* Add capability to recognize alias properties of the following common cases:

- A[c1] cannot alias A[c2] where constants c1 != c2
  - A[i] cannot alias B[j] if A & B are provably different arrays

This should help out array based codes.  For example, from bzip2 from spec,
3 additional loads can be GCSE'd, and _21_ additional loads can be LICMd due
to this change.

In a test example from the Spec GAP benchmark (vecffe.c), this change allows
_52_ additional loads to be GCSE'd and _224_ additional LICM'd loads.

Not bad for such a simple change.  Other testcases show no change at all
because they just don't use arrays.  Not too suprising there.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3616 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2002-09-08 18:45:18 +00:00
parent e4f318c7fc
commit a6299345ee

View File

@ -21,6 +21,7 @@
#include "llvm/BasicBlock.h" #include "llvm/BasicBlock.h"
#include "llvm/Support/InstVisitor.h" #include "llvm/Support/InstVisitor.h"
#include "llvm/iMemory.h" #include "llvm/iMemory.h"
#include "llvm/iOther.h"
#include "llvm/Constants.h" #include "llvm/Constants.h"
#include "llvm/GlobalValue.h" #include "llvm/GlobalValue.h"
#include "llvm/DerivedTypes.h" #include "llvm/DerivedTypes.h"
@ -124,6 +125,24 @@ static inline bool hasUniqueAddress(const Value *V) {
return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V); return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
} }
static const Value *getUnderlyingObject(const Value *V) {
if (!isa<PointerType>(V->getType())) return 0;
// If we are at some type of object... return it.
if (hasUniqueAddress(V)) return V;
// Traverse through different addressing mechanisms...
if (const Instruction *I = dyn_cast<Instruction>(V)) {
if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
return getUnderlyingObject(I->getOperand(0));
}
return 0;
}
// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
// as array references. Note that this function is heavily tail recursive.
// Hopefully we have a smart C++ compiler. :)
//
AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1, AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
const Value *V2) const { const Value *V2) const {
// Strip off constant pointer refs if they exist // Strip off constant pointer refs if they exist
@ -135,20 +154,68 @@ AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
// Are we checking for alias of the same value? // Are we checking for alias of the same value?
if (V1 == V2) return MustAlias; if (V1 == V2) return MustAlias;
if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
return NoAlias; // Scalars cannot alias each other return NoAlias; // Scalars cannot alias each other
bool V1Unique = hasUniqueAddress(V1); // Strip off cast instructions...
bool V2Unique = hasUniqueAddress(V2); if (const Instruction *I = dyn_cast<CastInst>(V1))
return alias(I->getOperand(0), V2);
if (const Instruction *I = dyn_cast<CastInst>(V2))
return alias(I->getOperand(0), V1);
if (V1Unique && V2Unique) // If we have two gep instructions with identical indices, return an alias
return NoAlias; // Can't alias if they are different unique values // result equal to the alias result of the original pointer...
//
if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
if (std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
return alias(GEP1->getOperand(0), GEP2->getOperand(0));
if ((V1Unique && isa<ConstantPointerNull>(V2)) || // If all of the indexes to the getelementptr are constant, but
(V2Unique && isa<ConstantPointerNull>(V1))) // different (well we already know they are different), then we know
return NoAlias; // Unique values don't alias null // that there cannot be an alias here if the two base pointers DO alias.
//
bool AllConstant = true;
for (unsigned i = 1, e = GEP1->getNumOperands(); i != e; ++i)
if (!isa<Constant>(GEP1->getOperand(i)) ||
!isa<Constant>(GEP2->getOperand(i))) {
AllConstant = false;
break;
}
// TODO: Handle getelementptr with nonzero offset // If we are all constant, then look at where the the base pointers
// alias. If they are known not to alias, then we are dealing with two
// different arrays or something, so no alias is possible. If they are
// known to be the same object, then we cannot alias because we are
// indexing into a different part of the object. As usual, MayAlias
// doesn't tell us anything.
//
if (AllConstant &&
alias(GEP1->getOperand(0), GEP2->getOperand(1)) != MayAlias)
return NoAlias;
}
// Figure out what objects these things are pointing to if we can...
const Value *O1 = getUnderlyingObject(V1);
const Value *O2 = getUnderlyingObject(V2);
// Pointing at a discernable object?
if (O1 && O2) {
// If they are two different objects, we know that we have no alias...
if (O1 != O2) return NoAlias;
// If they are the same object, they we can look at the indexes. If they
// index off of the object is the same for both pointers, they must alias.
// If they are provably different, they must not alias. Otherwise, we can't
// tell anything.
} else if (O1 && isa<ConstantPointerNull>(V2)) {
return NoAlias; // Unique values don't alias null
} else if (O2 && isa<ConstantPointerNull>(V1)) {
return NoAlias; // Unique values don't alias null
}
return MayAlias; return MayAlias;
} }