forked from OSchip/llvm-project
381 lines
13 KiB
C++
381 lines
13 KiB
C++
//===- FunctionAttrs.cpp - Pass which marks functions readnone or 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 file implements a simple interprocedural pass which walks the
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// call-graph, looking for functions which do not access or only read
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// non-local memory, and marking them readnone/readonly. In addition,
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// it marks function arguments (of pointer type) 'nocapture' if a call
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// to the function does not create any copies of the pointer value that
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// outlive the call. This more or less means that the pointer is only
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// dereferenced, and not returned from the function or stored in a global.
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// This pass is implemented as a bottom-up traversal of the call-graph.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "functionattrs"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/CallGraphSCCPass.h"
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#include "llvm/GlobalVariable.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/CallGraph.h"
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#include "llvm/Analysis/CaptureTracking.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/UniqueVector.h"
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#include "llvm/Support/InstIterator.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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STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
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STATISTIC(NumNoAlias, "Number of function returns marked noalias");
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namespace {
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struct FunctionAttrs : public CallGraphSCCPass {
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static char ID; // Pass identification, replacement for typeid
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FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
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initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
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}
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// runOnSCC - Analyze the SCC, performing the transformation if possible.
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bool runOnSCC(CallGraphSCC &SCC);
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// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
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bool AddReadAttrs(const CallGraphSCC &SCC);
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// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
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bool AddNoCaptureAttrs(const CallGraphSCC &SCC);
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// IsFunctionMallocLike - Does this function allocate new memory?
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bool IsFunctionMallocLike(Function *F,
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SmallPtrSet<Function*, 8> &) const;
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// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
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bool AddNoAliasAttrs(const CallGraphSCC &SCC);
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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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CallGraphSCCPass::getAnalysisUsage(AU);
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}
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private:
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AliasAnalysis *AA;
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};
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}
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char FunctionAttrs::ID = 0;
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INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
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"Deduce function attributes", false, false)
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INITIALIZE_AG_DEPENDENCY(CallGraph)
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INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
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"Deduce function attributes", false, false)
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Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
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/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
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bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
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SmallPtrSet<Function*, 8> SCCNodes;
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// Fill SCCNodes with the elements of the SCC. Used for quickly
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// looking up whether a given CallGraphNode is in this SCC.
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
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SCCNodes.insert((*I)->getFunction());
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// Check if any of the functions in the SCC read or write memory. If they
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// write memory then they can't be marked readnone or readonly.
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bool ReadsMemory = false;
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
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Function *F = (*I)->getFunction();
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if (F == 0)
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// External node - may write memory. Just give up.
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return false;
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AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
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if (MRB == AliasAnalysis::DoesNotAccessMemory)
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// Already perfect!
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continue;
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// Definitions with weak linkage may be overridden at linktime with
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// something that writes memory, so treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden()) {
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if (!AliasAnalysis::onlyReadsMemory(MRB))
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// May write memory. Just give up.
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return false;
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ReadsMemory = true;
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continue;
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}
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// Scan the function body for instructions that may read or write memory.
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for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
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Instruction *I = &*II;
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// Some instructions can be ignored even if they read or write memory.
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// Detect these now, skipping to the next instruction if one is found.
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CallSite CS(cast<Value>(I));
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if (CS) {
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// Ignore calls to functions in the same SCC.
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if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
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continue;
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AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(CS);
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// If the call doesn't access arbitrary memory, we may be able to
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// figure out something.
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if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
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// If the call does access argument pointees, check each argument.
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if (AliasAnalysis::doesAccessArgPointees(MRB))
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// Check whether all pointer arguments point to local memory, and
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// ignore calls that only access local memory.
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for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
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CI != CE; ++CI) {
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Value *Arg = *CI;
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if (Arg->getType()->isPointerTy()) {
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AliasAnalysis::Location Loc(Arg,
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AliasAnalysis::UnknownSize,
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I->getMetadata(LLVMContext::MD_tbaa));
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if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true)) {
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if (MRB & AliasAnalysis::Mod)
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// Writes non-local memory. Give up.
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return false;
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if (MRB & AliasAnalysis::Ref)
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// Ok, it reads non-local memory.
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ReadsMemory = true;
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}
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}
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}
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continue;
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}
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// The call could access any memory. If that includes writes, give up.
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if (MRB & AliasAnalysis::Mod)
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return false;
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// If it reads, note it.
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if (MRB & AliasAnalysis::Ref)
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ReadsMemory = true;
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continue;
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} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
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// Ignore non-volatile loads from local memory. (Atomic is okay here.)
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if (!LI->isVolatile()) {
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AliasAnalysis::Location Loc = AA->getLocation(LI);
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if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
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continue;
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}
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} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
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// Ignore non-volatile stores to local memory. (Atomic is okay here.)
