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https://github.com/c64scene-ar/llvm-6502.git
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e7a1e3ee82
This change is the first in a series of changes improving LLVM's Block Frequency propogation implementation to not lose probability mass in branchy code when propogating block frequency information from a basic block to its successors. This patch is a simple infrastructure improvement that does not actually modify the block frequency algorithm. The specific changes are: 1. Changes the division algorithm used when scaling block frequencies by branch probabilities to a short division algorithm. This gives us the remainder for free as well as provides a nice speed boost. When I benched the old routine and the new routine on a Sandy Bridge iMac with disabled turbo mode performing 8192 iterations on an array of length 32768, I saw ~600% increase in speed in mean/median performance. 2. Exposes a scale method that returns a remainder. This is important so we can ensure that when we scale a block frequency by some branch probability BP = N/D, the remainder from the division by D can be retrieved and propagated to other children to ensure no probability mass is lost (more to come on this). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194950 91177308-0d34-0410-b5e6-96231b3b80d8
90 lines
2.8 KiB
C++
90 lines
2.8 KiB
C++
//===-------- BlockFrequency.h - Block Frequency Wrapper --------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements Block Frequency class.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_SUPPORT_BLOCKFREQUENCY_H
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#define LLVM_SUPPORT_BLOCKFREQUENCY_H
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#include "llvm/Support/DataTypes.h"
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namespace llvm {
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class raw_ostream;
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class BranchProbability;
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// This class represents Block Frequency as a 64-bit value.
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class BlockFrequency {
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uint64_t Frequency;
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static const int64_t ENTRY_FREQ = 1 << 14;
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/// \brief Scale the given BlockFrequency by N/D. Return the remainder from
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/// the division by D. Upon overflow, the routine will saturate and
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/// additionally will return the remainder set to D.
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uint32_t scale(uint32_t N, uint32_t D);
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public:
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BlockFrequency(uint64_t Freq = 0) : Frequency(Freq) { }
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/// \brief Returns the frequency of the entry block of the function.
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static uint64_t getEntryFrequency() { return ENTRY_FREQ; }
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/// \brief Returns the maximum possible frequency, the saturation value.
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static uint64_t getMaxFrequency() { return -1ULL; }
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/// \brief Returns the frequency as a fixpoint number scaled by the entry
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/// frequency.
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uint64_t getFrequency() const { return Frequency; }
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/// \brief Multiplies with a branch probability. The computation will never
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/// overflow.
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BlockFrequency &operator*=(const BranchProbability &Prob);
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const BlockFrequency operator*(const BranchProbability &Prob) const;
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/// \brief Divide by a non-zero branch probability using saturating
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/// arithmetic.
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BlockFrequency &operator/=(const BranchProbability &Prob);
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BlockFrequency operator/(const BranchProbability &Prob) const;
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/// \brief Adds another block frequency using saturating arithmetic.
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BlockFrequency &operator+=(const BlockFrequency &Freq);
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const BlockFrequency operator+(const BlockFrequency &Freq) const;
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/// \brief Scale the given BlockFrequency by N/D. Return the remainder from
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/// the division by D. Upon overflow, the routine will saturate.
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uint32_t scale(const BranchProbability &Prob);
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bool operator<(const BlockFrequency &RHS) const {
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return Frequency < RHS.Frequency;
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}
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bool operator<=(const BlockFrequency &RHS) const {
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return Frequency <= RHS.Frequency;
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}
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bool operator>(const BlockFrequency &RHS) const {
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return Frequency > RHS.Frequency;
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}
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bool operator>=(const BlockFrequency &RHS) const {
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return Frequency >= RHS.Frequency;
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}
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void print(raw_ostream &OS) const;
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};
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raw_ostream &operator<<(raw_ostream &OS, const BlockFrequency &Freq);
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}
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#endif
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