Re: auto vectorizer

[Ira Rosen <IRAR at il dot ibm dot com>]

gcc-help-owner@gcc.gnu.org wrote on 24/08/2009 23:31:45:

> My loop doesn't vectorize. The message says:
>
> ./TestAutoVect.cpp:31: note: not vectorized: unhandled data-ref
>
> 22
> //-----------------------------------------------------------------------------

> 23 void MaskRGBA(
> 24       const int *in,
> 25       const int * const inEnd,
> 26       int *out,
> 27       const int * const m)
> 28 {
> 29   for (;in<inEnd; ++in, ++out)
> 30     {
> 31     *out=*in&*m;
> 32     }
> 33 }

The problem is that '*m' cannot be taken out of the loop because the alias
analysis cannot prove that it doesn't alias other memory accesses in the
loop. This causes data references analysis to fail since the above access
is loop invariant. Therefore, this loop is not vectorized.

>
> I found that when I change it to this
>
> 23
> //-----------------------------------------------------------------------------

> 24 void MaskRGBA(
> 25       const int *in,
> 26       const int * const inEnd,
> 27       int *out,
> 28       const int * const mask)
> 29 {
> 30   const int m=*mask;
> 31   for (;in<inEnd; ++in, ++out)
> 32     {
> 33     *out=*in&m;
> 34     }
> 35 }

Here there is no such problem.

>
> It gets vectorized. What is the reason behind that?
>

You can also use __restrict__ to help vectorization:

void MaskRGBA(
       const int *__restrict__ in,
       const int * const __restrict__  inEnd,
       int *__restrict__  out,
       const int * const __restrict__  m)
 {
   for (;in<inEnd; ++in, ++out)
     {
     *out=*in&*m;
     }
 }

Ira