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