lib/Transforms/IPO/FunctionAttrs.cpp - llvm - Git at Google

 //===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements a simple interprocedural pass which walks the
 // call-graph, looking for functions which do not access or only read
 // non-local memory, and marking them readnone/readonly.  In addition,
 // it marks function arguments (of pointer type) 'nocapture' if a call
 // to the function does not create any copies of the pointer value that
 // outlive the call.  This more or less means that the pointer is only
 // dereferenced, and not returned from the function or stored in a global.
 // This pass is implemented as a bottom-up traversal of the call-graph.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "functionattrs"
 #include "llvm/Transforms/IPO.h"
 #include "llvm/CallGraphSCCPass.h"
 #include "llvm/GlobalVariable.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/LLVMContext.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/UniqueVector.h"
 #include "llvm/Support/InstIterator.h"
 using namespace llvm;

 STATISTIC(NumReadNone, "Number of functions marked readnone");
 STATISTIC(NumReadOnly, "Number of functions marked readonly");
 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
 STATISTIC(NumNoAlias, "Number of function returns marked noalias");

 namespace {
   struct FunctionAttrs : public CallGraphSCCPass {
     static char ID; // Pass identification, replacement for typeid
     FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
       initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
     }

     // runOnSCC - Analyze the SCC, performing the transformation if possible.
     bool runOnSCC(CallGraphSCC &SCC);

     // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
     bool AddReadAttrs(const CallGraphSCC &SCC);

     // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
     bool AddNoCaptureAttrs(const CallGraphSCC &SCC);

     // IsFunctionMallocLike - Does this function allocate new memory?
     bool IsFunctionMallocLike(Function *F,
                               SmallPtrSet<Function*, 8> &) const;

     // AddNoAliasAttrs - Deduce noalias attributes for the SCC.
     bool AddNoAliasAttrs(const CallGraphSCC &SCC);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.setPreservesCFG();
       AU.addRequired<AliasAnalysis>();
       CallGraphSCCPass::getAnalysisUsage(AU);
     }

   private:
     AliasAnalysis *AA;
   };
 }

 char FunctionAttrs::ID = 0;
 INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
                 "Deduce function attributes", false, false)
 INITIALIZE_AG_DEPENDENCY(CallGraph)
 INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
                 "Deduce function attributes", false, false)

 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }


 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
 bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
   SmallPtrSet<Function*, 8> SCCNodes;

   // Fill SCCNodes with the elements of the SCC.  Used for quickly
   // looking up whether a given CallGraphNode is in this SCC.
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     SCCNodes.insert((*I)->getFunction());

   // Check if any of the functions in the SCC read or write memory.  If they
   // write memory then they can't be marked readnone or readonly.
   bool ReadsMemory = false;
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
     Function *F = (*I)->getFunction();

     if (F == 0)
       // External node - may write memory.  Just give up.
       return false;

     AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
     if (MRB == AliasAnalysis::DoesNotAccessMemory)
       // Already perfect!
       continue;

     // Definitions with weak linkage may be overridden at linktime with
     // something that writes memory, so treat them like declarations.
     if (F->isDeclaration() || F->mayBeOverridden()) {
       if (!AliasAnalysis::onlyReadsMemory(MRB))
         // May write memory.  Just give up.
         return false;

       ReadsMemory = true;
       continue;
     }

     // Scan the function body for instructions that may read or write memory.
     for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
       Instruction *I = &*II;

