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Branch profiling: floating-point avoidance.

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Patch by: Jakub Staszak!

Introduces BranchProbability. Changes unsigned to uint32_t all over and
uint64_t only when overflow is expected.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132867 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrew Trick 2011-06-11 01:05:22 +00:00
parent b5923db192
commit f289df2d95
5 changed files with 176 additions and 62 deletions
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26
include/llvm/Analysis/BranchProbabilityInfo.h
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@ -15,19 +15,24 @@
#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
#include "llvm/InitializePasses.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Support/Debug.h"
namespace llvm {
class raw_ostream;
class BranchProbabilityInfo : public FunctionPass {
// Default weight value. Used when we don't have information about the edge.
static const unsigned int DEFAULT_WEIGHT = 16;
static const uint32_t DEFAULT_WEIGHT = 16;
typedef std::pair<BasicBlock *, BasicBlock *> Edge;
DenseMap<Edge, unsigned> Weights;
DenseMap<Edge, uint32_t> Weights;
// Get sum of the block successors' weights.
uint32_t getSumForBlock(BasicBlock *BB) const;
public:
static char ID;
@ -43,12 +48,12 @@ public:
bool runOnFunction(Function &F);
// Returned value is between 1 and UINT_MAX. Look at BranchProbabilityInfo.cpp
// for details.
unsigned getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const;
// Returned value is between 1 and UINT32_MAX. Look at
// BranchProbabilityInfo.cpp for details.
uint32_t getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const;
// Look at BranchProbabilityInfo.cpp for details. Use it with caution!
void setEdgeWeight(BasicBlock *Src, BasicBlock *Dst, unsigned Weight);
void setEdgeWeight(BasicBlock *Src, BasicBlock *Dst, uint32_t Weight);
// A 'Hot' edge is an edge which probability is >= 80%.
bool isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const;
@ -56,11 +61,16 @@ public:
// Return a hot successor for the block BB or null if there isn't one.
BasicBlock *getHotSucc(BasicBlock *BB) const;
// Return a probability as a fraction between 0 (0% probability) and
// 1 (100% probability), however the value is never equal to 0, and can be 1
// only iff SRC block has only one successor.
BranchProbability getEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const;
// Print value between 0 (0% probability) and 1 (100% probability),
// however the value is never equal to 0, and can be 1 only iff SRC block
// has only one successor.
raw_ostream &printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
BasicBlock *Dst) const;
BasicBlock *Dst) const;
};
}

50
include/llvm/Support/BranchProbability.h Normal file
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@ -0,0 +1,50 @@
//===- BranchProbability.h - Branch Probability Analysis --------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Definition of BranchProbability shared by IR and Machine Instructions.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_SUPPORT_BRANCHPROBABILITY_H
#define LLVM_SUPPORT_BRANCHPROBABILITY_H
#include "llvm/Support/DataTypes.h"
namespace llvm {
class raw_ostream;
class BranchProbabilityInfo;
class MachineBranchProbabilityInfo;
class MachineBasicBlock;
// This class represents Branch Probability as a non-negative fraction.
class BranchProbability {
friend class BranchProbabilityInfo;
friend class MachineBranchProbabilityInfo;
friend class MachineBasicBlock;
// Numerator
uint32_t N;
// Denominator
uint32_t D;
BranchProbability(uint32_t n, uint32_t d);
public:
raw_ostream &print(raw_ostream &OS) const;
void dump() const;
};
raw_ostream &operator<<(raw_ostream &OS, const BranchProbability &Prob);
}
#endif

