Messages in this thread | | | Date | Tue, 30 Jun 2015 22:20:08 -0600 | From | Jeff Law <> | 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 be surprised if you couldn't clobber it.
jeff
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