[LLVMdev] Wrong AliasAnalysis::getModRefInfo result
Welson Sun
welson.sun at gmail.com
Thu Feb 16 11:38:29 PST 2012
Something must be wrong, more probable on my side. So the C source code is
unchanged, I just did another experiment to first extract all the GEPs in
the code, and call AliasAnalysis::alias on each pair of GEPs. Here is the
code:
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
TargetData &TD = getAnalysis<TargetData>();
for (Module::iterator it = M.begin(); it != M.end(); it++){
Function &F = *it;
errs().write_escaped(F.getName()) << '\n';
for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
{
Instruction *Inst = &*I;
if ( GetElementPtrInst *gep = dyn_cast<GetElementPtrInst>(Inst) )
geps.push_back(gep);
}
}
errs() << "List of GEPs:\n";
for(std::list<GetElementPtrInst* >::iterator itGep1
= geps.begin(); itGep1 != geps.end(); itGep1++){
GetElementPtrInst *gep1 = *itGep1;
Type *t1 = ((PointerType *)gep1->getType())->getElementType();
int64_t size1 = TD.getTypeAllocSize(t1);
std::list<GetElementPtrInst* >::iterator itGep2 = itGep1;
for(; itGep2 != geps.end(); itGep2++){
GetElementPtrInst *gep2 = *itGep2;
gep1->dump();
errs() << "-->";
gep2->dump();
Type *t2 = ((PointerType *)gep2->getType())->getElementType();
int64_t size2 = TD.getTypeAllocSize(t2);
AliasAnalysis::AliasResult aa = AA.alias(gep1, size1,
errs() << " alias: " << aa << "\n";
}
}
Here is the result:
%9 = getelementptr inbounds i32* %8, i32 %7
--> %9 = getelementptr inbounds i32* %8, i32 %7
alias: 1
%9 = getelementptr inbounds i32* %8, i32 %7
--> %13 = getelementptr inbounds i32* %12, i32 %11
alias: 1
%9 = getelementptr inbounds i32* %8, i32 %7
--> %18 = getelementptr inbounds i32* %17, i32 %16
alias: 1
%9 = getelementptr inbounds i32* %8, i32 %7
--> %8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
alias: 1
%9 = getelementptr inbounds i32* %8, i32 %7
--> %13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
alias: 1
%9 = getelementptr inbounds i32* %8, i32 %7
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
%13 = getelementptr inbounds i32* %12, i32 %11
--> %13 = getelementptr inbounds i32* %12, i32 %11
alias: 1
%13 = getelementptr inbounds i32* %12, i32 %11
--> %18 = getelementptr inbounds i32* %17, i32 %16
alias: 1
%13 = getelementptr inbounds i32* %12, i32 %11
--> %8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
alias: 1
%13 = getelementptr inbounds i32* %12, i32 %11
--> %13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
alias: 1
%13 = getelementptr inbounds i32* %12, i32 %11
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
%18 = getelementptr inbounds i32* %17, i32 %16
--> %18 = getelementptr inbounds i32* %17, i32 %16
alias: 1
%18 = getelementptr inbounds i32* %17, i32 %16
--> %8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
alias: 1
%18 = getelementptr inbounds i32* %17, i32 %16
--> %13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
alias: 1
%18 = getelementptr inbounds i32* %17, i32 %16
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
%8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
--> %8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
alias: 1
%8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
--> %13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
alias: 1
%8 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 %7
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
%13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
--> %13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
alias: 1
%13 = getelementptr inbounds [10 x i32]* %a, i32 0, i32 0
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
%14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
--> %14 = getelementptr inbounds [10 x i32]* %b, i32 0, i32 0
alias: 1
How can be each pair MayAlias? That's simply wrong! For example:
%13 = getelementptr inbounds [10 x i32]* *%a*, i32 0, i32 0
--> %14 = getelementptr inbounds [10 x i32]* *%b*, i32 0, i32 0
Must not alias,
while
%14 = getelementptr inbounds [10 x i32]* *%b*, i32 0, i32 0
--> %14 = getelementptr inbounds [10 x i32]* *%b*, i32 0, i32 0
Must alias.
Can someone help identifying where is wrong with my code?
Thanks!
Welson
On Tue, Feb 14, 2012 at 5:12 PM, Welson Sun <welson.sun at gmail.com> wrote:
> Just want to test out the LLVM's AliasAnalysis::getModRefInfo API. The
> input C code is very simple:
>
> void foo(int *a, int *b)
> {
> for(int i=0; i<10; i++)
> b[i] = a[i]*a[i];
> }
>
> int main()
> {
> int a[10];
> int b[10];
>
> for(int i=0; i<10; i++)
> a[i] = i;
>
> foo(a,b);
>
> return 0;
> }
>
> Obviously, for "foo", it only reads from array "a" and only writes to
> array "b".
>
> The LLVM pass:
> virtual bool runOnFunction(Function &F) {
> ++HelloCounter;
> errs() << "Hello: ";
> errs().write_escaped(F.getName()) << '\n';
>
> AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
> for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) {
> Instruction *Inst = &*I;
> if ( CallInst *ci = dyn_cast<CallInst>(Inst) ){
> ci->dump();
> for(int i = 0; i < ci->getNumArgOperands(); i++){
> Value *v = ci->getArgOperand(i);
> if (GetElementPtrInst *vi = dyn_cast<GetElementPtrInst>(v)){
> Value *vPtr = vi->getPointerOperand();
> vPtr->dump();
> if ( AllocaInst *allo = dyn_cast<AllocaInst>(vPtr) ) {
> const Type *t = allo->getAllocatedType();
> if ( const ArrayType *at = dyn_cast<ArrayType>(t) ) {
> int64_t size = at->getNumElements() *
> at->getElementType()->getPrimitiveSizeInBits() / 8;
> ImmutableCallSite cs(ci);
> AliasAnalysis::Location loc(v, size);
> errs() << AA.getModRefInfo(ci, loc) << "\n";
> }
> }
> }
> }
> }
> }
>
> return false;
> }
>
>
> However, the result is "3" for both a and b, which is both read and write.
> What's the problem? I am not quite sure if I get the
> AliasAnalysis::Location right, what is exactly "address-units" for the size
> of the location? And did I get the starting address of the Location right?
> I tried v, vi and vPtr, same result.
>
>
> Any insight helps,
> Welson
>
--
Welson
Phone: (408) 418-8385
Email: welson.sun at gmail.com
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