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Re: gcc feature request / RFC: extra clobbered regs

From: Jeff Law <law at redhat dot com>
To: "H. Peter Anvin" <hpa at zytor dot com>, 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 22:20:08 -0600
Subject: Re: gcc feature request / RFC: extra clobbered regs
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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@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.

For ia32, the PIC register really isn't special anymore. I'd besurprised if you couldn't clobber it.


jeff

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