[LLVMdev] alias analysis results

[Weihua Sheng]

Hi there,

I am trying to understand how AliasAnalysis works out in LLVM. I used
the following simple test-case (test4.c):
--
void test()
{
    int *jj, *kk;
    int aa = 100;
    jj = &aa;
    *jj = 300;
}

int main()
{
    test();
    return 0;
}
--
Then I did "llvm-gcc -emit-llvm -c -o test4.bc test4.c" to get bc. I
tried the following 2 ways to get what I expect to get as
"alias-sets":
1. opt -aa-eval -print-all-alias-modref-info test4.bc
Function: test: 4 pointers, 0 call sites
  NoAlias:      i32* %aa, i32** %kk
  NoAlias:      i32* %aa, i32** %jj
  NoAlias:      i32** %jj, i32** %kk
  MayAlias:     i32* %0, i32* %aa
  MayAlias:     i32* %0, i32** %kk
  MayAlias:     i32* %0, i32** %jj
Function: main: 2 pointers, 1 call sites
  NoAlias:      i32* %0, i32* %retval
  NoModRef:  Ptr: i32* %retval  <->     call void @test() nounwind
  NoModRef:  Ptr: i32* %0       <->     call void @test() nounwind

This way at least I could see " aa <- (kk, jj)" results from mayalias.

2. opt -anders-aa -aa-eval -print-all-alias-modref-info test4.bc
Function: test: 4 pointers, 0 call sites
  NoAlias:      i32* %aa, i32** %kk
  NoAlias:      i32* %aa, i32** %jj
  NoAlias:      i32** %jj, i32** %kk
  MayAlias:     i32* %0, i32* %aa
  NoAlias:      i32* %0, i32** %kk
  NoAlias:      i32* %0, i32** %jj
Function: main: 2 pointers, 1 call sites
  NoAlias:      i32* %0, i32* %retval
  NoModRef:  Ptr: i32* %retval  <->     call void @test() nounwind
  NoModRef:  Ptr: i32* %0       <->     call void @test() nounwind

I actullay would expect the more accurate results from applying
anders-aa, but I could not interpret what has been returned - at least
I should see something like jj->aa, right?

Maybe I missed something out, could someone on the mailing list share
some thoughts?

Thanks a lot!

Weihua