[LLVMdev] Autovectorization questions

[Arnold Schwaighofer]

On Mar 12, 2014, at 4:48 PM, Chandler Carruth <chandlerc at google.com> wrote:

> 
> On Wed, Mar 12, 2014 at 4:45 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:
> The loop vectorizer relies on scev’s nowrap flags. We need to improve SCEV for this.
> 
>   %conv = sext i32 %k to i64
>   -->  (sext i32 %k to i64)
>   %i.06 = phi i64 [ 0, %entry ], [ %inc, %for.body ]
>   -->  {0,+,1}<nuw><nsw><%for.body>             Exits: 1023
>   %mul1 = mul nsw i64 %i.06, 7
>   -->  {0,+,7}<%for.body>               Exits: 7161
>   %arrayidx2 = getelementptr inbounds i32* %A, i64 %mul1
>   -->  {%A,+,28}<%for.body>           <== we want to see a nw flag here.
> 
> 
> Scev sometimes drops new flags for safety (cannonicalization can make them invalid if the same expression is used in different contexts) . See past discussions on this.
> 
> Sure, but I think its really important to clarify that the *example* is fine, and there is nothing fundamental about it that prevents vectorization.
> 
 
Sure. I thought this was clear in my answer (obviously not :). Rereading, I should probably have added that the code is vectorizable (I assumed the Zinovy knows this).

> In order to vectorize code like this LLVM needs to prove that “A[i*7]” does not wrap in the address space. It fails to do so and so LLVM doesn’t vectorize this loop even if we try to force it.