llvm-6502/lib/Analysis/AliasAnalysis.cpp
Dan Gohman de0eb19248 Move isKnownNonNull out of AliasAnalysis.h and into ValueTracking.cpp since
it isn't really an AliasAnalysis concept, and ValueTracking has similar things
that it could plausibly share code with some day.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174027 91177308-0d34-0410-b5e6-96231b3b80d8
2013-01-31 02:40:59 +00:00

558 lines
20 KiB
C++

//===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the generic AliasAnalysis interface which is used as the
// common interface used by all clients and implementations of alias analysis.
//
// This file also implements the default version of the AliasAnalysis interface
// that is to be used when no other implementation is specified. This does some
// simple tests that detect obvious cases: two different global pointers cannot
// alias, a global cannot alias a malloc, two different mallocs cannot alias,
// etc.
//
// This alias analysis implementation really isn't very good for anything, but
// it is very fast, and makes a nice clean default implementation. Because it
// handles lots of little corner cases, other, more complex, alias analysis
// implementations may choose to rely on this pass to resolve these simple and
// easy cases.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Type.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetLibraryInfo.h"
using namespace llvm;
// Register the AliasAnalysis interface, providing a nice name to refer to.
INITIALIZE_ANALYSIS_GROUP(AliasAnalysis, "Alias Analysis", NoAA)
char AliasAnalysis::ID = 0;
//===----------------------------------------------------------------------===//
// Default chaining methods
//===----------------------------------------------------------------------===//
AliasAnalysis::AliasResult
AliasAnalysis::alias(const Location &LocA, const Location &LocB) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
return AA->alias(LocA, LocB);
}
bool AliasAnalysis::pointsToConstantMemory(const Location &Loc,
bool OrLocal) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
return AA->pointsToConstantMemory(Loc, OrLocal);
}
void AliasAnalysis::deleteValue(Value *V) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
AA->deleteValue(V);
}
void AliasAnalysis::copyValue(Value *From, Value *To) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
AA->copyValue(From, To);
}
void AliasAnalysis::addEscapingUse(Use &U) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
AA->addEscapingUse(U);
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
const Location &Loc) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
ModRefBehavior MRB = getModRefBehavior(CS);
if (MRB == DoesNotAccessMemory)
return NoModRef;
ModRefResult Mask = ModRef;
if (onlyReadsMemory(MRB))
Mask = Ref;
if (onlyAccessesArgPointees(MRB)) {
bool doesAlias = false;
if (doesAccessArgPointees(MRB)) {
MDNode *CSTag = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
AI != AE; ++AI) {
const Value *Arg = *AI;
if (!Arg->getType()->isPointerTy())
continue;
Location CSLoc(Arg, UnknownSize, CSTag);
if (!isNoAlias(CSLoc, Loc)) {
doesAlias = true;
break;
}
}
}
if (!doesAlias)
return NoModRef;
}
// If Loc is a constant memory location, the call definitely could not
// modify the memory location.
if ((Mask & Mod) && pointsToConstantMemory(Loc))
Mask = ModRefResult(Mask & ~Mod);
// If this is the end of the chain, don't forward.
if (!AA) return Mask;
// Otherwise, fall back to the next AA in the chain. But we can merge
// in any mask we've managed to compute.
return ModRefResult(AA->getModRefInfo(CS, Loc) & Mask);
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
// If CS1 or CS2 are readnone, they don't interact.
ModRefBehavior CS1B = getModRefBehavior(CS1);
if (CS1B == DoesNotAccessMemory) return NoModRef;
ModRefBehavior CS2B = getModRefBehavior(CS2);
if (CS2B == DoesNotAccessMemory) return NoModRef;
// If they both only read from memory, there is no dependence.
if (onlyReadsMemory(CS1B) && onlyReadsMemory(CS2B))
return NoModRef;
AliasAnalysis::ModRefResult Mask = ModRef;
// If CS1 only reads memory, the only dependence on CS2 can be
// from CS1 reading memory written by CS2.
if (onlyReadsMemory(CS1B))
Mask = ModRefResult(Mask & Ref);
// If CS2 only access memory through arguments, accumulate the mod/ref
// information from CS1's references to the memory referenced by
// CS2's arguments.
