>(I->getOperand(1)), true);
if (!CmpLHSConst)
break;
}
if (!CmpLHSConst)
continue;
// Now constant-evaluate the compare
Constant *Result = ConstantExpr::getCompare(CI->getPredicate(),
CmpLHSConst, CmpConst, true);
// If the result means we don't branch to the block then that block is
// unlikely.
if (Result &&
((Result->isZeroValue() && B == BI->getSuccessor(0)) ||
(Result->isOneValue() && B == BI->getSuccessor(1))))
UnlikelyBlocks.insert(B);
}
}
}
// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
bool BranchProbabilityInfo::calcLoopBranchHeuristics(const BasicBlock *BB,
const LoopInfo &LI,
SccInfo &SccI) {
int SccNum;
Loop *L = LI.getLoopFor(BB);
if (!L) {
SccNum = getSCCNum(BB, SccI);
if (SccNum < 0)
return false;
}
SmallPtrSet<const BasicBlock*, 8> UnlikelyBlocks;
if (L)
computeUnlikelySuccessors(BB, L, UnlikelyBlocks);
SmallVector<unsigned, 8> BackEdges;
SmallVector<unsigned, 8> ExitingEdges;
SmallVector<unsigned, 8> InEdges; // Edges from header to the loop.
SmallVector<unsigned, 8> UnlikelyEdges;
for (const_succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
// Use LoopInfo if we have it, otherwise fall-back to SCC info to catch
// irreducible loops.
if (L) {
if (UnlikelyBlocks.count(*I) != 0)
UnlikelyEdges.push_back(I.getSuccessorIndex());
else if (!L->contains(*I))
ExitingEdges.push_back(I.getSuccessorIndex());
else if (L->getHeader() == *I)
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
} else {
if (getSCCNum(*I, SccI) != SccNum)
ExitingEdges.push_back(I.getSuccessorIndex());
else if (isSCCHeader(*I, SccNum, SccI))
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
}
}
if (BackEdges.empty() && ExitingEdges.empty() && UnlikelyEdges.empty())
return false;
// Collect the sum of probabilities of back-edges/in-edges/exiting-edges, and
// normalize them so that they sum up to one.
unsigned Denom = (BackEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
(InEdges.empty() ? 0 : LBH_TAKEN_WEIGHT) +
(UnlikelyEdges.empty() ? 0 : LBH_UNLIKELY_WEIGHT) +
(ExitingEdges.empty() ? 0 : LBH_NONTAKEN_WEIGHT);
SmallVector<BranchProbability, 4> EdgeProbabilities(
BB->getTerminator()->getNumSuccessors(), BranchProbability::getUnknown());
if (uint32_t numBackEdges = BackEdges.size()) {
BranchProbability TakenProb = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
auto Prob = TakenProb / numBackEdges;
for (unsigned SuccIdx : BackEdges)
EdgeProbabilities[SuccIdx] = Prob;
}
if (uint32_t numInEdges = InEdges.size()) {
BranchProbability TakenProb = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
auto Prob = TakenProb / numInEdges;
for (unsigned SuccIdx : InEdges)
EdgeProbabilities[SuccIdx] = Prob;
}
if (uint32_t numExitingEdges = ExitingEdges.size()) {
BranchProbability NotTakenProb = BranchProbability(LBH_NONTAKEN_WEIGHT,
Denom);
auto Prob = NotTakenProb / numExitingEdges;
for (unsigned SuccIdx : ExitingEdges)
EdgeProbabilities[SuccIdx] = Prob;
}
if (uint32_t numUnlikelyEdges = UnlikelyEdges.size()) {
BranchProbability UnlikelyProb = BranchProbability(LBH_UNLIKELY_WEIGHT,
Denom);
auto Prob = UnlikelyProb / numUnlikelyEdges;
for (unsigned SuccIdx : UnlikelyEdges)
EdgeProbabilities[SuccIdx] = Prob;
}
setEdgeProbability(BB, EdgeProbabilities);
return true;
}
bool BranchProbabilityInfo::calcZeroHeuristics(const BasicBlock *BB,
const TargetLibraryInfo *TLI) {
const 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;
auto GetConstantInt = [](Value *V) {
if (auto *I = dyn_cast<BitCastInst>(V))
return dyn_cast<ConstantInt>(I->getOperand(0));
return dyn_cast<ConstantInt>(V);
};
Value *RHS = CI->getOperand(1);
ConstantInt *CV = GetConstantInt(RHS);
if (!CV)
return false;
// If the LHS is the result of AND'ing a value with a single bit bitmask,
// we don't have information about probabilities.
