llvm/lib/CodeGen/Analysis.cpp - toolchain/llvm-project - Gitiles

 //===-- Analysis.cpp - CodeGen LLVM IR Analysis Utilities --*- C++ ------*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines several CodeGen-specific LLVM IR analysis utilties.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/DerivedTypes.h"
 #include "llvm/Function.h"
 #include "llvm/Instructions.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/LLVMContext.h"
 #include "llvm/Module.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 using namespace llvm;

 /// ComputeLinearIndex - Given an LLVM IR aggregate type and a sequence
 /// of insertvalue or extractvalue indices that identify a member, return
 /// the linearized index of the start of the member.
 ///
 unsigned llvm::ComputeLinearIndex(const TargetLowering &TLI, const Type *Ty,
                                   const unsigned *Indices,
                                   const unsigned *IndicesEnd,
                                   unsigned CurIndex) {
   // Base case: We're done.
   if (Indices && Indices == IndicesEnd)
     return CurIndex;

   // Given a struct type, recursively traverse the elements.
   if (const StructType *STy = dyn_cast<StructType>(Ty)) {
     for (StructType::element_iterator EB = STy->element_begin(),
                                       EI = EB,
                                       EE = STy->element_end();
         EI != EE; ++EI) {
       if (Indices && *Indices == unsigned(EI - EB))
         return ComputeLinearIndex(TLI, *EI, Indices+1, IndicesEnd, CurIndex);
       CurIndex = ComputeLinearIndex(TLI, *EI, 0, 0, CurIndex);
     }
     return CurIndex;
   }
   // Given an array type, recursively traverse the elements.
   else if (const ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
     const Type *EltTy = ATy->getElementType();
     for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i) {
       if (Indices && *Indices == i)
         return ComputeLinearIndex(TLI, EltTy, Indices+1, IndicesEnd, CurIndex);
       CurIndex = ComputeLinearIndex(TLI, EltTy, 0, 0, CurIndex);
     }
     return CurIndex;
   }
   // We haven't found the type we're looking for, so keep searching.
   return CurIndex + 1;
 }

 /// ComputeValueVTs - Given an LLVM IR type, compute a sequence of
 /// EVTs that represent all the individual underlying
 /// non-aggregate types that comprise it.
 ///
 /// If Offsets is non-null, it points to a vector to be filled in
 /// with the in-memory offsets of each of the individual values.
 ///
 void llvm::ComputeValueVTs(const TargetLowering &TLI, const Type *Ty,
                            SmallVectorImpl<EVT> &ValueVTs,
                            SmallVectorImpl<uint64_t> *Offsets,
                            uint64_t StartingOffset) {
   // Given a struct type, recursively traverse the elements.
   if (const StructType *STy = dyn_cast<StructType>(Ty)) {
     const StructLayout *SL = TLI.getTargetData()->getStructLayout(STy);
     for (StructType::element_iterator EB = STy->element_begin(),
                                       EI = EB,
                                       EE = STy->element_end();
          EI != EE; ++EI)
       ComputeValueVTs(TLI, *EI, ValueVTs, Offsets,
                       StartingOffset + SL->getElementOffset(EI - EB));
     return;
   }
   // Given an array type, recursively traverse the elements.
   if (const ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
     const Type *EltTy = ATy->getElementType();
     uint64_t EltSize = TLI.getTargetData()->getTypeAllocSize(EltTy);
     for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i)
       ComputeValueVTs(TLI, EltTy, ValueVTs, Offsets,
                       StartingOffset + i * EltSize);
     return;
   }
   // Interpret void as zero return values.
   if (Ty->isVoidTy())
     return;
   // Base case: we can get an EVT for this LLVM IR type.
   ValueVTs.push_back(TLI.getValueType(Ty));
   if (Offsets)
     Offsets->push_back(StartingOffset);
 }

