[LLVMdev] Question about Alias Analysis

[Chris Lattner]

On Wed, 27 Jun 2007, Ben Chambers wrote:
> Thanks, that's a lot more like the output I was expecting.  But, I'm
> now wondering if I'm doing something wrong because the output confuses
> me.

It won't make a lot of sense to you unless you look at the LLVM IR.  In 
particular "%a" represents the address of a, not its contents.

-Chris

> When run on the program:
>
> int main() {
>  int a = 5;
>  int* b = &a;
>  int* c = &a;
>  return *b;
> }
>
> I would expect to see either b may alias c or b must alias c.  But, it
> doesn't even appear in the list.  Furthermore, I would expect to see
> **b (may/must) alias *c, but it instead says that this isn't true.  Am
> I not running the right passes (I used -basicaa -anders-aa) or am I
> misreading things.
>
> Results:
>
> Function: main: 6 pointers, 0 call sites
>  NoAlias:  i32* %retval, i32* %tmp
>  NoAlias:  i32* %a, i32* %tmp
>  NoAlias:  i32* %a, i32* %retval
>  NoAlias:  i32** %b, i32* %tmp
>  NoAlias:  i32** %b, i32* %retval
>  NoAlias:  i32** %b, i32* %a
>  NoAlias:  i32** %c, i32* %tmp
>  NoAlias:  i32** %c, i32* %retval
>  NoAlias:  i32** %c, i32* %a
>  NoAlias:  i32** %c, i32** %b
>  NoAlias:  i32* %tmp1, i32* %tmp
>  NoAlias:  i32* %tmp1, i32* %retval
>  MayAlias: i32* %tmp1, i32* %a
>  NoAlias:  i32* %tmp1, i32** %b
>  NoAlias:  i32* %tmp1, i32** %c
> ===== Alias Analysis Evaluator Report =====
>  15 Total Alias Queries Performed
>  14 no alias responses (93.3%)
>  1 may alias responses (6.6%)
>  0 must alias responses (0.0%)
>  Alias Analysis Evaluator Pointer Alias Summary: 93%/6%/0%
>  Alias Analysis Mod/Ref Evaluator Summary: no mod/ref!
>
> On 6/26/07, Chris Lattner <sabre at nondot.org> wrote:
>> On Mon, 25 Jun 2007, Ben Chambers wrote:
>>> I guess what confuses me is that it doesn't seem like it was able to
>>> figure out that *b = a.  Am I looking at this wrong?  Is there a more
>>> accurate way of getting the alias information out of the pass?
>>
>> The answer is to not look at the alias sets.  I'd suggest looking at the
>> raw results of alias queries.  To do this, use the -aa-eval pass with the
>> -print-all-alias-modref-info option.  The alias set builder uses
>> unification to build the sets, so it will give you less precise
>> information than the raw queries do.
>>
>> -Chris
>>
>>> Thanks,
>>> Ben Chambers
>>>
>>> Output:
>>> AliasSet[0x601e70,2] may  alias, Mod/Ref   Pointers: (i32* %a, 4),
>>> (i32* %tmp1, 4)
>>> AliasSet[0x601ea0,1] must alias, Mod/Ref   Pointers: (i32** %b, 4)
>>> AliasSet[0x601f10,1] must alias, Mod/Ref   Pointers: (i32* %tmp, 4)
>>> AliasSet[0x601f60,1] must alias, Mod/Ref   Pointers: (i32* %retval, 4)
>>>
>>> Input:
>>>
>>> int main() {
>>> int a = 5;
>>> int* b = &a;
>>> return *b;
>>> }
>>>
>>> Assembly:
>>>
>>> define i32 @main() {
>>> entry:
>>> %retval = alloca i32, align 4   ; <i32*> [#uses=2]
>>> %tmp = alloca i32, align 4    ; <i32*> [#uses=2]
>>> %a = alloca i32, align 4    ; <i32*> [#uses=2]
>>> %b = alloca i32*, align 4   ; <i32**> [#uses=2]
>>> "alloca point" = bitcast i32 0 to i32   ; <i32> [#uses=0]
>>> store i32 5, i32* %a
>>> store i32* %a, i32** %b
>>> %tmp1 = load i32** %b   ; <i32*> [#uses=1]
>>> %tmp2 = load i32* %tmp1   ; <i32> [#uses=1]
>>> store i32 %tmp2, i32* %tmp
>>> %tmp3 = load i32* %tmp    ; <i32> [#uses=1]
>>> store i32 %tmp3, i32* %retval
>>> br label %return
>>>
>>> return:   ; preds = %entry
>>> %retval4 = load i32* %retval    ; <i32> [#uses=1]
>>> ret i32 %retval4
>>> }
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>
>> -Chris
>>
>> --
>> http://nondot.org/sabre/
>> http://llvm.org/
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-Chris

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