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

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Message-ID: <55936A78.1060206@redhat.com>
Date:	Tue, 30 Jun 2015 22:20:08 -0600
From:	Jeff Law <law@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>
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 06/30/2015 04:02 PM, H. Peter Anvin wrote:
> On 06/30/2015 02:55 PM, Andy Lutomirski wrote:
>> 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.
>>
>
> No, but you need to be able to call other functions, or you just push
> the issue down one level.
For ia32, the PIC register really isn't special anymore.  I'd be 
surprised if you couldn't clobber it.

jeff
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