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119 lines
3.9 KiB
C++
119 lines
3.9 KiB
C++
//===- MachineBranchProbabilityInfo.cpp - Machine Branch Probability Info -===//
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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 analysis uses probability info stored in Machine Basic Blocks.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Instructions.h"
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#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
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#include "llvm/CodeGen/MachineBasicBlock.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
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"Machine Branch Probability Analysis", false, true)
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INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
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"Machine Branch Probability Analysis", false, true)
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char MachineBranchProbabilityInfo::ID = 0;
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void MachineBranchProbabilityInfo::anchor() { }
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uint32_t MachineBranchProbabilityInfo::
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getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
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// First we compute the sum with 64-bits of precision, ensuring that cannot
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// overflow by bounding the number of weights considered. Hopefully no one
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// actually needs 2^32 successors.
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assert(MBB->succ_size() < UINT32_MAX);
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uint64_t Sum = 0;
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Scale = 1;
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for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
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E = MBB->succ_end(); I != E; ++I) {
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uint32_t Weight = getEdgeWeight(MBB, *I);
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Sum += Weight;
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}
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// If the computed sum fits in 32-bits, we're done.
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if (Sum <= UINT32_MAX)
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return Sum;
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// Otherwise, compute the scale necessary to cause the weights to fit, and
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// re-sum with that scale applied.
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assert((Sum / UINT32_MAX) < UINT32_MAX);
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Scale = (Sum / UINT32_MAX) + 1;
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Sum = 0;
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for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
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E = MBB->succ_end(); I != E; ++I) {
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uint32_t Weight = getEdgeWeight(MBB, *I);
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Sum += Weight / Scale;
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}
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assert(Sum <= UINT32_MAX);
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return Sum;
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}
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uint32_t
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MachineBranchProbabilityInfo::getEdgeWeight(const MachineBasicBlock *Src,
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const MachineBasicBlock *Dst) const {
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uint32_t Weight = Src->getSuccWeight(Dst);
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if (!Weight)
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return DEFAULT_WEIGHT;
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return Weight;
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}
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bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
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MachineBasicBlock *Dst) const {
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// Hot probability is at least 4/5 = 80%
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// FIXME: Compare against a static "hot" BranchProbability.
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return getEdgeProbability(Src, Dst) > BranchProbability(4, 5);
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}
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MachineBasicBlock *
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MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
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uint32_t MaxWeight = 0;
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MachineBasicBlock *MaxSucc = 0;
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for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
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E = MBB->succ_end(); I != E; ++I) {
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uint32_t Weight = getEdgeWeight(MBB, *I);
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if (Weight > MaxWeight) {
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MaxWeight = Weight;
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MaxSucc = *I;
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}
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}
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if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
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return MaxSucc;
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return 0;
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}
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BranchProbability
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MachineBranchProbabilityInfo::getEdgeProbability(MachineBasicBlock *Src,
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MachineBasicBlock *Dst) const {
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uint32_t Scale = 1;
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uint32_t D = getSumForBlock(Src, Scale);
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uint32_t N = getEdgeWeight(Src, Dst) / Scale;
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return BranchProbability(N, D);
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}
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raw_ostream &MachineBranchProbabilityInfo::
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printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
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MachineBasicBlock *Dst) const {
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const BranchProbability Prob = getEdgeProbability(Src, Dst);
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OS << "edge MBB#" << Src->getNumber() << " -> MBB#" << Dst->getNumber()
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<< " probability is " << Prob
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<< (isEdgeHot(Src, Dst) ? " [HOT edge]\n" : "\n");
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return OS;
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}
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