if (onlyAccessesArgPointees(CS2B)) {
AliasAnalysis::ModRefResult R = NoModRef;
if (doesAccessArgPointees(CS2B)) {
MDNode *CS2Tag = CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
for (ImmutableCallSite::arg_iterator
I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
const Value *Arg = *I;
if (!Arg->getType()->isPointerTy())
continue;
Location CS2Loc(Arg, UnknownSize, CS2Tag);
R = ModRefResult((R | getModRefInfo(CS1, CS2Loc)) & Mask);
if (R == Mask)
break;
}
}
return R;
}
// If CS1 only accesses memory through arguments, check if CS2 references
// any of the memory referenced by CS1's arguments. If not, return NoModRef.
if (onlyAccessesArgPointees(CS1B)) {
AliasAnalysis::ModRefResult R = NoModRef;
if (doesAccessArgPointees(CS1B)) {
MDNode *CS1Tag = CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa);
for (ImmutableCallSite::arg_iterator
I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I) {
const Value *Arg = *I;
if (!Arg->getType()->isPointerTy())
continue;
Location CS1Loc(Arg, UnknownSize, CS1Tag);
if (getModRefInfo(CS2, CS1Loc) != NoModRef) {
R = Mask;
break;
}
}
}
if (R == NoModRef)
return R;
}
// If this is the end of the chain, don't forward.
if (!AA) return Mask;
// Otherwise, fall back to the next AA in the chain. But we can merge
// in any mask we've managed to compute.
return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
}
AliasAnalysis::ModRefBehavior
AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
ModRefBehavior Min = UnknownModRefBehavior;
// Call back into the alias analysis with the other form of getModRefBehavior
// to see if it can give a better response.
if (const Function *F = CS.getCalledFunction())
Min = getModRefBehavior(F);
// If this is the end of the chain, don't forward.
if (!AA) return Min;
// Otherwise, fall back to the next AA in the chain. But we can merge
// in any result we've managed to compute.
return ModRefBehavior(AA->getModRefBehavior(CS) & Min);
}
AliasAnalysis::ModRefBehavior
AliasAnalysis::getModRefBehavior(const Function *F) {
assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
return AA->getModRefBehavior(F);
}
//===----------------------------------------------------------------------===//
// AliasAnalysis non-virtual helper method implementation
//===----------------------------------------------------------------------===//
AliasAnalysis::Location AliasAnalysis::getLocation(const LoadInst *LI) {
return Location(LI->getPointerOperand(),
getTypeStoreSize(LI->getType()),
LI->getMetadata(LLVMContext::MD_tbaa));
}
AliasAnalysis::Location AliasAnalysis::getLocation(const StoreInst *SI) {
return Location(SI->getPointerOperand(),
getTypeStoreSize(SI->getValueOperand()->getType()),
SI->getMetadata(LLVMContext::MD_tbaa));
}
AliasAnalysis::Location AliasAnalysis::getLocation(const VAArgInst *VI) {
return Location(VI->getPointerOperand(),
UnknownSize,
VI->getMetadata(LLVMContext::MD_tbaa));
}
AliasAnalysis::Location
AliasAnalysis::getLocation(const AtomicCmpXchgInst *CXI) {
return Location(CXI->getPointerOperand(),
getTypeStoreSize(CXI->getCompareOperand()->getType()),
CXI->getMetadata(LLVMContext::MD_tbaa));
}
AliasAnalysis::Location
AliasAnalysis::getLocation(const AtomicRMWInst *RMWI) {
return Location(RMWI->getPointerOperand(),
getTypeStoreSize(RMWI->getValOperand()->getType()),
RMWI->getMetadata(LLVMContext::MD_tbaa));
}
AliasAnalysis::Location
AliasAnalysis::getLocationForSource(const MemTransferInst *MTI) {
uint64_t Size = UnknownSize;
if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
Size = C->getValue().getZExtValue();
// memcpy/memmove can have TBAA tags. For memcpy, they apply
// to both the source and the destination.
MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
return Location(MTI->getRawSource(), Size, TBAATag);
}
AliasAnalysis::Location
AliasAnalysis::getLocationForDest(const MemIntrinsic *MTI) {
uint64_t Size = UnknownSize;
if (ConstantInt *C = dyn_cast<ConstantInt>(MTI->getLength()))
Size = C->getValue().getZExtValue();
// memcpy/memmove can have TBAA tags. For memcpy, they apply
// to both the source and the destination.
