AVX generic mode tuning discussion.

[Richard Guenther]

On Tue, Jul 12, 2011 at 11:56 PM, Richard Henderson <rth@redhat.com> wrote:
> On 07/12/2011 02:22 PM, harsha.jagasia@amd.com wrote:
>> We would like to propose changing AVX generic mode tuning to generate 128-bit
>> AVX instead of 256-bit AVX.
>
> You indicate a 3% reduction on bulldozer with avx256.
> How does avx128 compare to -mno-avx -msse4.2?
> Will the next AMD generation have a useable avx256?
>
> I'm not keen on the idea of generic mode being tune
> for a single processor revision that maybe shouldn't
> actually be using avx at all.

Btw, it looks like the data is massively skewed by
436.cactusADM.  What are the overall numbers if you
disregard cactus?  It's also for sure the case that the vectorizer
cost model has not been touched for avx256 vs. avx128 vs. sse,
so a more sensible approach would be to look at differentiating
things there to improve the cactus numbers.  Harsha, did you
investigate why avx256 is such a loss for cactus or why it is
so much of a win for SB?

I suppose generic tuning is of less importance for AVX as
people need to enable that manually anyway (and will possibly
do so only via means of -march=native).

Thanks,
Richard.

>
> r~
>