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Re: combine permutations in gimple
On 15 August 2012 13:07, Jakub Jelinek <jakub@redhat.com> wrote:
> On Wed, Aug 15, 2012 at 01:46:05PM +0200, Richard Guenther wrote:
>> Well, we're waiting for someone to break the tie ... I'd go with the original
>> patch, improving the backends where necessary.
>
> E.g. i?86/x86_64 with just plain -msse2 has only very small subset of
> constant shuffles (and no variable shuffle), so by doing the transformation
> you could end up with a scalarized shuffle instead of two constant vector
> shuffles. Expecting the backend to figure it out and doing the two constant
> vector shuffles in every case is not realistic, i386/x86_64 has way too many
> different shuffling instructions (and worse different ISA levels have
> different subsets of them) and while a lot of effort has been spent on
> it already by Richard, me, Marc and others, we are nowhere close to having
> optimal sequence in many cases for various modes and ISA levels. Doing a
> brute force might be too expensive.
>
Neon has atleast 7-8 such forms. Brute force computing this would be expensive.
I don't know what happens on other vector targets - surely other SIMD
targets in GCC will also have the same problem.
Ramana
> Jakub