if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
if (LHS->getOpcode() == Instruction::And)
if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
if (AndRHS->getValue().isPowerOf2())
return false;
// Check if the LHS is the return value of a library function
LibFunc Func = NumLibFuncs;
if (TLI)
if (CallInst *Call = dyn_cast<CallInst>(CI->getOperand(0)))
if (Function *CalledFn = Call->getCalledFunction())
TLI->getLibFunc(*CalledFn, Func);
bool isProb;
if (Func == LibFunc_strcasecmp ||
Func == LibFunc_strcmp ||
Func == LibFunc_strncasecmp ||
Func == LibFunc_strncmp ||
Func == LibFunc_memcmp) {
// strcmp and similar functions return zero, negative, or positive, if the
// first string is equal, less, or greater than the second. We consider it
// likely that the strings are not equal, so a comparison with zero is
// probably false, but also a comparison with any other number is also
// probably false given that what exactly is returned for nonzero values is
// not specified. Any kind of comparison other than equality we know
// nothing about.
switch (CI->getPredicate()) {
case CmpInst::ICMP_EQ:
isProb = false;
break;
case CmpInst::ICMP_NE:
isProb = true;
break;
default:
return false;
}
} else 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->isMinusOne()) {
switch (CI->getPredicate()) {
case CmpInst::ICMP_EQ:
// X == -1 -> Unlikely
isProb = false;
break;
case CmpInst::ICMP_NE:
// X != -1 -> Likely
isProb = true;
break;
case CmpInst::ICMP_SGT:
// InstCombine canonicalizes X >= 0 into X > -1.
// X >= 0 -> Likely
isProb = true;
break;
default:
return false;
}
} else {
return false;
}
BranchProbability TakenProb(ZH_TAKEN_WEIGHT,
ZH_TAKEN_WEIGHT + ZH_NONTAKEN_WEIGHT);
BranchProbability UntakenProb(ZH_NONTAKEN_WEIGHT,
ZH_TAKEN_WEIGHT + ZH_NONTAKEN_WEIGHT);
if (!isProb)
std::swap(TakenProb, UntakenProb);
setEdgeProbability(
BB, SmallVector<BranchProbability, 2>({TakenProb, UntakenProb}));
return true;
}
bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
const 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;
uint32_t TakenWeight = FPH_TAKEN_WEIGHT;
uint32_t NontakenWeight = FPH_NONTAKEN_WEIGHT;
bool isProb;
if (FCmp->isEquality()) {
// f1 == f2 -> Unlikely
// f1 != f2 -> Likely
isProb = !FCmp->isTrueWhenEqual();
} else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
// !isnan -> Likely
isProb = true;
TakenWeight = FPH_ORD_WEIGHT;
NontakenWeight = FPH_UNO_WEIGHT;
} else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
// isnan -> Unlikely
isProb = false;
TakenWeight = FPH_ORD_WEIGHT;
NontakenWeight = FPH_UNO_WEIGHT;
} else {
return false;
}
BranchProbability TakenProb(TakenWeight, TakenWeight + NontakenWeight);
BranchProbability UntakenProb(NontakenWeight, TakenWeight + NontakenWeight);
if (!isProb)
std::swap(TakenProb, UntakenProb);
setEdgeProbability(
BB, SmallVector<BranchProbability, 2>({TakenProb, UntakenProb}));
return true;
}
bool BranchProbabilityInfo::calcInvokeHeuristics(const BasicBlock *BB) {
const InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator());
if (!II)
return false;
BranchProbability TakenProb(IH_TAKEN_WEIGHT,
IH_TAKEN_WEIGHT + IH_NONTAKEN_WEIGHT);
setEdgeProbability(
BB, SmallVector<BranchProbability, 2>({TakenProb, TakenProb.getCompl()}));
return true;
}
void BranchProbabilityInfo::releaseMemory() {
Probs.clear();
Handles.clear();
}
bool BranchProbabilityInfo::invalidate(Function &, const PreservedAnalyses &PA,
FunctionAnalysisManager::Invalidator &) {
// Check whether the analysis, all analyses on functions, or the function's
// CFG have been preserved.
auto PAC = PA.getChecker<BranchProbabilityAnalysis>();
return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
PAC.preservedSet<CFGAnalyses>());
}
void BranchProbabilityInfo::print(raw_ostream &OS) const {
OS << "---- Branch Probabilities ----\n";
// We print the probabilities from the last function the analysis ran over,
// or the function it is currently running over.
