linux-kernel - Re: gcc feature request / RFC: extra clobbered regs

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Message-ID: <CALCETrWJO_3mmsywSXjHFuL8bmbDo3okRsAdGYWX2bGfBSHEew@mail.gmail.com>
Date:	Tue, 30 Jun 2015 14:55:43 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jakub Jelinek <jakub@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, gcc@....gnu.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Makarov <vmakarov@...hat.com>
Subject: Re: gcc feature request / RFC: extra clobbered regs

On Tue, Jun 30, 2015 at 2:52 PM, H. Peter Anvin <hpa@...or.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@...or.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
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