On 15 August 2012 13:07, Jakub Jelinek <ja...@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
