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Re: [PATCH, rs6000] Prefer vspltisw/h over xxspltib+instruction when available
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, David Edelsohn <dje dot gcc at gmail dot com>
- Date: Wed, 22 Jun 2016 09:22:22 -0500
- Subject: Re: [PATCH, rs6000] Prefer vspltisw/h over xxspltib+instruction when available
- Authentication-results: sourceware.org; auth=none
- References: <73436B77-BC85-4919-975E-915A6D93F585 at linux dot vnet dot ibm dot com> <20160621223440 dot GA18969 at gate dot crashing dot org> <17065109-773A-402B-9550-2827CAEE4E3C at linux dot vnet dot ibm dot com>
On Tue, Jun 21, 2016 at 06:46:57PM -0500, Bill Schmidt wrote:
> >> When I did this, I ran into a problem with an existing test case. We end up matching the *vsx_splat_v4si_internal pattern instead of falling back to the altivec_vspltisw pattern. The constraints don't match for constant input. To avoid this, I added a pattern ahead of this one that will match for VMX output registers and produce the vspltisw as desired. This corrected the failing test and produces the expected code.
> >
> > Why does the predicate allow constant input, while the constraints do not?
>
> I have no idea why it was built that way. The predicate seems to provide for all sorts of things, but this and the subsequent pattern both handle only a subset of the constraints implied by it. To be honest, I didn't feel competent to try to fix the existing patterns. Do you have any suggestions for what to do instead?
Don't give up so easily? ;-)
The predicate should be tightened, the expander should use a new predicate
that allows all those other things. The hardest part is figuring a good
name for it ;-)
Segher