       // Some instructions can be ignored even if they read or write memory.
       // Detect these now, skipping to the next instruction if one is found.
       CallSite CS(cast<Value>(I));
       if (CS) {
         // Ignore calls to functions in the same SCC.
         if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
           continue;
         AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(CS);
         // If the call doesn't access arbitrary memory, we may be able to
         // figure out something.
         if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
           // If the call does access argument pointees, check each argument.
           if (AliasAnalysis::doesAccessArgPointees(MRB))
             // Check whether all pointer arguments point to local memory, and
             // ignore calls that only access local memory.
             for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
                  CI != CE; ++CI) {
               Value *Arg = *CI;
               if (Arg->getType()->isPointerTy()) {
                 AliasAnalysis::Location Loc(Arg,
                                             AliasAnalysis::UnknownSize,
                                             I->getMetadata(LLVMContext::MD_tbaa));
                 if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true)) {
                   if (MRB & AliasAnalysis::Mod)
                     // Writes non-local memory.  Give up.
                     return false;
                   if (MRB & AliasAnalysis::Ref)
                     // Ok, it reads non-local memory.
                     ReadsMemory = true;
                 }
               }
             }
           continue;
         }
         // The call could access any memory. If that includes writes, give up.
         if (MRB & AliasAnalysis::Mod)
           return false;
         // If it reads, note it.
         if (MRB & AliasAnalysis::Ref)
           ReadsMemory = true;
         continue;
       } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
         // Ignore non-volatile loads from local memory. (Atomic is okay here.)
         if (!LI->isVolatile()) {
           AliasAnalysis::Location Loc = AA->getLocation(LI);
           if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
             continue;
         }
       } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
         // Ignore non-volatile stores to local memory. (Atomic is okay here.)
         if (!SI->isVolatile()) {
           AliasAnalysis::Location Loc = AA->getLocation(SI);
           if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
             continue;
         }
       } else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
         // Ignore vaargs on local memory.
         AliasAnalysis::Location Loc = AA->getLocation(VI);
         if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
           continue;
       }

       // Any remaining instructions need to be taken seriously!  Check if they
       // read or write memory.
       if (I->mayWriteToMemory())
         // Writes memory.  Just give up.
         return false;

       // If this instruction may read memory, remember that.
       ReadsMemory |= I->mayReadFromMemory();
     }
   }

   // Success!  Functions in this SCC do not access memory, or only read memory.
   // Give them the appropriate attribute.
   bool MadeChange = false;
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
     Function *F = (*I)->getFunction();

     if (F->doesNotAccessMemory())
       // Already perfect!
       continue;

     if (F->onlyReadsMemory() && ReadsMemory)
       // No change.
       continue;

     MadeChange = true;

     // Clear out any existing attributes.
     F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);

     // Add in the new attribute.
     F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);

     if (ReadsMemory)
       ++NumReadOnly;
     else
       ++NumReadNone;
   }

   return MadeChange;
 }

 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
 bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
   bool Changed = false;

   // Check each function in turn, determining which pointer arguments are not
   // captured.
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
     Function *F = (*I)->getFunction();

     if (F == 0)
       // External node - skip it;
       continue;

     // Definitions with weak linkage may be overridden at linktime with
     // something that writes memory, so treat them like declarations.
     if (F->isDeclaration() || F->mayBeOverridden())
       continue;

     for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
       if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
           !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) {
         A->addAttr(Attribute::NoCapture);
         ++NumNoCapture;
         Changed = true;
       }
   }

   return Changed;
 }

 /// IsFunctionMallocLike - A function is malloc-like if it returns either null
 /// or a pointer that doesn't alias any other pointer visible to the caller.
 bool FunctionAttrs::IsFunctionMallocLike(Function *F,
                               SmallPtrSet<Function*, 8> &SCCNodes) const {
   UniqueVector<Value *> FlowsToReturn;
   for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
     if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
       FlowsToReturn.insert(Ret->getReturnValue());

   for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
     Value *RetVal = FlowsToReturn[i+1];   // UniqueVector[0] is reserved.

     if (Constant *C = dyn_cast<Constant>(RetVal)) {
       if (!C->isNullValue() && !isa<UndefValue>(C))
         return false;

       continue;
     }

     if (isa<Argument>(RetVal))
       return false;

     if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
       switch (RVI->getOpcode()) {
         // Extend the analysis by looking upwards.
         case Instruction::BitCast:
         case Instruction::GetElementPtr:
           FlowsToReturn.insert(RVI->getOperand(0));
           continue;
         case Instruction::Select: {
           SelectInst *SI = cast<SelectInst>(RVI);
           FlowsToReturn.insert(SI->getTrueValue());
           FlowsToReturn.insert(SI->getFalseValue());
           continue;
         }
         case Instruction::PHI: {
           PHINode *PN = cast<PHINode>(RVI);
           for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
             FlowsToReturn.insert(PN->getIncomingValue(i));
           continue;
         }