 /// ExtractTypeInfo - Returns the type info, possibly bitcast, encoded in V.
 GlobalVariable *llvm::ExtractTypeInfo(Value *V) {
   V = V->stripPointerCasts();
   GlobalVariable *GV = dyn_cast<GlobalVariable>(V);

   if (GV && GV->getName() == ".llvm.eh.catch.all.value") {
     assert(GV->hasInitializer() &&
            "The EH catch-all value must have an initializer");
     Value *Init = GV->getInitializer();
     GV = dyn_cast<GlobalVariable>(Init);
     if (!GV) V = cast<ConstantPointerNull>(Init);
   }

   assert((GV || isa<ConstantPointerNull>(V)) &&
          "TypeInfo must be a global variable or NULL");
   return GV;
 }

 /// hasInlineAsmMemConstraint - Return true if the inline asm instruction being
 /// processed uses a memory 'm' constraint.
 bool
 llvm::hasInlineAsmMemConstraint(std::vector<InlineAsm::ConstraintInfo> &CInfos,
                                 const TargetLowering &TLI) {
   for (unsigned i = 0, e = CInfos.size(); i != e; ++i) {
     InlineAsm::ConstraintInfo &CI = CInfos[i];
     for (unsigned j = 0, ee = CI.Codes.size(); j != ee; ++j) {
       TargetLowering::ConstraintType CType = TLI.getConstraintType(CI.Codes[j]);
       if (CType == TargetLowering::C_Memory)
         return true;
     }

     // Indirect operand accesses access memory.
     if (CI.isIndirect)
       return true;
   }

   return false;
 }

 /// getFCmpCondCode - Return the ISD condition code corresponding to
 /// the given LLVM IR floating-point condition code.  This includes
 /// consideration of global floating-point math flags.
 ///
 ISD::CondCode llvm::getFCmpCondCode(FCmpInst::Predicate Pred) {
   ISD::CondCode FPC, FOC;
   switch (Pred) {
   case FCmpInst::FCMP_FALSE: FOC = FPC = ISD::SETFALSE; break;
   case FCmpInst::FCMP_OEQ:   FOC = ISD::SETEQ; FPC = ISD::SETOEQ; break;
   case FCmpInst::FCMP_OGT:   FOC = ISD::SETGT; FPC = ISD::SETOGT; break;
   case FCmpInst::FCMP_OGE:   FOC = ISD::SETGE; FPC = ISD::SETOGE; break;
   case FCmpInst::FCMP_OLT:   FOC = ISD::SETLT; FPC = ISD::SETOLT; break;
   case FCmpInst::FCMP_OLE:   FOC = ISD::SETLE; FPC = ISD::SETOLE; break;
   case FCmpInst::FCMP_ONE:   FOC = ISD::SETNE; FPC = ISD::SETONE; break;
   case FCmpInst::FCMP_ORD:   FOC = FPC = ISD::SETO;   break;
   case FCmpInst::FCMP_UNO:   FOC = FPC = ISD::SETUO;  break;
   case FCmpInst::FCMP_UEQ:   FOC = ISD::SETEQ; FPC = ISD::SETUEQ; break;
   case FCmpInst::FCMP_UGT:   FOC = ISD::SETGT; FPC = ISD::SETUGT; break;
   case FCmpInst::FCMP_UGE:   FOC = ISD::SETGE; FPC = ISD::SETUGE; break;
   case FCmpInst::FCMP_ULT:   FOC = ISD::SETLT; FPC = ISD::SETULT; break;
   case FCmpInst::FCMP_ULE:   FOC = ISD::SETLE; FPC = ISD::SETULE; break;
   case FCmpInst::FCMP_UNE:   FOC = ISD::SETNE; FPC = ISD::SETUNE; break;
   case FCmpInst::FCMP_TRUE:  FOC = FPC = ISD::SETTRUE; break;
   default:
     llvm_unreachable("Invalid FCmp predicate opcode!");
     FOC = FPC = ISD::SETFALSE;
     break;
   }
   if (FiniteOnlyFPMath())
     return FOC;
   else
     return FPC;
 }