assert(LastF && "Cannot print prior to running over a function");
for (const auto &BI : *LastF) {
for (const_succ_iterator SI = succ_begin(&BI), SE = succ_end(&BI); SI != SE;
++SI) {
printEdgeProbability(OS << " ", &BI, *SI);
}
}
}
bool BranchProbabilityInfo::
isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const {
// Hot probability is at least 4/5 = 80%
// FIXME: Compare against a static "hot" BranchProbability.
return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
}
const BasicBlock *
BranchProbabilityInfo::getHotSucc(const BasicBlock *BB) const {
auto MaxProb = BranchProbability::getZero();
const BasicBlock *MaxSucc = nullptr;
for (const_succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
const BasicBlock *Succ = *I;
auto Prob = getEdgeProbability(BB, Succ);
if (Prob > MaxProb) {
MaxProb = Prob;
MaxSucc = Succ;
}
}
// Hot probability is at least 4/5 = 80%
if (MaxProb > BranchProbability(4, 5))
return MaxSucc;
return nullptr;
}
/// Get the raw edge probability for the edge. If can't find it, return a
/// default probability 1/N where N is the number of successors. Here an edge is
/// specified using PredBlock and an
/// index to the successors.
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
unsigned IndexInSuccessors) const {
auto I = Probs.find(std::make_pair(Src, IndexInSuccessors));
if (I != Probs.end())
return I->second;
return {1, static_cast<uint32_t>(succ_size(Src))};
}
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
const_succ_iterator Dst) const {
return getEdgeProbability(Src, Dst.getSuccessorIndex());
}
/// Get the raw edge probability calculated for the block pair. This returns the
/// sum of all raw edge probabilities from Src to Dst.
BranchProbability
BranchProbabilityInfo::getEdgeProbability(const BasicBlock *Src,
const BasicBlock *Dst) const {
auto Prob = BranchProbability::getZero();
bool FoundProb = false;
uint32_t EdgeCount = 0;
for (const_succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
if (*I == Dst) {
++EdgeCount;
auto MapI = Probs.find(std::make_pair(Src, I.getSuccessorIndex()));
if (MapI != Probs.end()) {
FoundProb = true;
Prob += MapI->second;
}
}
uint32_t succ_num = std::distance(succ_begin(Src), succ_end(Src));
return FoundProb ? Prob : BranchProbability(EdgeCount, succ_num);
}
/// Set the edge probability for a given edge specified by PredBlock and an
/// index to the successors.
void BranchProbabilityInfo::setEdgeProbability(const BasicBlock *Src,
unsigned IndexInSuccessors,
BranchProbability Prob) {
Probs[std::make_pair(Src, IndexInSuccessors)] = Prob;
Handles.insert(BasicBlockCallbackVH(Src, this));
LLVM_DEBUG(dbgs() << "set edge " << Src->getName() << " -> "
<< IndexInSuccessors << " successor probability to " << Prob
<< "\n");
}
/// Set the edge probability for all edges at once.
void BranchProbabilityInfo::setEdgeProbability(
const BasicBlock *Src, const SmallVectorImpl<BranchProbability> &Probs) {
assert(Src->getTerminator()->getNumSuccessors() == Probs.size());
if (Probs.size() == 0)
return; // Nothing to set.
uint64_t TotalNumerator = 0;
for (unsigned SuccIdx = 0; SuccIdx < Probs.size(); ++SuccIdx) {
setEdgeProbability(Src, SuccIdx, Probs[SuccIdx]);
TotalNumerator += Probs[SuccIdx].getNumerator();
}
// Because of rounding errors the total probability cannot be checked to be
// 1.0 exactly. That is TotalNumerator == BranchProbability::getDenominator.
// Instead, every single probability in Probs must be as accurate as possible.
