GCC vectorization problem on X86

Ira Rosen IRAR@il.ibm.com
Sun Jun 6 08:02:00 GMT 2010



gcc-help-owner@gcc.gnu.org wrote on 03/06/2010 09:37:01 PM:

> Hi. I'm having a problem with GCC vectorization on an Opteron 165.
>
> I have two codes, which are, unfortunately, machine-generated
> and large, which differ, as far as I tell, only in the source
> of the loop size, N, for a loop roughly of this form:
>
>   for( i=0; i<N; i++) {
>     vec[i] = i;
>    }
>
> In both cases, N comes from another function and is theoretically
> not inlined. In the first case, N is generated by an identity
> function that hides its value; this case vectorizes nicely,
> if the presence of punpckldq instructions is suitable evidence.
> (papiex confirms vectorization with high PAPI_VEC_INS counts.)
>
> In the other case, N comes from a sscanf, and is very well hidden,
> since it comes from the command line, ultimately. This case
> does not vectorize, at present. It did vectorize some months ago...
>
> This is on:  gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu12)
> Neither the compiler nor the OS have changed in that time; the
> code going into gcc has, of course, changed as the sac2c compiler
> has evolved.
>
> So, are there some subtle (or not subtle...) criteria that gcc has
> for deciding when to emit vector ops, based on array size, perhaps?
>
> Alternately, if someone can point me at the relevant gcc source code,
> maybe I can get an idea as to what's going on. Or, if there is
> a bugzilla site for it, I'll take a look there.

Auto-vectorization can fail if number of iterations can't be computed. The
vectorizer calls number_of_exit_cond_executions() in tree-
scalar-evolution.c to determine loop bound.

HTH,
Ira


>
> Thanks,
> Robert
>
>
>
>
>
>
>



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