 /// getICmpCondCode - Return the ISD condition code corresponding to
 /// the given LLVM IR integer condition code.
 ///
 ISD::CondCode llvm::getICmpCondCode(ICmpInst::Predicate Pred) {
   switch (Pred) {
   case ICmpInst::ICMP_EQ:  return ISD::SETEQ;
   case ICmpInst::ICMP_NE:  return ISD::SETNE;
   case ICmpInst::ICMP_SLE: return ISD::SETLE;
   case ICmpInst::ICMP_ULE: return ISD::SETULE;
   case ICmpInst::ICMP_SGE: return ISD::SETGE;
   case ICmpInst::ICMP_UGE: return ISD::SETUGE;
   case ICmpInst::ICMP_SLT: return ISD::SETLT;
   case ICmpInst::ICMP_ULT: return ISD::SETULT;
   case ICmpInst::ICMP_SGT: return ISD::SETGT;
   case ICmpInst::ICMP_UGT: return ISD::SETUGT;
   default:
     llvm_unreachable("Invalid ICmp predicate opcode!");
     return ISD::SETNE;
   }
 }

 /// Test if the given instruction is in a position to be optimized
 /// with a tail-call. This roughly means that it's in a block with
 /// a return and there's nothing that needs to be scheduled
 /// between it and the return.
 ///
 /// This function only tests target-independent requirements.
 bool llvm::isInTailCallPosition(ImmutableCallSite CS, Attributes CalleeRetAttr,
                                 const TargetLowering &TLI) {
   const Instruction *I = CS.getInstruction();
   const BasicBlock *ExitBB = I->getParent();
   const TerminatorInst *Term = ExitBB->getTerminator();
   const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);
   const Function *F = ExitBB->getParent();

   // The block must end in a return statement or unreachable.
   //
   // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
   // an unreachable, for now. The way tailcall optimization is currently
   // implemented means it will add an epilogue followed by a jump. That is
   // not profitable. Also, if the callee is a special function (e.g.
   // longjmp on x86), it can end up causing miscompilation that has not
   // been fully understood.
   if (!Ret &&
       (!GuaranteedTailCallOpt || !isa<UnreachableInst>(Term))) return false;

   // If I will have a chain, make sure no other instruction that will have a
   // chain interposes between I and the return.
   if (I->mayHaveSideEffects() || I->mayReadFromMemory() ||
       !I->isSafeToSpeculativelyExecute())
     for (BasicBlock::const_iterator BBI = prior(prior(ExitBB->end())); ;
          --BBI) {
       if (&*BBI == I)
         break;
       // Debug info intrinsics do not get in the way of tail call optimization.
       if (isa<DbgInfoIntrinsic>(BBI))
         continue;
       if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
           !BBI->isSafeToSpeculativelyExecute())
         return false;
     }

   // If the block ends with a void return or unreachable, it doesn't matter
   // what the call's return type is.
   if (!Ret || Ret->getNumOperands() == 0) return true;

   // If the return value is undef, it doesn't matter what the call's
   // return type is.
   if (isa<UndefValue>(Ret->getOperand(0))) return true;

   // Conservatively require the attributes of the call to match those of
   // the return. Ignore noalias because it doesn't affect the call sequence.
   unsigned CallerRetAttr = F->getAttributes().getRetAttributes();
   if ((CalleeRetAttr ^ CallerRetAttr) & ~Attribute::NoAlias)
     return false;

   // It's not safe to eliminate the sign / zero extension of the return value.
   if ((CallerRetAttr & Attribute::ZExt) || (CallerRetAttr & Attribute::SExt))
     return false;

   // Otherwise, make sure the unmodified return value of I is the return value.
   for (const Instruction *U = dyn_cast<Instruction>(Ret->getOperand(0)); ;
        U = dyn_cast<Instruction>(U->getOperand(0))) {
     if (!U)
       return false;
     if (!U->hasOneUse())
       return false;
     if (U == I)
       break;
     // Check for a truly no-op truncate.
     if (isa<TruncInst>(U) &&
         TLI.isTruncateFree(U->getOperand(0)->getType(), U->getType()))
       continue;
     // Check for a truly no-op bitcast.
     if (isa<BitCastInst>(U) &&
         (U->getOperand(0)->getType() == U->getType() ||
          (U->getOperand(0)->getType()->isPointerTy() &&
           U->getType()->isPointerTy())))
       continue;
     // Otherwise it's not a true no-op.
     return false;
   }

   return true;
 }
	//===-- Analysis.cpp - CodeGen LLVM IR Analysis Utilities --- C++ -------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file defines several CodeGen-specific LLVM IR analysis utilties.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/Analysis.h"
	#include "llvm/DerivedTypes.h"
	#include "llvm/Function.h"
	#include "llvm/Instructions.h"
	#include "llvm/IntrinsicInst.h"
	#include "llvm/LLVMContext.h"
	#include "llvm/Module.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/Target/TargetData.h"
	#include "llvm/Target/TargetLowering.h"
	#include "llvm/Target/TargetOptions.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/MathExtras.h"
	using namespace llvm;