MDNode *TBAATag = MTI->getMetadata(LLVMContext::MD_tbaa);
return Location(MTI->getRawDest(), Size, TBAATag);
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(const LoadInst *L, const Location &Loc) {
// Be conservative in the face of volatile/atomic.
if (!L->isUnordered())
return ModRef;
// If the load address doesn't alias the given address, it doesn't read
// or write the specified memory.
if (!alias(getLocation(L), Loc))
return NoModRef;
// Otherwise, a load just reads.
return Ref;
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(const StoreInst *S, const Location &Loc) {
// Be conservative in the face of volatile/atomic.
if (!S->isUnordered())
return ModRef;
// If the store address cannot alias the pointer in question, then the
// specified memory cannot be modified by the store.
if (!alias(getLocation(S), Loc))
return NoModRef;
// If the pointer is a pointer to constant memory, then it could not have been
// modified by this store.
if (pointsToConstantMemory(Loc))
return NoModRef;
// Otherwise, a store just writes.
return Mod;
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(const VAArgInst *V, const Location &Loc) {
// If the va_arg address cannot alias the pointer in question, then the
// specified memory cannot be accessed by the va_arg.
if (!alias(getLocation(V), Loc))
return NoModRef;
// If the pointer is a pointer to constant memory, then it could not have been
// modified by this va_arg.
if (pointsToConstantMemory(Loc))
return NoModRef;
// Otherwise, a va_arg reads and writes.
return ModRef;
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(const AtomicCmpXchgInst *CX, const Location &Loc) {
// Acquire/Release cmpxchg has properties that matter for arbitrary addresses.
if (CX->getOrdering() > Monotonic)
return ModRef;
// If the cmpxchg address does not alias the location, it does not access it.
if (!alias(getLocation(CX), Loc))
return NoModRef;
return ModRef;
}
AliasAnalysis::ModRefResult
AliasAnalysis::getModRefInfo(const AtomicRMWInst *RMW, const Location &Loc) {
// Acquire/Release atomicrmw has properties that matter for arbitrary addresses.
if (RMW->getOrdering() > Monotonic)
return ModRef;
// If the atomicrmw address does not alias the location, it does not access it.
if (!alias(getLocation(RMW), Loc))
return NoModRef;
return ModRef;
}
namespace {
// Conservatively return true. Return false, if there is a single path
// starting from "From" and the path does not reach "To".
static bool hasPath(const BasicBlock *From, const BasicBlock *To) {
const unsigned MaxCheck = 5;
const BasicBlock *Current = From;
for (unsigned I = 0; I < MaxCheck; I++) {
unsigned NumSuccs = Current->getTerminator()->getNumSuccessors();
if (NumSuccs > 1)
return true;
if (NumSuccs == 0)
return false;
Current = Current->getTerminator()->getSuccessor(0);
if (Current == To)
return true;
}
return true;
}
/// Only find pointer captures which happen before the given instruction. Uses
/// the dominator tree to determine whether one instruction is before another.
/// Only support the case where the Value is defined in the same basic block
/// as the given instruction and the use.
struct CapturesBefore : public CaptureTracker {
CapturesBefore(const Instruction *I, DominatorTree *DT)
: BeforeHere(I), DT(DT), Captured(false) {}
void tooManyUses() { Captured = true; }
bool shouldExplore(Use *U) {
Instruction *I = cast<Instruction>(U->getUser());
BasicBlock *BB = I->getParent();
// We explore this usage only if the usage can reach "BeforeHere".
// If use is not reachable from entry, there is no need to explore.
if (BeforeHere != I && !DT->isReachableFromEntry(BB))
return false;
// If the value is defined in the same basic block as use and BeforeHere,
// there is no need to explore the use if BeforeHere dominates use.
// Check whether there is a path from I to BeforeHere.
if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
!hasPath(BB, BeforeHere->getParent()))
return false;
return true;
}
bool captured(Use *U) {
Instruction *I = cast<Instruction>(U->getUser());
BasicBlock *BB = I->getParent();
// Same logic as in shouldExplore.
if (BeforeHere != I && !DT->isReachableFromEntry(BB))
return false;
if (BeforeHere != I && DT->dominates(BeforeHere, I) &&
!hasPath(BB, BeforeHere->getParent()))
return false;
Captured = true;
return true;
}
const Instruction *BeforeHere;
DominatorTree *DT;
bool Captured;
};
}
// FIXME: this is really just shoring-up a deficiency in alias analysis.
// BasicAA isn't willing to spend linear time determining whether an alloca
// was captured before or after this particular call, while we are. However,
// with a smarter AA in place, this test is just wasting compile time.
