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Add a small pass that sets the readnone/readonly
attributes on functions, based on the result of alias analysis. It's not hardwired to use GlobalsModRef even though this is the only (AFAIK) alias analysis that results in this pass actually doing something. Enable as follows: opt ... -globalsmodref-aa -markmodref ... Advantages of this pass: (1) records the result of globalsmodref in the bitcode, meaning it is available for use by later passes (currently the pass manager isn't smart enough to magically make an advanced alias analysis available to all later passes), which may expose more optimization opportunities; (2) hopefully speeds up compilation when code is optimized twice, for example when a file is compiled to bitcode, then later LTO is done on it: marking functions readonly/readnone when producing the initial bitcode should speed up alias analysis during LTO; (3) good for discovering that globalsmodref doesn't work very well :) Not currently turned on by default. llvm-svn: 55604
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@ -85,6 +85,7 @@ namespace {
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(void) llvm::createLowerInvokePass();
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(void) llvm::createLowerSetJmpPass();
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(void) llvm::createLowerSwitchPass();
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(void) llvm::createMarkModRefPass();
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(void) llvm::createNoAAPass();
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(void) llvm::createNoProfileInfoPass();
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(void) llvm::createProfileLoaderPass();
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@ -137,6 +137,11 @@ LoopPass *createLoopRotatePass();
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//
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LoopPass *createLoopIndexSplitPass();
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//===----------------------------------------------------------------------===//
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//
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// MarkModRef - This pass marks functions readnone/readonly.
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//
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FunctionPass *createMarkModRefPass();
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//===----------------------------------------------------------------------===//
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//
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69
lib/Transforms/Scalar/MarkModRef.cpp
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69
lib/Transforms/Scalar/MarkModRef.cpp
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@ -0,0 +1,69 @@
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//===--------- MarkModRef.cpp - Mark functions readnone/readonly ----------===//
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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 pass marks functions readnone/readonly based on the results of alias
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// analysis. This requires a sufficiently powerful alias analysis, such as
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// GlobalsModRef (invoke as "opt ... -globalsmodref-aa -markmodref ...").
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "markmodref"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Function.h"
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#include "llvm/Pass.h"
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using namespace llvm;
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STATISTIC(NumReadNone, "Number of functions marked readnone");
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STATISTIC(NumReadOnly, "Number of functions marked readonly");
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namespace {
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struct VISIBILITY_HIDDEN MarkModRef : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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MarkModRef() : FunctionPass((intptr_t)&ID) {}
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bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequired<AliasAnalysis>();
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AU.addPreserved<AliasAnalysis>();
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}
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};
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}
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char MarkModRef::ID = 0;
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static RegisterPass<MarkModRef>
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X("markmodref", "Mark functions readnone/readonly");
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bool MarkModRef::runOnFunction(Function &F) {
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// FIXME: Wrong for functions with weak linkage.
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if (F.doesNotAccessMemory())
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// Cannot do better.
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return false;
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AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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AliasAnalysis::ModRefBehavior ModRef = AA.getModRefBehavior(&F);
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if (ModRef == AliasAnalysis::DoesNotAccessMemory) {
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F.setDoesNotAccessMemory();
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NumReadNone++;
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return true;
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} else if (ModRef == AliasAnalysis::OnlyReadsMemory && !F.onlyReadsMemory()) {
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F.setOnlyReadsMemory();
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NumReadOnly++;
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return true;
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}
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return false;
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}
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FunctionPass *llvm::createMarkModRefPass() {
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return new MarkModRef();
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}
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