	/// ComputeLinearIndex - Given an LLVM IR aggregate type and a sequence
	/// of insertvalue or extractvalue indices that identify a member, return
	/// the linearized index of the start of the member.
	///
	unsigned llvm::ComputeLinearIndex(const TargetLowering &TLI, const Type *Ty,
	const unsigned *Indices,
	const unsigned *IndicesEnd,
	unsigned CurIndex) {
	// Base case: We're done.
	if (Indices && Indices == IndicesEnd)
	return CurIndex;

	// Given a struct type, recursively traverse the elements.
	if (const StructType *STy = dyn_cast<StructType>(Ty)) {
	for (StructType::element_iterator EB = STy->element_begin(),
	EI = EB,
	EE = STy->element_end();
	EI != EE; ++EI) {
	if (Indices && *Indices == unsigned(EI - EB))
	return ComputeLinearIndex(TLI, *EI, Indices+1, IndicesEnd, CurIndex);
	CurIndex = ComputeLinearIndex(TLI, *EI, 0, 0, CurIndex);
	}
	return CurIndex;
	}
	// Given an array type, recursively traverse the elements.
	else if (const ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
	const Type *EltTy = ATy->getElementType();
	for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i) {
	if (Indices && *Indices == i)
	return ComputeLinearIndex(TLI, EltTy, Indices+1, IndicesEnd, CurIndex);
	CurIndex = ComputeLinearIndex(TLI, EltTy, 0, 0, CurIndex);
	}
	return CurIndex;
	}
	// We haven't found the type we're looking for, so keep searching.
	return CurIndex + 1;
	}

	/// ComputeValueVTs - Given an LLVM IR type, compute a sequence of
	/// EVTs that represent all the individual underlying
	/// non-aggregate types that comprise it.
	///
	/// If Offsets is non-null, it points to a vector to be filled in
	/// with the in-memory offsets of each of the individual values.
	///
	void llvm::ComputeValueVTs(const TargetLowering &TLI, const Type *Ty,
	SmallVectorImpl<EVT> &ValueVTs,
	SmallVectorImpl<uint64_t> *Offsets,
	uint64_t StartingOffset) {
	// Given a struct type, recursively traverse the elements.
	if (const StructType *STy = dyn_cast<StructType>(Ty)) {
	const StructLayout *SL = TLI.getTargetData()->getStructLayout(STy);
	for (StructType::element_iterator EB = STy->element_begin(),
	EI = EB,
	EE = STy->element_end();
	EI != EE; ++EI)
	ComputeValueVTs(TLI, *EI, ValueVTs, Offsets,
	StartingOffset + SL->getElementOffset(EI - EB));
	return;
	}
	// Given an array type, recursively traverse the elements.
	if (const ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
	const Type *EltTy = ATy->getElementType();
	uint64_t EltSize = TLI.getTargetData()->getTypeAllocSize(EltTy);
	for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i)
	ComputeValueVTs(TLI, EltTy, ValueVTs, Offsets,
	StartingOffset + i * EltSize);
	return;
	}
	// Interpret void as zero return values.
	if (Ty->isVoidTy())
	return;
	// Base case: we can get an EVT for this LLVM IR type.
	ValueVTs.push_back(TLI.getValueType(Ty));
	if (Offsets)
	Offsets->push_back(StartingOffset);
	}