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if (!SI->isVolatile()) {
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AliasAnalysis::Location Loc = AA->getLocation(SI);
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if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
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continue;
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}
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} else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
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// Ignore vaargs on local memory.
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AliasAnalysis::Location Loc = AA->getLocation(VI);
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if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
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continue;
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}
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// Any remaining instructions need to be taken seriously! Check if they
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// read or write memory.
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if (I->mayWriteToMemory())
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// Writes memory. Just give up.
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return false;
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// If this instruction may read memory, remember that.
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ReadsMemory |= I->mayReadFromMemory();
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}
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}
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// Success! Functions in this SCC do not access memory, or only read memory.
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// Give them the appropriate attribute.
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bool MadeChange = false;
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
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Function *F = (*I)->getFunction();
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if (F->doesNotAccessMemory())
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// Already perfect!
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continue;
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if (F->onlyReadsMemory() && ReadsMemory)
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// No change.
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continue;
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MadeChange = true;
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// Clear out any existing attributes.
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F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
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// Add in the new attribute.
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F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
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if (ReadsMemory)
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++NumReadOnly;
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else
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++NumReadNone;
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}
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return MadeChange;
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}
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/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
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bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
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bool Changed = false;
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// Check each function in turn, determining which pointer arguments are not
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// captured.
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
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Function *F = (*I)->getFunction();
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if (F == 0)
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// External node - skip it;
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continue;
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// Definitions with weak linkage may be overridden at linktime with
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// something that writes memory, so treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden())
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continue;
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for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
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if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
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!PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) {
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A->addAttr(Attribute::NoCapture);
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++NumNoCapture;
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Changed = true;
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}
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}
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return Changed;
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}
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/// IsFunctionMallocLike - A function is malloc-like if it returns either null
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/// or a pointer that doesn't alias any other pointer visible to the caller.
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bool FunctionAttrs::IsFunctionMallocLike(Function *F,
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SmallPtrSet<Function*, 8> &SCCNodes) const {
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UniqueVector<Value *> FlowsToReturn;
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for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
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if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
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FlowsToReturn.insert(Ret->getReturnValue());
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for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
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Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved.
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if (Constant *C = dyn_cast<Constant>(RetVal)) {
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if (!C->isNullValue() && !isa<UndefValue>(C))
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return false;
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continue;
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}
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if (isa<Argument>(RetVal))
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return false;
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if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
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switch (RVI->getOpcode()) {
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// Extend the analysis by looking upwards.
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case Instruction::BitCast:
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case Instruction::GetElementPtr:
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FlowsToReturn.insert(RVI->getOperand(0));
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continue;
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case Instruction::Select: {
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SelectInst *SI = cast<SelectInst>(RVI);
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FlowsToReturn.insert(SI->getTrueValue());
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FlowsToReturn.insert(SI->getFalseValue());
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continue;
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}
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case Instruction::PHI: {
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PHINode *PN = cast<PHINode>(RVI);
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for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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FlowsToReturn.insert(PN->getIncomingValue(i));
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continue;
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}
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// Check whether the pointer came from an allocation.
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case Instruction::Alloca:
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break;
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case Instruction::Call:
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case Instruction::Invoke: {
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CallSite CS(RVI);
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if (CS.paramHasAttr(0, Attribute::NoAlias))
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break;
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if (CS.getCalledFunction() &&
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SCCNodes.count(CS.getCalledFunction()))
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break;
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} // fall-through
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default:
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return false; // Did not come from an allocation.
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}
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if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
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return false;
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}
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return true;
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}
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/// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
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bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
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SmallPtrSet<Function*, 8> SCCNodes;
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// Fill SCCNodes with the elements of the SCC. Used for quickly
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// looking up whether a given CallGraphNode is in this SCC.
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
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SCCNodes.insert((*I)->getFunction());
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// Check each function in turn, determining which functions return noalias
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// pointers.
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
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Function *F = (*I)->getFunction();
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if (F == 0)
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// External node - skip it;
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return false;
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// Already noalias.
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if (F->doesNotAlias(0))
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continue;
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// Definitions with weak linkage may be overridden at linktime, so
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// treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden())
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return false;
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// We annotate noalias return values, which are only applicable to
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// pointer types.
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if (!F->getReturnType()->isPointerTy())
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continue;
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if (!IsFunctionMallocLike(F, SCCNodes))
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return false;
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}
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bool MadeChange = false;
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for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
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Function *F = (*I)->getFunction();
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if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
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continue;
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F->setDoesNotAlias(0);
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++NumNoAlias;
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MadeChange = true;
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}
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return MadeChange;
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}
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bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
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AA = &getAnalysis<AliasAnalysis>();
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bool Changed = AddReadAttrs(SCC);
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Changed |= AddNoCaptureAttrs(SCC);
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Changed |= AddNoAliasAttrs(SCC);
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return Changed;
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}
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