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

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Message-ID: <CALCETrV12pF7ksZ8Gzm-oWssJB2pLjeq5GM4gx5GPBba9H=bng@mail.gmail.com>
Date:	Tue, 30 Jun 2015 14:48:08 -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 2:41 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.)

--Andy
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