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

[De Azevedo Piovezan, Felipe via llvm-dev]

Hi Johannes,

I followed your advice and got the same result: NoAlias and No dependence. Would you say AA is faulty for saying NoAlias or DA is faulty for saying no dependence? Or both?
(revised example below)

Thanks!

define float @f() {
entry:
  %g = alloca float, align 4
  %h = alloca float, align 4
  br label %for.body

for.cond.cleanup:                                 ; preds = %for.body
  ret float undef

for.body:                                         ; preds = %for.body, %entry
  %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
  %branch_cond = fcmp ugt float %0, 0.0
  br i1 %branch_cond, label %for.cond.cleanup, label %for.body
}

Alias Set Tracker: 2 alias sets for 2 pointer values.
  AliasSet[0x83e1fe0, 1] must alias, Ref       Pointers: (float* %p, LocationSize::precise(4))
  AliasSet[0x83e3390, 1] must alias, Mod       Pointers: (float* %q, LocationSize::precise(4))

da analyze -  
   %0 = load float, float* %p, align 4
  store float undef, float* %q, align 4
none!

-----Original Message-----
From: Doerfert, Johannes [mailto:jdoerfert at anl.gov] 
Sent: Friday, May 31, 2019 9:07 PM
To: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Question about a AA result and its use in Dependence Analysis

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
> 

> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov