Re: combine permutations in gimple

[Jakub Jelinek <jakub at redhat dot com>]

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.

	Jakub