117
lib/Analysis/BranchProbabilityInfo.cpp
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@ -13,7 +13,7 @@
#include "llvm/Instructions.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include <climits>
#include "llvm/Support/Debug.h"
using namespace llvm;
@ -34,7 +34,7 @@ class BranchProbabilityAnalysis {
typedef std::pair<BasicBlock *, BasicBlock *> Edge;
DenseMap<Edge, unsigned> *Weights;
DenseMap<Edge, uint32_t> *Weights;
BranchProbabilityInfo *BP;
@ -62,15 +62,15 @@ class BranchProbabilityAnalysis {
// Probability of the edge BB2->BB1 = 128 / (128 + 4) = 0.9696..
// Probability of the edge BB2->BB3 = 4 / (128 + 4) = 0.0303..
static const unsigned int LBH_TAKEN_WEIGHT = 128;
static const unsigned int LBH_NONTAKEN_WEIGHT = 4;
static const uint32_t LBH_TAKEN_WEIGHT = 128;
static const uint32_t LBH_NONTAKEN_WEIGHT = 4;
// Standard weight value. Used when none of the heuristics set weight for
// the edge.
static const unsigned int NORMAL_WEIGHT = 16;
static const uint32_t NORMAL_WEIGHT = 16;
// Minimum weight of an edge. Please note, that weight is NEVER 0.
static const unsigned int MIN_WEIGHT = 1;
static const uint32_t MIN_WEIGHT = 1;
// Return TRUE if BB leads directly to a Return Instruction.
static bool isReturningBlock(BasicBlock *BB) {
@ -101,8 +101,8 @@ class BranchProbabilityAnalysis {
// Multiply Edge Weight by two.
void incEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
unsigned Weight = BP->getEdgeWeight(Src, Dst);
unsigned MaxWeight = getMaxWeightFor(Src);
uint32_t Weight = BP->getEdgeWeight(Src, Dst);
uint32_t MaxWeight = getMaxWeightFor(Src);
if (Weight * 2 > MaxWeight)
BP->setEdgeWeight(Src, Dst, MaxWeight);
@ -112,7 +112,7 @@ class BranchProbabilityAnalysis {
// Divide Edge Weight by two.
void decEdgeWeight(BasicBlock *Src, BasicBlock *Dst) {
unsigned Weight = BP->getEdgeWeight(Src, Dst);
uint32_t Weight = BP->getEdgeWeight(Src, Dst);
assert(Weight > 0);
if (Weight / 2 < MIN_WEIGHT)
@ -122,12 +122,12 @@ class BranchProbabilityAnalysis {
}
unsigned getMaxWeightFor(BasicBlock *BB) const {
return UINT_MAX / BB->getTerminator()->getNumSuccessors();
uint32_t getMaxWeightFor(BasicBlock *BB) const {
return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
}
public:
BranchProbabilityAnalysis(DenseMap<Edge, unsigned> *W,
BranchProbabilityAnalysis(DenseMap<Edge, uint32_t> *W,
BranchProbabilityInfo *BP, LoopInfo *LI)
: Weights(W), BP(BP), LI(LI) {
}
@ -195,7 +195,7 @@ void BranchProbabilityAnalysis::calcPointerHeuristics(BasicBlock *BB) {
// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
unsigned numSuccs = BB->getTerminator()->getNumSuccessors();
uint32_t numSuccs = BB->getTerminator()->getNumSuccessors();
Loop *L = LI->getLoopFor(BB);
if (!L)
@ -213,8 +213,8 @@ void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
BackEdges.push_back(Succ);
}
if (unsigned numBackEdges = BackEdges.size()) {
unsigned backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
if (uint32_t numBackEdges = BackEdges.size()) {
uint32_t backWeight = LBH_TAKEN_WEIGHT / numBackEdges;
if (backWeight < NORMAL_WEIGHT)
backWeight = NORMAL_WEIGHT;
@ -225,9 +225,9 @@ void BranchProbabilityAnalysis::calcLoopBranchHeuristics(BasicBlock *BB) {
}
}
unsigned numExitingEdges = ExitingEdges.size();
if (unsigned numNonExitingEdges = numSuccs - numExitingEdges) {
unsigned exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
uint32_t numExitingEdges = ExitingEdges.size();
if (uint32_t numNonExitingEdges = numSuccs - numExitingEdges) {
uint32_t exitWeight = LBH_NONTAKEN_WEIGHT / numNonExitingEdges;
if (exitWeight < MIN_WEIGHT)
exitWeight = MIN_WEIGHT;
@ -260,36 +260,43 @@ bool BranchProbabilityAnalysis::runOnFunction(Function &F) {
bool BranchProbabilityInfo::runOnFunction(Function &F) {
LoopInfo &LI = getAnalysis<LoopInfo>();
BranchProbabilityAnalysis BPA(&Weights, this, &LI);
bool ret = BPA.runOnFunction(F);
return ret;
return BPA.runOnFunction(F);
}
// TODO: This currently hardcodes 80% as a fraction 4/5. We will soon add a
// BranchProbability class to encapsulate the fractional probability and
// define a few static instances of the class for use as predefined thresholds.
bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
unsigned Sum = 0;
for (succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I) {
uint32_t BranchProbabilityInfo::getSumForBlock(BasicBlock *BB) const {