AliasAnalysis::ModRefResult
AliasAnalysis::callCapturesBefore(const Instruction *I,
const AliasAnalysis::Location &MemLoc,
DominatorTree *DT) {
if (!DT || !TD) return AliasAnalysis::ModRef;
const Value *Object = GetUnderlyingObject(MemLoc.Ptr, TD);
if (!isIdentifiedObject(Object) || isa<GlobalValue>(Object) ||
isa<Constant>(Object))
return AliasAnalysis::ModRef;
ImmutableCallSite CS(I);
if (!CS.getInstruction() || CS.getInstruction() == Object)
return AliasAnalysis::ModRef;
CapturesBefore CB(I, DT);
llvm::PointerMayBeCaptured(Object, &CB);
if (CB.Captured)
return AliasAnalysis::ModRef;
unsigned ArgNo = 0;
for (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
CI != CE; ++CI, ++ArgNo) {
// Only look at the no-capture or byval pointer arguments. If this
// pointer were passed to arguments that were neither of these, then it
// couldn't be no-capture.
if (!(*CI)->getType()->isPointerTy() ||
(!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
continue;
// If this is a no-capture pointer argument, see if we can tell that it
// is impossible to alias the pointer we're checking. If not, we have to
// assume that the call could touch the pointer, even though it doesn't
// escape.
if (!isNoAlias(AliasAnalysis::Location(*CI),
AliasAnalysis::Location(Object))) {
return AliasAnalysis::ModRef;
}
}
return AliasAnalysis::NoModRef;
}
// AliasAnalysis destructor: DO NOT move this to the header file for
// AliasAnalysis or else clients of the AliasAnalysis class may not depend on
// the AliasAnalysis.o file in the current .a file, causing alias analysis
// support to not be included in the tool correctly!
//
AliasAnalysis::~AliasAnalysis() {}
/// InitializeAliasAnalysis - Subclasses must call this method to initialize the
/// AliasAnalysis interface before any other methods are called.
///
void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
TD = P->getAnalysisIfAvailable<DataLayout>();
TLI = P->getAnalysisIfAvailable<TargetLibraryInfo>();
AA = &P->getAnalysis<AliasAnalysis>();
}
// getAnalysisUsage - All alias analysis implementations should invoke this
// directly (using AliasAnalysis::getAnalysisUsage(AU)).
void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AliasAnalysis>(); // All AA's chain
}
/// getTypeStoreSize - Return the DataLayout store size for the given type,
/// if known, or a conservative value otherwise.
///
uint64_t AliasAnalysis::getTypeStoreSize(Type *Ty) {
return TD ? TD->getTypeStoreSize(Ty) : UnknownSize;
}
/// canBasicBlockModify - Return true if it is possible for execution of the
/// specified basic block to modify the value pointed to by Ptr.
///
bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
const Location &Loc) {
return canInstructionRangeModify(BB.front(), BB.back(), Loc);
}
/// canInstructionRangeModify - Return true if it is possible for the execution
/// of the specified instructions to modify the value pointed to by Ptr. The
/// instructions to consider are all of the instructions in the range of [I1,I2]
/// INCLUSIVE. I1 and I2 must be in the same basic block.
///
bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
const Instruction &I2,
const Location &Loc) {
assert(I1.getParent() == I2.getParent() &&
"Instructions not in same basic block!");
BasicBlock::const_iterator I = &I1;
BasicBlock::const_iterator E = &I2;
++E; // Convert from inclusive to exclusive range.
for (; I != E; ++I) // Check every instruction in range
if (getModRefInfo(I, Loc) & Mod)
return true;
return false;
}
/// isNoAliasCall - Return true if this pointer is returned by a noalias
/// function.
bool llvm::isNoAliasCall(const Value *V) {
if (isa<CallInst>(V) || isa<InvokeInst>(V))
return ImmutableCallSite(cast<Instruction>(V))
.paramHasAttr(0, Attribute::NoAlias);
return false;
}
/// isIdentifiedObject - Return true if this pointer refers to a distinct and
/// identifiable object. This returns true for:
/// Global Variables and Functions (but not Global Aliases)
/// Allocas and Mallocs
/// ByVal and NoAlias Arguments
/// NoAlias returns
///
bool llvm::isIdentifiedObject(const Value *V) {
if (isa<AllocaInst>(V))
return true;
if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
return true;
if (isNoAliasCall(V))
return true;
if (const Argument *A = dyn_cast<Argument>(V))
return A->hasNoAliasAttr() || A->hasByValAttr();
return false;
}