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llvm-mirror/lib/Analysis/BranchProbabilityInfo.cpp
Chandler Carruth 1c28e7f745 [TI removal] Make variables declared as TerminatorInst and initialized 
by `getTerminator()` calls instead be declared as `Instruction`.

This is the biggest remaining chunk of the usage of `getTerminator()`
that insists on the narrow type and so is an easy batch of updates.
Several files saw more extensive updates where this would cascade to
requiring API updates within the file to use `Instruction` instead of
`TerminatorInst`. All of these were trivial in nature (pervasively using
`Instruction` instead just worked).

llvm-svn: 344502
2018-10-15 10:04:59 +00:00

1040 lines
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//===- BranchProbabilityInfo.cpp - Branch Probability Analysis ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Loops should be simplified before this analysis.
//
//===----------------------------------------------------------------------===//
#include "llvm/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/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/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.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_END(BranchProbabilityInfoWrapperPass, "branch-prob",
"Branch Probability Analysis", false, true)
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 equally 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;
/// 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;
/// Add \p BB to PostDominatedByUnreachable set if applicable.
void
BranchProbabilityInfo::updatePostDominatedByUnreachable(const BasicBlock *BB) {
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())
PostDominatedByUnreachable.insert(BB);
return;
}
// 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>(TI)) {
if (PostDominatedByUnreachable.count(II->getNormalDest()))
PostDominatedByUnreachable.insert(BB);
return;
}
for (auto *I : successors(BB))
// If any of successor is not post dominated then BB is also not.
if (!PostDominatedByUnreachable.count(I))
return;
PostDominatedByUnreachable.insert(BB);
}
/// Add \p BB to PostDominatedByColdCall set if applicable.
void
BranchProbabilityInfo::updatePostDominatedByColdCall(const BasicBlock *BB) {
assert(!PostDominatedByColdCall.count(BB));
const Instruction *TI = BB->getTerminator();
if (TI->getNumSuccessors() == 0)
return;
// If all of successor are post dominated then BB is also done.
if (llvm::all_of(successors(BB), [&](const BasicBlock *SuccBB) {
return PostDominatedByColdCall.count(SuccBB);
})) {
PostDominatedByColdCall.insert(BB);
return;
}
// 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>(TI))
if (PostDominatedByColdCall.count(II->getNormalDest())) {
PostDominatedByColdCall.insert(BB);
return;
}
// Otherwise, if the block itself contains a cold function, add it to the
// set of blocks post-dominated by a cold call.
for (auto &I : *BB)
if (const CallInst *CI = dyn_cast<CallInst>(&I))
if (CI->hasFnAttr(Attribute::Cold)) {
PostDominatedByColdCall.insert(BB);
return;
}
}
/// 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 (succ_const_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;
if (ReachableEdges.empty()) {
BranchProbability Prob(1, UnreachableEdges.size());
for (unsigned SuccIdx : UnreachableEdges)
setEdgeProbability(BB, SuccIdx, Prob);
return true;
}
auto UnreachableProb = UR_TAKEN_PROB;
auto ReachableProb =
(BranchProbability::getOne() - UR_TAKEN_PROB * UnreachableEdges.size()) /
ReachableEdges.size();
for (unsigned SuccIdx : UnreachableEdges)
setEdgeProbability(BB, SuccIdx, UnreachableProb);
for (unsigned SuccIdx : ReachableEdges)
setEdgeProbability(BB, SuccIdx, ReachableProb);
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)))
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) {
auto ToDistribute = BranchProbability::getZero();
auto UnreachableProb = UR_TAKEN_PROB;
for (auto i : UnreachableIdxs)
if (UnreachableProb < BP[i]) {
ToDistribute += BP[i] - UnreachableProb;
BP[i] = UnreachableProb;
}
// If we modified the probability of some edges then we must distribute
// the difference between reachable blocks.
if (ToDistribute > BranchProbability::getZero()) {
BranchProbability PerEdge = ToDistribute / ReachableIdxs.size();
for (auto i : ReachableIdxs)
BP[i] += PerEdge;
}
}
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
setEdgeProbability(BB, i, BP[i]);
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 (succ_const_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;
if (NormalEdges.empty()) {
BranchProbability Prob(1, ColdEdges.size());
for (unsigned SuccIdx : ColdEdges)
setEdgeProbability(BB, SuccIdx, Prob);
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)
setEdgeProbability(BB, SuccIdx, ColdProb);
for (unsigned SuccIdx : NormalEdges)
setEdgeProbability(BB, SuccIdx, NormalProb);
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());
// p != 0 -> isProb = true
// p == 0 -> isProb = false
// p != q -> isProb = true
// p == q -> isProb = false;
unsigned TakenIdx = 0, NonTakenIdx = 1;
bool isProb = CI->getPredicate() == ICmpInst::ICMP_NE;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(PH_TAKEN_WEIGHT,
PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
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 (succ_const_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);
if (uint32_t numBackEdges = BackEdges.size()) {
BranchProbability TakenProb = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
auto Prob = TakenProb / numBackEdges;
for (unsigned SuccIdx : BackEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
if (uint32_t numInEdges = InEdges.size()) {
BranchProbability TakenProb = BranchProbability(LBH_TAKEN_WEIGHT, Denom);
auto Prob = TakenProb / numInEdges;
for (unsigned SuccIdx : InEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
if (uint32_t numExitingEdges = ExitingEdges.size()) {
BranchProbability NotTakenProb = BranchProbability(LBH_NONTAKEN_WEIGHT,
Denom);
auto Prob = NotTakenProb / numExitingEdges;
for (unsigned SuccIdx : ExitingEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
if (uint32_t numUnlikelyEdges = UnlikelyEdges.size()) {
BranchProbability UnlikelyProb = BranchProbability(LBH_UNLIKELY_WEIGHT,
Denom);
auto Prob = UnlikelyProb / numUnlikelyEdges;
for (unsigned SuccIdx : UnlikelyEdges)
setEdgeProbability(BB, SuccIdx, Prob);
}
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;
Value *RHS = CI->getOperand(1);
ConstantInt *CV = dyn_cast<ConstantInt>(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;
}
unsigned TakenIdx = 0, NonTakenIdx = 1;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(ZH_TAKEN_WEIGHT,
ZH_TAKEN_WEIGHT + ZH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
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;
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;
} else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
// isnan -> Unlikely
isProb = false;
} else {
return false;
}
unsigned TakenIdx = 0, NonTakenIdx = 1;
if (!isProb)
std::swap(TakenIdx, NonTakenIdx);
BranchProbability TakenProb(FPH_TAKEN_WEIGHT,
FPH_TAKEN_WEIGHT + FPH_NONTAKEN_WEIGHT);
setEdgeProbability(BB, TakenIdx, TakenProb);
setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
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, 0 /*Index for Normal*/, TakenProb);
setEdgeProbability(BB, 1 /*Index for Unwind*/, TakenProb.getCompl());
return true;
}
void BranchProbabilityInfo::releaseMemory() {
Probs.clear();
}
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 (succ_const_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 (succ_const_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,
succ_const_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;
for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
if (*I == Dst) {
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(1, 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");
}
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) {
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");
}
// 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");
updatePostDominatedByUnreachable(BB);
updatePostDominatedByColdCall(BB);
// 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.setPreservesAll();
}
bool BranchProbabilityInfoWrapperPass::runOnFunction(Function &F) {
const LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
BPI.calculate(F, LI, &TLI);
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));
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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