	/// ExtractTypeInfo - Returns the type info, possibly bitcast, encoded in V.
	GlobalVariable llvm::ExtractTypeInfo(Value V) {
	V = V->stripPointerCasts();
	GlobalVariable *GV = dyn_cast<GlobalVariable>(V);

	if (GV && GV->getName() == ".llvm.eh.catch.all.value") {
	assert(GV->hasInitializer() &&
	"The EH catch-all value must have an initializer");
	Value *Init = GV->getInitializer();
	GV = dyn_cast<GlobalVariable>(Init);
	if (!GV) V = cast<ConstantPointerNull>(Init);
	}

	assert((GV \|\| isa<ConstantPointerNull>(V)) &&
	"TypeInfo must be a global variable or NULL");
	return GV;
	}

	/// hasInlineAsmMemConstraint - Return true if the inline asm instruction being
	/// processed uses a memory 'm' constraint.
	bool
	llvm::hasInlineAsmMemConstraint(std::vector<InlineAsm::ConstraintInfo> &CInfos,
	const TargetLowering &TLI) {
	for (unsigned i = 0, e = CInfos.size(); i != e; ++i) {
	InlineAsm::ConstraintInfo &CI = CInfos[i];
	for (unsigned j = 0, ee = CI.Codes.size(); j != ee; ++j) {
	TargetLowering::ConstraintType CType = TLI.getConstraintType(CI.Codes[j]);
	if (CType == TargetLowering::C_Memory)
	return true;
	}

	// Indirect operand accesses access memory.
	if (CI.isIndirect)
	return true;
	}

	return false;
	}

	/// getFCmpCondCode - Return the ISD condition code corresponding to
	/// the given LLVM IR floating-point condition code. This includes
	/// consideration of global floating-point math flags.
	///
	ISD::CondCode llvm::getFCmpCondCode(FCmpInst::Predicate Pred) {
	ISD::CondCode FPC, FOC;
	switch (Pred) {
	case FCmpInst::FCMP_FALSE: FOC = FPC = ISD::SETFALSE; break;
	case FCmpInst::FCMP_OEQ: FOC = ISD::SETEQ; FPC = ISD::SETOEQ; break;
	case FCmpInst::FCMP_OGT: FOC = ISD::SETGT; FPC = ISD::SETOGT; break;
	case FCmpInst::FCMP_OGE: FOC = ISD::SETGE; FPC = ISD::SETOGE; break;
	case FCmpInst::FCMP_OLT: FOC = ISD::SETLT; FPC = ISD::SETOLT; break;
	case FCmpInst::FCMP_OLE: FOC = ISD::SETLE; FPC = ISD::SETOLE; break;
	case FCmpInst::FCMP_ONE: FOC = ISD::SETNE; FPC = ISD::SETONE; break;
	case FCmpInst::FCMP_ORD: FOC = FPC = ISD::SETO; break;
	case FCmpInst::FCMP_UNO: FOC = FPC = ISD::SETUO; break;
	case FCmpInst::FCMP_UEQ: FOC = ISD::SETEQ; FPC = ISD::SETUEQ; break;
	case FCmpInst::FCMP_UGT: FOC = ISD::SETGT; FPC = ISD::SETUGT; break;
	case FCmpInst::FCMP_UGE: FOC = ISD::SETGE; FPC = ISD::SETUGE; break;
	case FCmpInst::FCMP_ULT: FOC = ISD::SETLT; FPC = ISD::SETULT; break;
	case FCmpInst::FCMP_ULE: FOC = ISD::SETLE; FPC = ISD::SETULE; break;
	case FCmpInst::FCMP_UNE: FOC = ISD::SETNE; FPC = ISD::SETUNE; break;
	case FCmpInst::FCMP_TRUE: FOC = FPC = ISD::SETTRUE; break;
	default:
	llvm_unreachable("Invalid FCmp predicate opcode!");
	FOC = FPC = ISD::SETFALSE;
	break;
	}
	if (FiniteOnlyFPMath())
	return FOC;
	else
	return FPC;
	}

