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9f05fb5e02
The zero heuristic assumes that integers are more likely positive than negative, but this also has the effect of assuming that strcmp return values are more likely positive than negative. Given that for nonzero strcmp return values it's the ordering of arguments that determines the sign of the result there's no reason to assume that's true. Fix this by inspecting the LHS of the compare and using TargetLibraryInfo to decide if it's strcmp-like, and if so only assume that nonzero is more likely than zero i.e. strings are more often different than the same. This causes a slight code generation change in the spec2006 benchmark 403.gcc, but with no noticeable performance impact. The intent of this patch is to allow better optimisation of dhrystone on Cortex-M cpus, but currently it won't as there are also some changes that need to be made to if-conversion. Differential Revision: https://reviews.llvm.org/D33934 llvm-svn: 304970
70 lines
2.6 KiB
C++
70 lines
2.6 KiB
C++
//===- LazyBranchProbabilityInfo.cpp - Lazy Branch Probability Analysis ---===//
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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 is an alternative analysis pass to BranchProbabilityInfoWrapperPass.
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// The difference is that with this pass the branch probabilities are not
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// computed when the analysis pass is executed but rather when the BPI results
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// is explicitly requested by the analysis client.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/LazyBranchProbabilityInfo.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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using namespace llvm;
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#define DEBUG_TYPE "lazy-branch-prob"
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INITIALIZE_PASS_BEGIN(LazyBranchProbabilityInfoPass, DEBUG_TYPE,
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"Lazy Branch Probability Analysis", true, true)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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INITIALIZE_PASS_END(LazyBranchProbabilityInfoPass, DEBUG_TYPE,
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"Lazy Branch Probability Analysis", true, true)
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char LazyBranchProbabilityInfoPass::ID = 0;
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LazyBranchProbabilityInfoPass::LazyBranchProbabilityInfoPass()
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: FunctionPass(ID) {
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initializeLazyBranchProbabilityInfoPassPass(*PassRegistry::getPassRegistry());
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}
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void LazyBranchProbabilityInfoPass::print(raw_ostream &OS,
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const Module *) const {
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LBPI->getCalculated().print(OS);
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}
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void LazyBranchProbabilityInfoPass::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addRequired<TargetLibraryInfoWrapperPass>();
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AU.setPreservesAll();
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}
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void LazyBranchProbabilityInfoPass::releaseMemory() { LBPI.reset(); }
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bool LazyBranchProbabilityInfoPass::runOnFunction(Function &F) {
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LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
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LBPI = llvm::make_unique<LazyBranchProbabilityInfo>(&F, &LI, &TLI);
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return false;
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}
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void LazyBranchProbabilityInfoPass::getLazyBPIAnalysisUsage(AnalysisUsage &AU) {
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AU.addRequired<LazyBranchProbabilityInfoPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addRequired<TargetLibraryInfoWrapperPass>();
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}
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void llvm::initializeLazyBPIPassPass(PassRegistry &Registry) {
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INITIALIZE_PASS_DEPENDENCY(LazyBranchProbabilityInfoPass);
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass);
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INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass);
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}
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