[LLVMdev] Autovectorization questions

[Arnold Schwaighofer]

On Mar 12, 2014, at 4:05 PM, Chandler Carruth <chandlerc at google.com> wrote:

> 
> On Wed, Mar 12, 2014 at 3:50 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:
> 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
> 
> But, why?
> 
> I'm moderately sure that neither C nor C++ allow wrapping around the end of the address space. If they do, we will fix C++ at least to disallow this. 'i' is a signed integer, so we can't wrap in the index space either. So why can't LLVM prove this?

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.  

We are thinking about doing something like described here: http://permalink.gmane.org/gmane.comp.compilers.llvm.devel/67476 or in this thread:(http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20131007/190703.html.