	/// getICmpCondCode - Return the ISD condition code corresponding to
	/// the given LLVM IR integer condition code.
	///
	ISD::CondCode llvm::getICmpCondCode(ICmpInst::Predicate Pred) {
	switch (Pred) {
	case ICmpInst::ICMP_EQ: return ISD::SETEQ;
	case ICmpInst::ICMP_NE: return ISD::SETNE;
	case ICmpInst::ICMP_SLE: return ISD::SETLE;
	case ICmpInst::ICMP_ULE: return ISD::SETULE;
	case ICmpInst::ICMP_SGE: return ISD::SETGE;
	case ICmpInst::ICMP_UGE: return ISD::SETUGE;
	case ICmpInst::ICMP_SLT: return ISD::SETLT;
	case ICmpInst::ICMP_ULT: return ISD::SETULT;
	case ICmpInst::ICMP_SGT: return ISD::SETGT;
	case ICmpInst::ICMP_UGT: return ISD::SETUGT;
	default:
	llvm_unreachable("Invalid ICmp predicate opcode!");
	return ISD::SETNE;
	}
	}

	/// Test if the given instruction is in a position to be optimized
	/// with a tail-call. This roughly means that it's in a block with
	/// a return and there's nothing that needs to be scheduled
	/// between it and the return.
	///
	/// This function only tests target-independent requirements.
	bool llvm::isInTailCallPosition(ImmutableCallSite CS, Attributes CalleeRetAttr,
	const TargetLowering &TLI) {
	const Instruction *I = CS.getInstruction();
	const BasicBlock *ExitBB = I->getParent();
	const TerminatorInst *Term = ExitBB->getTerminator();
	const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);
	const Function *F = ExitBB->getParent();

	// The block must end in a return statement or unreachable.
	//
	// FIXME: Decline tailcall if it's not guaranteed and if the block ends in
	// an unreachable, for now. The way tailcall optimization is currently
	// implemented means it will add an epilogue followed by a jump. That is
	// not profitable. Also, if the callee is a special function (e.g.
	// longjmp on x86), it can end up causing miscompilation that has not
	// been fully understood.
	if (!Ret &&
	(!GuaranteedTailCallOpt \|\| !isa<UnreachableInst>(Term))) return false;

	// If I will have a chain, make sure no other instruction that will have a
	// chain interposes between I and the return.
	if (I->mayHaveSideEffects() \|\| I->mayReadFromMemory() \|\|
	!I->isSafeToSpeculativelyExecute())
	for (BasicBlock::const_iterator BBI = prior(prior(ExitBB->end())); ;
	--BBI) {
	if (&*BBI == I)
	break;
	// Debug info intrinsics do not get in the way of tail call optimization.
	if (isa<DbgInfoIntrinsic>(BBI))
	continue;
	if (BBI->mayHaveSideEffects() \|\| BBI->mayReadFromMemory() \|\|
	!BBI->isSafeToSpeculativelyExecute())
	return false;
	}

	// If the block ends with a void return or unreachable, it doesn't matter
	// what the call's return type is.
	if (!Ret \|\| Ret->getNumOperands() == 0) return true;

	// If the return value is undef, it doesn't matter what the call's
	// return type is.
	if (isa<UndefValue>(Ret->getOperand(0))) return true;

	// Conservatively require the attributes of the call to match those of
	// the return. Ignore noalias because it doesn't affect the call sequence.
	unsigned CallerRetAttr = F->getAttributes().getRetAttributes();
	if ((CalleeRetAttr ^ CallerRetAttr) & ~Attribute::NoAlias)
	return false;

	// It's not safe to eliminate the sign / zero extension of the return value.
	if ((CallerRetAttr & Attribute::ZExt) \|\| (CallerRetAttr & Attribute::SExt))
	return false;

	// Otherwise, make sure the unmodified return value of I is the return value.
	for (const Instruction *U = dyn_cast<Instruction>(Ret->getOperand(0)); ;
	U = dyn_cast<Instruction>(U->getOperand(0))) {
	if (!U)
	return false;
	if (!U->hasOneUse())
	return false;
	if (U == I)
	break;
	// Check for a truly no-op truncate.
	if (isa<TruncInst>(U) &&
	TLI.isTruncateFree(U->getOperand(0)->getType(), U->getType()))
	continue;
	// Check for a truly no-op bitcast.
	if (isa<BitCastInst>(U) &&
	(U->getOperand(0)->getType() == U->getType() \|\|
	(U->getOperand(0)->getType()->isPointerTy() &&
	U->getType()->isPointerTy())))
	continue;
	// Otherwise it's not a true no-op.
	return false;
	}

	return true;
	}