Message-ID: <55930FB9.2070904@zytor.com>
Date: Tue, 30 Jun 2015 14:52:57 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Jakub Jelinek <jakub@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        gcc@gcc.gnu.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Vladimir Makarov <vmakarov@redhat.com>
Subject: Re: gcc feature request / RFC: extra clobbered regs
References: <CALCETrX6j9vBZR7RirXf8usz1Y4f-1TnVaYTVg0_PgQCeWZnRg@mail.gmail.com> <20150630213736.GQ10247@tucnak.redhat.com> <55930D24.4040105@zytor.com> <CALCETrV12pF7ksZ8Gzm-oWssJB2pLjeq5GM4gx5GPBba9H=bng@mail.gmail.com>
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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".

	-hpa


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