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Re: [patch, ARM] Make 128 bits the default vector size for NEON


On 6 April 2011 16:07, Hans-Peter Nilsson <hans-peter.nilsson@axis.com> wrote:
>> Date: Thu, 31 Mar 2011 13:39:05 +0200
>> From: Ira Rosen <ira.rosen@linaro.org>
>
>> This patch changes NEON's default vector size from 64 to 128 bits.
>
> I'm wondering, are there NEON-specific measurements to support
> this change?
>
> A colleague of mine implemented support for 64- and 128-bit NEON
> for RAPP <http://savannah.nongnu.org/projects/rapp/>, but found
> that the 128-bit version was slower.

The best vector size depends on the particular target (a version of
ARM) and the benchmark. The vectorizer has a cost model that allows it
to fall back to 64-bit vectors if 128-bit version is not profitable
(-fvect-cost-model). We plan to enhance the cost model to estimate
both versions and vectorize with the most profitable vector size. For
now, you can use preferred-vector-size param if needed. Having 64 as a
default doesn't allow us to use different NEON instructions that use
both types of vectors.

Ira

>
> brgds, H-P
>


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