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Re: gcc feature request / RFC: extra clobbered regs
- From: Andy Lutomirski <luto at amacapital dot net>
- To: "H. Peter Anvin" <hpa at zytor dot com>
- Cc: Jakub Jelinek <jakub at redhat dot com>, Andy Lutomirski <luto at kernel dot org>, gcc at gcc dot gnu dot org, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, Linus Torvalds <torvalds at linux-foundation dot org>, Ingo Molnar <mingo at kernel dot org>, Thomas Gleixner <tglx at linutronix dot de>, Vladimir Makarov <vmakarov at redhat dot com>
- Date: Tue, 30 Jun 2015 14:55:43 -0700
- Subject: Re: gcc feature request / RFC: extra clobbered regs
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- References: <CALCETrX6j9vBZR7RirXf8usz1Y4f-1TnVaYTVg0_PgQCeWZnRg at mail dot gmail dot com> <20150630213736 dot GQ10247 at tucnak dot redhat dot com> <55930D24 dot 4040105 at zytor dot com> <CALCETrV12pF7ksZ8Gzm-oWssJB2pLjeq5GM4gx5GPBba9H=bng at mail dot gmail dot com> <55930FB9 dot 2070904 at zytor dot com>
On Tue, Jun 30, 2015 at 2:52 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 06/30/2015 02:48 PM, Andy Lutomirski wrote:
>> On Tue, Jun 30, 2015 at 2:41 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>>> On 06/30/2015 02:37 PM, Jakub Jelinek wrote:
>>>> I'd say the most natural API for this would be to allow
>>>> f{fixed,call-{used,saved}}-REG in target attribute.
>>>
>>> Either that or
>>>
>>> __attribute__((fixed(rbp,rcx),used(rax,rbx),saved(r11)))
>>>
>>> ... just to be shorter. Either way, I would consider this to be
>>> desirable -- I have myself used this to good effect in a past life
>>> (*cough* Transmeta *cough*) -- but not a high priority feature.
>>
>> I think I mean the per-function equivalent of -fcall-used-reg, so
>> hpa's "used" suggestion would do the trick.
>>
>> I guess that clobbering the frame pointer is a non-starter, but five
>> out of six isn't so bad. It would be nice to error out instead of
>> producing "disastrous results", though, if another bad reg is chosen.
>> (Presumably the PIC register on PIC builds would be an example of
>> that.)
>>
>
> Clobbering the frame pointer is perfectly fine, as is the PIC register.
> However, gcc might need to handle them as "fixed" rather than "clobbered".
Hmm. True, I guess, although I wouldn't necessarily expect gcc to be
able to generate code to call a function like that.
--Andy