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Re: [PATCH AArch64 1/3] Don't disparage add/sub in SIMD registers
- From: Marcus Shawcroft <marcus dot shawcroft at gmail dot com>
- To: Alan Lawrence <alan dot lawrence at arm dot com>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 2 Sep 2014 16:08:43 +0100
- Subject: Re: [PATCH AArch64 1/3] Don't disparage add/sub in SIMD registers
- Authentication-results: sourceware.org; auth=none
- References: <53EA26D1 dot 4010506 at arm dot com> <53EA2767 dot 5060301 at arm dot com> <FEA046E9-88D1-41C7-9DC6-EDAFBA6789C9 at gmail dot com> <53F22EBF dot 40901 at arm dot com>
On 18 August 2014 17:50, Alan Lawrence <alan.lawrence@arm.com> wrote:
> Well, you're right that it could be. So I presented the wrong justification.
>
> Clearly we would benefit from some better cost infrastructure here, ideally
> that is expressive, taken into account at all appropriate stages of the
> compiler, and tunable per core. I imagine that steps (patches) towards such
> infrastructure would be welcomed by both AArch64 maintainers and more
> widely.
>
> In the meantime, however, we must work with what we have. I'll still argue
> that we should remove the '!' (as per patch), however. As James has said,
> even if your add is more expensive in SIMD registers, the '!' still doesn't
> express that; and leaving it in affects code-generation on all cores. And it
> is inconsistent with other instructions.
Agreed and OK. /Marcus