         // Check whether the pointer came from an allocation.
         case Instruction::Alloca:
           break;
         case Instruction::Call:
         case Instruction::Invoke: {
           CallSite CS(RVI);
           if (CS.paramHasAttr(0, Attribute::NoAlias))
             break;
           if (CS.getCalledFunction() &&
               SCCNodes.count(CS.getCalledFunction()))
             break;
         } // fall-through
         default:
           return false;  // Did not come from an allocation.
       }

     if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
       return false;
   }

   return true;
 }

 /// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
 bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
   SmallPtrSet<Function*, 8> SCCNodes;

   // Fill SCCNodes with the elements of the SCC.  Used for quickly
   // looking up whether a given CallGraphNode is in this SCC.
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
     SCCNodes.insert((*I)->getFunction());

   // Check each function in turn, determining which functions return noalias
   // pointers.
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
     Function *F = (*I)->getFunction();

     if (F == 0)
       // External node - skip it;
       return false;

     // Already noalias.
     if (F->doesNotAlias(0))
       continue;

     // Definitions with weak linkage may be overridden at linktime, so
     // treat them like declarations.
     if (F->isDeclaration() || F->mayBeOverridden())
       return false;

     // We annotate noalias return values, which are only applicable to
     // pointer types.
     if (!F->getReturnType()->isPointerTy())
       continue;

     if (!IsFunctionMallocLike(F, SCCNodes))
       return false;
   }

   bool MadeChange = false;
   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
     Function *F = (*I)->getFunction();
     if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
       continue;

     F->setDoesNotAlias(0);
     ++NumNoAlias;
     MadeChange = true;
   }

   return MadeChange;
 }

 bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
   AA = &getAnalysis<AliasAnalysis>();

   bool Changed = AddReadAttrs(SCC);
   Changed |= AddNoCaptureAttrs(SCC);
   Changed |= AddNoAliasAttrs(SCC);
   return Changed;
 }
	//===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file implements a simple interprocedural pass which walks the
	// call-graph, looking for functions which do not access or only read
	// non-local memory, and marking them readnone/readonly. In addition,
	// it marks function arguments (of pointer type) 'nocapture' if a call
	// to the function does not create any copies of the pointer value that
	// outlive the call. This more or less means that the pointer is only
	// dereferenced, and not returned from the function or stored in a global.
	// This pass is implemented as a bottom-up traversal of the call-graph.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "functionattrs"
	#include "llvm/Transforms/IPO.h"
	#include "llvm/CallGraphSCCPass.h"
	#include "llvm/GlobalVariable.h"
	#include "llvm/IntrinsicInst.h"
	#include "llvm/LLVMContext.h"
	#include "llvm/Analysis/AliasAnalysis.h"
	#include "llvm/Analysis/CallGraph.h"
	#include "llvm/Analysis/CaptureTracking.h"
	#include "llvm/ADT/SmallSet.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/ADT/UniqueVector.h"
	#include "llvm/Support/InstIterator.h"
	using namespace llvm;

	STATISTIC(NumReadNone, "Number of functions marked readnone");
	STATISTIC(NumReadOnly, "Number of functions marked readonly");
	STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
	STATISTIC(NumNoAlias, "Number of function returns marked noalias");

	namespace {
	struct FunctionAttrs : public CallGraphSCCPass {
	static char ID; // Pass identification, replacement for typeid
	FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
	initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
	}

	// runOnSCC - Analyze the SCC, performing the transformation if possible.
	bool runOnSCC(CallGraphSCC &SCC);

	// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
	bool AddReadAttrs(const CallGraphSCC &SCC);

	// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
	bool AddNoCaptureAttrs(const CallGraphSCC &SCC);

	// IsFunctionMallocLike - Does this function allocate new memory?
	bool IsFunctionMallocLike(Function *F,
	SmallPtrSet<Function*, 8> &) const;

	// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
	bool AddNoAliasAttrs(const CallGraphSCC &SCC);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesCFG();
	AU.addRequired<AliasAnalysis>();
	CallGraphSCCPass::getAnalysisUsage(AU);
	}

	private:
	AliasAnalysis *AA;
	};
	}

	char FunctionAttrs::ID = 0;
	INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
	"Deduce function attributes", false, false)
	INITIALIZE_AG_DEPENDENCY(CallGraph)
	INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
	"Deduce function attributes", false, false)

	Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }


	/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
	bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
	SmallPtrSet<Function*, 8> SCCNodes;

	// Fill SCCNodes with the elements of the SCC. Used for quickly
	// looking up whether a given CallGraphNode is in this SCC.
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
	SCCNodes.insert((*I)->getFunction());

	// Check if any of the functions in the SCC read or write memory. If they
	// write memory then they can't be marked readnone or readonly.
	bool ReadsMemory = false;
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
	Function F = (I)->getFunction();

	if (F == 0)
	// External node - may write memory. Just give up.
	return false;

	AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
	if (MRB == AliasAnalysis::DoesNotAccessMemory)
	// Already perfect!
	continue;

	// Definitions with weak linkage may be overridden at linktime with
	// something that writes memory, so treat them like declarations.
	if (F->isDeclaration() \|\| F->mayBeOverridden()) {
	if (!AliasAnalysis::onlyReadsMemory(MRB))
	// May write memory. Just give up.
	return false;

	ReadsMemory = true;
	continue;
	}

	// Scan the function body for instructions that may read or write memory.
	for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
	Instruction I = &II;

	// Some instructions can be ignored even if they read or write memory.
	// Detect these now, skipping to the next instruction if one is found.
	CallSite CS(cast<Value>(I));
	if (CS) {
	// Ignore calls to functions in the same SCC.
	if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
	continue;
	AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(CS);
	// If the call doesn't access arbitrary memory, we may be able to
	// figure out something.
	if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
	// If the call does access argument pointees, check each argument.
	if (AliasAnalysis::doesAccessArgPointees(MRB))
	// Check whether all pointer arguments point to local memory, and
	// ignore calls that only access local memory.
	for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
	CI != CE; ++CI) {
	Value Arg = CI;
	if (Arg->getType()->isPointerTy()) {
	AliasAnalysis::Location Loc(Arg,
	AliasAnalysis::UnknownSize,
	I->getMetadata(LLVMContext::MD_tbaa));
	if (!AA->pointsToConstantMemory(Loc, /OrLocal=/true)) {
	if (MRB & AliasAnalysis::Mod)
	// Writes non-local memory. Give up.
	return false;
	if (MRB & AliasAnalysis::Ref)
	// Ok, it reads non-local memory.
	ReadsMemory = true;
	}
	}
	}
	continue;
	}
	// The call could access any memory. If that includes writes, give up.
	if (MRB & AliasAnalysis::Mod)
	return false;
	// If it reads, note it.
	if (MRB & AliasAnalysis::Ref)
	ReadsMemory = true;
	continue;
	} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
	// Ignore non-volatile loads from local memory. (Atomic is okay here.)
	if (!LI->isVolatile()) {
	AliasAnalysis::Location Loc = AA->getLocation(LI);
	if (AA->pointsToConstantMemory(Loc, /OrLocal=/true))
	continue;
	}
	} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
	// Ignore non-volatile stores to local memory. (Atomic is okay here.)
	if (!SI->isVolatile()) {
	AliasAnalysis::Location Loc = AA->getLocation(SI);
	if (AA->pointsToConstantMemory(Loc, /OrLocal=/true))
	continue;
	}
	} else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
	// Ignore vaargs on local memory.
	AliasAnalysis::Location Loc = AA->getLocation(VI);
	if (AA->pointsToConstantMemory(Loc, /OrLocal=/true))
	continue;
	}

	// Any remaining instructions need to be taken seriously! Check if they
	// read or write memory.
	if (I->mayWriteToMemory())
	// Writes memory. Just give up.
	return false;

	// If this instruction may read memory, remember that.
	ReadsMemory \|= I->mayReadFromMemory();
	}
	}

	// Success! Functions in this SCC do not access memory, or only read memory.
	// Give them the appropriate attribute.
	bool MadeChange = false;
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
	Function F = (I)->getFunction();

	if (F->doesNotAccessMemory())
	// Already perfect!
	continue;

	if (F->onlyReadsMemory() && ReadsMemory)
	// No change.
	continue;

	MadeChange = true;

	// Clear out any existing attributes.
	F->removeAttribute(~0, Attribute::ReadOnly \| Attribute::ReadNone);