uint32_t Sum = 0;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
unsigned Weight = getEdgeWeight(Src, Succ);
unsigned PrevSum = Sum;
uint32_t Weight = getEdgeWeight(BB, Succ);
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
}
return getEdgeWeight(Src, Dst) * 5 > Sum * 4;
return Sum;
}
bool BranchProbabilityInfo::isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const {
// Hot probability is at least 4/5 = 80%
uint32_t Weight = getEdgeWeight(Src, Dst);
uint32_t Sum = getSumForBlock(Src);
// FIXME: Implement BranchProbability::compare then change this code to
// compare this BranchProbability against a static "hot" BranchProbability.
return (uint64_t)Weight * 5 > (uint64_t)Sum * 4;
}
BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
unsigned Sum = 0;
unsigned MaxWeight = 0;
uint32_t Sum = 0;
uint32_t MaxWeight = 0;
BasicBlock *MaxSucc = 0;
for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
BasicBlock *Succ = *I;
unsigned Weight = getEdgeWeight(BB, Succ);
unsigned PrevSum = Sum;
uint32_t Weight = getEdgeWeight(BB, Succ);
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
@ -300,17 +307,18 @@ BasicBlock *BranchProbabilityInfo::getHotSucc(BasicBlock *BB) const {
}
}
if (MaxWeight * 5 > Sum * 4)
// FIXME: Use BranchProbability::compare.
if ((uint64_t)MaxWeight * 5 > (uint64_t)Sum * 4)
return MaxSucc;
return 0;
}
// Return edge's weight. If can't find it, return DEFAULT_WEIGHT value.
unsigned
uint32_t
BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
Edge E(Src, Dst);
DenseMap<Edge, unsigned>::const_iterator I = Weights.find(E);
DenseMap<Edge, uint32_t>::const_iterator I = Weights.find(E);
if (I != Weights.end())
return I->second;
@ -319,30 +327,31 @@ BranchProbabilityInfo::getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const {
}
void BranchProbabilityInfo::setEdgeWeight(BasicBlock *Src, BasicBlock *Dst,
unsigned Weight) {
uint32_t Weight) {
Weights[std::make_pair(Src, Dst)] = Weight;
DEBUG(dbgs() << "setEdgeWeight: " << Src->getNameStr() << " -> "
<< Dst->getNameStr() << " to " << Weight
<< (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
DEBUG(dbgs() << "set edge " << Src->getNameStr() << " -> "
<< Dst->getNameStr() << " weight to " << Weight
<< (isEdgeHot(Src, Dst) ? " [is HOT now]\n" : "\n"));
}
BranchProbability BranchProbabilityInfo::
getEdgeProbability(BasicBlock *Src, BasicBlock *Dst) const {
uint32_t N = getEdgeWeight(Src, Dst);
uint32_t D = getSumForBlock(Src);
return BranchProbability(N, D);
}
raw_ostream &
BranchProbabilityInfo::printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
BasicBlock *Dst) const {
BasicBlock *Dst) const {
BranchProbability Prob = getEdgeProbability(Src, Dst);
unsigned Sum = 0;
for (succ_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I) {
BasicBlock *Succ = *I;
unsigned Weight = getEdgeWeight(Src, Succ);
unsigned PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
}
double Prob = (double)getEdgeWeight(Src, Dst) / Sum;
OS << "probability (" << Src->getNameStr() << " --> " << Dst->getNameStr()
<< ") = " << Prob << "\n";
OS << "edge " << Src->getNameStr() << " -> " << Dst->getNameStr()
<< " probability is " << Prob
<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
return OS;
}

44
lib/Support/BranchProbability.cpp Normal file
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@ -0,0 +1,44 @@
//===-------------- lib/Support/BranchProbability.cpp -----------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Branch Probability class.
//
//===----------------------------------------------------------------------===//
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
BranchProbability::BranchProbability(uint32_t n, uint32_t d) {
assert(d > 0 && "Denomiator cannot be 0!");
assert(n <= d && "Probability cannot be bigger than 1!");
N = n;
D = d;
}
raw_ostream &BranchProbability::print(raw_ostream &OS) const {
OS << N << " / " << D << " = " << ((double)N / D);
return OS;
}
void BranchProbability::dump() const {
print(dbgs());
dbgs() << "\n";
}
namespace llvm {
raw_ostream &operator<<(raw_ostream &OS, const BranchProbability &Prob) {
Prob.print(OS);
return OS;
}
}

1
lib/Support/CMakeLists.txt
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@ -9,6 +9,7 @@ add_llvm_library(LLVMSupport
APInt.cpp
APSInt.cpp
Allocator.cpp
BranchProbability.cpp
circular_raw_ostream.cpp
CommandLine.cpp
ConstantRange.cpp