[LLVMdev] alias analysis results

[Weihua Sheng]

Hi Eli, thanks for the answers helping out.

I tried to understand further - got another example:
void test()
{
    int *jj, *kk;
    int aa = 100;

    jj = &aa;

    *jj = 300;

    kk = jj;

    *kk = 400;
}


int main()
{
    test();
    return 0;
}

bc looks like the following (only test() part)
define void @test() nounwind {
entry:
	%aa = alloca i32		; <i32*> [#uses=2]
	%kk = alloca i32*		; <i32**> [#uses=2]
	%jj = alloca i32*		; <i32**> [#uses=3]
	%"alloca point" = bitcast i32 0 to i32		; <i32> [#uses=0]
	store i32 100, i32* %aa, align 4
	store i32* %aa, i32** %jj, align 8
	%0 = load i32** %jj, align 8		; <i32*> [#uses=1]
	store i32 300, i32* %0, align 4
	%1 = load i32** %jj, align 8		; <i32*> [#uses=1]
	store i32* %1, i32** %kk, align 8
	%2 = load i32** %kk, align 8		; <i32*> [#uses=1]
	store i32 400, i32* %2, align 4
	br label %return

return:		; preds = %entry
	ret void
}

the andersens-aa gives out the following results:
Function: test: 6 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
  MayAlias:     i32* %1, i32* %aa
  NoAlias:      i32* %1, i32** %kk
  NoAlias:      i32* %1, i32** %jj
  MayAlias:     i32* %0, i32* %1
  MayAlias:     i32* %2, i32* %aa
  NoAlias:      i32* %2, i32** %kk
  NoAlias:      i32* %2, i32** %jj
  MayAlias:     i32* %0, i32* %2
  MayAlias:     i32* %1, i32* %2

which I interpret as (%0, %1, %aa, %2) as the alias set.  Here comes
my beginner's question: will the AA give the result that kk and jj
alias (that's what I understand from C and possibly could see from
bc)? Thanks a lot again!

Weihua








On Fri, May 15, 2009 at 1:33 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Thu, May 14, 2009 at 2:19 PM, Weihua Sheng <weihua.sheng at gmail.com> wrote:
>> 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?
>
> I'm not quite following... jj and aa don't alias; both versions show
> that.  The results returned for anders-aa are completely precise
> except that it doesn't know that "%0" and "%aa" must point to exactly
> the same object.
>
> -Eli
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