Re: gcc feature request / RFC: extra clobbered regs

H. Peter Anvin
2015-06-30T22:03:14Z

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.

-hpa