Re: gcc feature request / RFC: extra clobbered regs

H. Peter Anvin
2015-06-30T21:53:44Z

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