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llvm-mirror/lib/Analysis/ScalarEvolutionAliasAnalysis.cpp
Chandler Carruth dad102bcc9 [PM] Change the static object whose address is used to uniquely identify
analyses to have a common type which is enforced rather than using
a char object and a `void *` type when used as an identifier.

This has a number of advantages. First, it at least helps some of the
confusion raised in Justin Lebar's code review of why `void *` was being
used everywhere by having a stronger type that connects to documentation
about this.

However, perhaps more importantly, it addresses a serious issue where
the alignment of these pointer-like identifiers was unknown. This made
it hard to use them in pointer-like data structures. We were already
dodging this in dangerous ways to create the "all analyses" entry. In
a subsequent patch I attempted to use these with TinyPtrVector and
things fell apart in a very bad way.

And it isn't just a compile time or type system issue. Worse than that,
the actual alignment of these pointer-like opaque identifiers wasn't
guaranteed to be a useful alignment as they were just characters.

This change introduces a type to use as the "key" object whose address
forms the opaque identifier. This both forces the objects to have proper
alignment, and provides type checking that we get it right everywhere.
It also makes the types somewhat less mysterious than `void *`.

We could go one step further and introduce a truly opaque pointer-like
type to return from the `ID()` static function rather than returning
`AnalysisKey *`, but that didn't seem to be a clear win so this is just
the initial change to get to a reliably typed and aligned object serving
is a key for all the analyses.

Thanks to Richard Smith and Justin Lebar for helping pick plausible
names and avoid making this refactoring many times. =] And thanks to
Sean for the super fast review!

While here, I've tried to move away from the "PassID" nomenclature
entirely as it wasn't really helping and is overloaded with old pass
manager constructs. Now we have IDs for analyses, and key objects whose
address can be used as IDs. Where possible and clear I've shortened this
to just "ID". In a few places I kept "AnalysisID" to make it clear what
was being identified.

Differential Revision: https://reviews.llvm.org/D27031

llvm-svn: 287783
2016-11-23 17:53:26 +00:00

144 lines
5.6 KiB
C++

//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
// simple alias analysis implemented in terms of ScalarEvolution queries.
//
// This differs from traditional loop dependence analysis in that it tests
// for dependencies within a single iteration of a loop, rather than
// dependencies between different iterations.
//
// ScalarEvolution has a more complete understanding of pointer arithmetic
// than BasicAliasAnalysis' collection of ad-hoc analyses.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
using namespace llvm;
AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
// If either of the memory references is empty, it doesn't matter what the
// pointer values are. This allows the code below to ignore this special
// case.
if (LocA.Size == 0 || LocB.Size == 0)
return NoAlias;
// This is SCEVAAResult. Get the SCEVs!
const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
// If they evaluate to the same expression, it's a MustAlias.
if (AS == BS)
return MustAlias;
// If something is known about the difference between the two addresses,
// see if it's enough to prove a NoAlias.
if (SE.getEffectiveSCEVType(AS->getType()) ==
SE.getEffectiveSCEVType(BS->getType())) {
unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
APInt ASizeInt(BitWidth, LocA.Size);
APInt BSizeInt(BitWidth, LocB.Size);
// Compute the difference between the two pointers.
const SCEV *BA = SE.getMinusSCEV(BS, AS);
// Test whether the difference is known to be great enough that memory of
// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
// are non-zero, which is special-cased above.
if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
(-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
return NoAlias;
// Folding the subtraction while preserving range information can be tricky
// (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
// and try again to see if things fold better that way.
// Compute the difference between the two pointers.
const SCEV *AB = SE.getMinusSCEV(AS, BS);
// Test whether the difference is known to be great enough that memory of
// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
// are non-zero, which is special-cased above.
if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
(-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
return NoAlias;
}
// If ScalarEvolution can find an underlying object, form a new query.
// The correctness of this depends on ScalarEvolution not recognizing
// inttoptr and ptrtoint operators.
Value *AO = GetBaseValue(AS);
Value *BO = GetBaseValue(BS);
if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
AO ? +MemoryLocation::UnknownSize : LocA.Size,
AO ? AAMDNodes() : LocA.AATags),
MemoryLocation(BO ? BO : LocB.Ptr,
BO ? +MemoryLocation::UnknownSize : LocB.Size,
BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
return NoAlias;
// Forward the query to the next analysis.
return AAResultBase::alias(LocA, LocB);
}
/// Given an expression, try to find a base value.
///
/// Returns null if none was found.
Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// In an addrec, assume that the base will be in the start, rather
// than the step.
return GetBaseValue(AR->getStart());
} else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
// If there's a pointer operand, it'll be sorted at the end of the list.
const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
if (Last->getType()->isPointerTy())
return GetBaseValue(Last);
} else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
// This is a leaf node.
return U->getValue();
}
// No Identified object found.
return nullptr;
}
AnalysisKey SCEVAA::Key;
SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
}
char SCEVAAWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
"ScalarEvolution-based Alias Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
"ScalarEvolution-based Alias Analysis", false, true)
FunctionPass *llvm::createSCEVAAWrapperPass() {
return new SCEVAAWrapperPass();
}
SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
bool SCEVAAWrapperPass::runOnFunction(Function &F) {
Result.reset(
new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
return false;
}
void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<ScalarEvolutionWrapperPass>();
}