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Allow basicaa to walk through geps with identical indices in

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parallel, allowing it to decide that P/Q must alias if A/B
must alias in things like:
 P = gep A, 0, i, 1
 Q = gep B, 0, i, 1

This allows GVN to delete 62 more instructions out of 403.gcc.

llvm-svn: 60820
This commit is contained in:
Chris Lattner 2008-12-10 01:04:47 +00:00
parent a3b718a3c9
commit e2b5854e41
2 changed files with 47 additions and 17 deletions
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48
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -85,7 +85,7 @@ static const User *isGEP(const Value *V) {
}
static const Value *GetGEPOperands(const Value *V,
SmallVector<Value*, 16> &GEPOps){
SmallVector<Value*, 16> &GEPOps) {
assert(GEPOps.empty() && "Expect empty list to populate!");
GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
cast<User>(V)->op_end());
@ -369,8 +369,7 @@ BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
// 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. :)
// as array references.
//
AliasAnalysis::AliasResult
BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
@ -389,13 +388,14 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
return NoAlias; // Scalars cannot alias each other
// Strip off cast instructions...
// Strip off cast instructions. Since V1 and V2 are pointers, they must be
// pointer<->pointer bitcasts.
if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
return alias(I->getOperand(0), V1Size, V2, V2Size);
if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
return alias(V1, V1Size, I->getOperand(0), V2Size);
// Figure out what objects these things are pointing to if we can...
// Figure out what objects these things are pointing to if we can.
const Value *O1 = V1->getUnderlyingObject();
const Value *O2 = V2->getUnderlyingObject();
@ -438,21 +438,35 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
// constant expression getelementptrs here.
//
if (isGEP(V1) && isGEP(V2)) {
const User *GEP1 = cast<User>(V1);
const User *GEP2 = cast<User>(V2);
// If V1 and V2 are identical GEPs, just recurse down on both of them.
// This allows us to analyze things like:
// P = gep A, 0, i, 1
// Q = gep B, 0, i, 1
// by just analyzing A and B. This is even safe for variable indices.
if (GEP1->getType() == GEP2->getType() &&
GEP1->getNumOperands() == GEP2->getNumOperands() &&
GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
// All operands are the same, ignoring the base.
std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
// Drill down into the first non-gep value, to test for must-aliasing of
// the base pointers.
const User *G = cast<User>(V1);
while (isGEP(G->getOperand(0)) &&
G->getOperand(1) ==
Constant::getNullValue(G->getOperand(1)->getType()))
G = cast<User>(G->getOperand(0));
const Value *BasePtr1 = G->getOperand(0);
while (isGEP(GEP1->getOperand(0)) &&
GEP1->getOperand(1) ==
Constant::getNullValue(GEP1->getOperand(1)->getType()))
GEP1 = cast<User>(GEP1->getOperand(0));
const Value *BasePtr1 = GEP1->getOperand(0);
G = cast<User>(V2);
while (isGEP(G->getOperand(0)) &&
G->getOperand(1) ==
Constant::getNullValue(G->getOperand(1)->getType()))
G = cast<User>(G->getOperand(0));
const Value *BasePtr2 = G->getOperand(0);
while (isGEP(GEP2->getOperand(0)) &&
GEP2->getOperand(1) ==
Constant::getNullValue(GEP2->getOperand(1)->getType()))
GEP2 = cast<User>(GEP2->getOperand(0));
const Value *BasePtr2 = GEP2->getOperand(0);
// Do the base pointers alias?
AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);

16
test/Analysis/BasicAA/2008-12-09-GEP-IndicesAlias.ll Normal file
View File

@ -0,0 +1,16 @@
; RUN: llvm-as < %s | opt -aa-eval -print-all-alias-modref-info -disable-output |& grep {MustAlias:.*%R,.*%r}
; Make sure that basicaa thinks R and r are must aliases.
define i32 @test(i8 * %P) {
entry:
%Q = bitcast i8* %P to {i32, i32}*
%R = getelementptr {i32, i32}* %Q, i32 0, i32 1
%S = load i32* %R
%q = bitcast i8* %P to {i32, i32}*
%r = getelementptr {i32, i32}* %q, i32 0, i32 1
%s = load i32* %r
%t = sub i32 %S, %s
ret i32 %t
}