[llvm-dev] Question about a AA result and its use in Dependence Analysis

[Doerfert, Johannes via llvm-dev]

Can you try it without the undef branch condition. If you get still the
same result I'd argue its a bug. In the program as it is, I'd say it is
not because the back edge is never taken.

On 05/31, De Azevedo Piovezan, Felipe via llvm-dev wrote:
> Hello llvm-dev,
> 
> I would appreciate your feedback on the following problem. We're trying to determine whether this is a bug in LLVM or not.
> 
> In the IR snippet below, we have two pointers (p and q) which initially point to two completely non-overlapping locations. Then, on every iteration of a loop, we swap the pointers and load from the first, followed by a store to the second.
> 
> 1) AA says the two pointers are NoAlias, even though they do point to the same location if we consider them in distinct loop iterations. Is this right?
> 2) Dependence Analysis says there is no dependence between the load and the store. Is this right?
> 
> define float @f() {
> entry:
>   %g = alloca float, align 4
>   %h = alloca float, align 4
>   br label %for.body
> 
> for.cond.cleanup:
>   ret float undef
> 
> for.body:
>   %p = phi float* [ %g, %entry ], [ %q, %for.body ]
>   %q = phi float* [ %h, %entry ], [ %p, %for.body ]
>   %0 = load float, float* %p, align 4
>   store float undef, float* %q, align 4
>   br i1 undef, label %for.cond.cleanup, label %for.body
> }
> 
> AliasSet[0x872d800, 1] must alias, Ref       Pointers: (float* %p, LocationSize::precise(4))
> AliasSet[0x872d8b0, 1] must alias, Mod       Pointers: (float* %q, LocationSize::precise(4))
> 
> da analyze -
>    %0 = load float, float* %p, align 4   ; I added these two debug statements, DA doesn't print the values it is looking at...
>   store float undef, float* %q, align 4
> none!
> 
> --
> Felipe
> 

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-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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