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Re: gcc feature request / RFC: extra clobbered regs
- From: "H. Peter Anvin" <hpa at zytor dot com>
- To: Andy Lutomirski <luto at amacapital dot net>
- 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 15:02:43 -0700
- Subject: Re: gcc feature request / RFC: extra clobbered regs
- Authentication-results: sourceware.org; auth=none
- 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> <CALCETrWJO_3mmsywSXjHFuL8bmbDo3okRsAdGYWX2bGfBSHEew at mail dot gmail dot com>
On 06/30/2015 02:55 PM, Andy Lutomirski wrote:
> 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.
>
No, but you need to be able to call other functions, or you just push
the issue down one level.
-hpa