// This results in error 1/denominator at most, thus the total absolute error
// should be within Probs.size / BranchProbability::getDenominator.
assert(TotalNumerator <= BranchProbability::getDenominator() + Probs.size());
assert(TotalNumerator >= BranchProbability::getDenominator() - Probs.size());
}
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;
}
void BranchProbabilityInfo::eraseBlock(const BasicBlock *BB) {
for (auto I = Probs.begin(), E = Probs.end(); I != E; ++I) {
auto Key = I->first;
if (Key.first == BB)
Probs.erase(Key);
}
}
void BranchProbabilityInfo::calculate(const Function &F, const LoopInfo &LI,
const TargetLibraryInfo *TLI,
PostDominatorTree *PDT) {
LLVM_DEBUG(dbgs() << "---- Branch Probability Info : " << F.getName()
<< " ----\n\n");
LastF = &F; // Store the last function we ran on for printing.
assert(PostDominatedByUnreachable.empty());
assert(PostDominatedByColdCall.empty());
// Record SCC numbers of blocks in the CFG to identify irreducible loops.
// FIXME: We could only calculate this if the CFG is known to be irreducible
// (perhaps cache this info in LoopInfo if we can easily calculate it there?).
int SccNum = 0;
SccInfo SccI;
for (scc_iterator<const Function *> It = scc_begin(&F); !It.isAtEnd();
++It, ++SccNum) {
// Ignore single-block SCCs since they either aren't loops or LoopInfo will
// catch them.
const std::vector<const BasicBlock *> &Scc = *It;
if (Scc.size() == 1)
continue;
LLVM_DEBUG(dbgs() << "BPI: SCC " << SccNum << ":");
for (auto *BB : Scc) {
LLVM_DEBUG(dbgs() << " " << BB->getName());
SccI.SccNums[BB] = SccNum;
}
LLVM_DEBUG(dbgs() << "\n");
}
std::unique_ptr<PostDominatorTree> PDTPtr;
if (!PDT) {
PDTPtr = std::make_unique<PostDominatorTree>(const_cast<Function &>(F));
PDT = PDTPtr.get();
}
computePostDominatedByUnreachable(F, PDT);
computePostDominatedByColdCall(F, PDT);
// Walk the basic blocks in post-order so that we can build up state about
// the successors of a block iteratively.
for (auto BB : post_order(&F.getEntryBlock())) {
LLVM_DEBUG(dbgs() << "Computing probabilities for " << BB->getName()
<< "\n");
// If there is no at least two successors, no sense to set probability.
if (BB->getTerminator()->getNumSuccessors() < 2)
continue;
if (calcMetadataWeights(BB))
continue;
if (calcInvokeHeuristics(BB))
continue;
if (calcUnreachableHeuristics(BB))
continue;
if (calcColdCallHeuristics(BB))
continue;
if (calcLoopBranchHeuristics(BB, LI, SccI))
continue;
if (calcPointerHeuristics(BB))
continue;
if (calcZeroHeuristics(BB, TLI))
continue;
if (calcFloatingPointHeuristics(BB))
continue;
}
PostDominatedByUnreachable.clear();
PostDominatedByColdCall.clear();
if (PrintBranchProb &&
(PrintBranchProbFuncName.empty() ||
F.getName().equals(PrintBranchProbFuncName))) {
print(dbgs());
}
}
void BranchProbabilityInfoWrapperPass::getAnalysisUsage(
AnalysisUsage &AU) const {
// We require DT so it's available when LI is available. The LI updating code
// asserts that DT is also present so if we don't make sure that we have DT
// here, that assert will trigger.
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<PostDominatorTreeWrapperPass>();
AU.setPreservesAll();
}
bool BranchProbabilityInfoWrapperPass::runOnFunction(Function &F) {
const LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
const TargetLibraryInfo &TLI =
getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
PostDominatorTree &PDT =
getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
BPI.calculate(F, LI, &TLI, &PDT);
return false;
}
void BranchProbabilityInfoWrapperPass::releaseMemory() { BPI.releaseMemory(); }
void BranchProbabilityInfoWrapperPass::print(raw_ostream &OS,
const Module *) const {
BPI.print(OS);
}
AnalysisKey BranchProbabilityAnalysis::Key;
BranchProbabilityInfo
BranchProbabilityAnalysis::run(Function &F, FunctionAnalysisManager &AM) {
BranchProbabilityInfo BPI;
BPI.calculate(F, AM.getResult<LoopAnalysis>(F),
&AM.getResult<TargetLibraryAnalysis>(F),
&AM.getResult<PostDominatorTreeAnalysis>(F));
return BPI;
}
PreservedAnalyses
BranchProbabilityPrinterPass::run(Function &F, FunctionAnalysisManager &AM) {
OS << "Printing analysis results of BPI for function "
<< "'" << F.getName() << "':"
<< "\n";
AM.getResult<BranchProbabilityAnalysis>(F).print(OS);
return PreservedAnalyses::all();
}
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