	// Add in the new attribute.
	F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);

	if (ReadsMemory)
	++NumReadOnly;
	else
	++NumReadNone;
	}

	return MadeChange;
	}

	/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
	bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
	bool Changed = false;

	// Check each function in turn, determining which pointer arguments are not
	// captured.
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
	Function F = (I)->getFunction();

	if (F == 0)
	// External node - skip it;
	continue;

	// Definitions with weak linkage may be overridden at linktime with
	// something that writes memory, so treat them like declarations.
	if (F->isDeclaration() \|\| F->mayBeOverridden())
	continue;

	for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
	if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
	!PointerMayBeCaptured(A, true, /StoreCaptures=/false)) {
	A->addAttr(Attribute::NoCapture);
	++NumNoCapture;
	Changed = true;
	}
	}

	return Changed;
	}

	/// IsFunctionMallocLike - A function is malloc-like if it returns either null
	/// or a pointer that doesn't alias any other pointer visible to the caller.
	bool FunctionAttrs::IsFunctionMallocLike(Function *F,
	SmallPtrSet<Function*, 8> &SCCNodes) const {
	UniqueVector<Value *> FlowsToReturn;
	for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
	if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
	FlowsToReturn.insert(Ret->getReturnValue());

	for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
	Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved.

	if (Constant *C = dyn_cast<Constant>(RetVal)) {
	if (!C->isNullValue() && !isa<UndefValue>(C))
	return false;

	continue;
	}

	if (isa<Argument>(RetVal))
	return false;

	if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
	switch (RVI->getOpcode()) {
	// Extend the analysis by looking upwards.
	case Instruction::BitCast:
	case Instruction::GetElementPtr:
	FlowsToReturn.insert(RVI->getOperand(0));
	continue;
	case Instruction::Select: {
	SelectInst *SI = cast<SelectInst>(RVI);
	FlowsToReturn.insert(SI->getTrueValue());
	FlowsToReturn.insert(SI->getFalseValue());
	continue;
	}
	case Instruction::PHI: {
	PHINode *PN = cast<PHINode>(RVI);
	for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
	FlowsToReturn.insert(PN->getIncomingValue(i));
	continue;
	}

	// Check whether the pointer came from an allocation.
	case Instruction::Alloca:
	break;
	case Instruction::Call:
	case Instruction::Invoke: {
	CallSite CS(RVI);
	if (CS.paramHasAttr(0, Attribute::NoAlias))
	break;
	if (CS.getCalledFunction() &&
	SCCNodes.count(CS.getCalledFunction()))
	break;
	} // fall-through
	default:
	return false; // Did not come from an allocation.
	}

	if (PointerMayBeCaptured(RetVal, false, /StoreCaptures=/false))
	return false;
	}

	return true;
	}

	/// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
	bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
	SmallPtrSet<Function*, 8> SCCNodes;

	// Fill SCCNodes with the elements of the SCC. Used for quickly
	// looking up whether a given CallGraphNode is in this SCC.
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
	SCCNodes.insert((*I)->getFunction());

	// Check each function in turn, determining which functions return noalias
	// pointers.
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
	Function F = (I)->getFunction();

	if (F == 0)
	// External node - skip it;
	return false;

	// Already noalias.
	if (F->doesNotAlias(0))
	continue;

	// Definitions with weak linkage may be overridden at linktime, so
	// treat them like declarations.
	if (F->isDeclaration() \|\| F->mayBeOverridden())
	return false;

	// We annotate noalias return values, which are only applicable to
	// pointer types.
	if (!F->getReturnType()->isPointerTy())
	continue;

	if (!IsFunctionMallocLike(F, SCCNodes))
	return false;
	}

	bool MadeChange = false;
	for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
	Function F = (I)->getFunction();
	if (F->doesNotAlias(0) \|\| !F->getReturnType()->isPointerTy())
	continue;

	F->setDoesNotAlias(0);
	++NumNoAlias;
	MadeChange = true;
	}

	return MadeChange;
	}

	bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
	AA = &getAnalysis<AliasAnalysis>();

	bool Changed = AddReadAttrs(SCC);
	Changed \|= AddNoCaptureAttrs(SCC);
	Changed \|= AddNoAliasAttrs(SCC);
	return Changed;
	}