Re: PING: [RFA/i386] 2 more patterns in i386_analyze_stack_align

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> Date: Fri, 5 Jan 2007 10:49:16 +0400
> From: Joel Brobecker <brobecker@adacore.com>
> 
> Hi Mark,
> 
> A followup on a recent discussion:
> 
> > Hmm, you're missing the %ebx case here.  Now on ELF systems, you'll
> > probably never see it since %ebx is used for GOT access, but on other
> > object formats I don't think there is any reason why GCC wouldn't
> > choose to use %ebx as well.
> 
> I consulted with Olivier Hainque and here is what I learnt:
> 
>   . The current FSF GCC only uses %ecx, and punts on any realignment
>     request for a function which needs ecx for other purposes, like
>     neted functions with a static chain.
> 
>   . We have a local enhancement that takes advantage of the fact
>     that when ecx is not available, edx and then eax are used.
> 
>     I wasn't aware of the fact that this change was local when
>     I submitted my patch.  I don't know yet why this change was
>     not contributed, probably lack of time. Hopefully it will be
>     included soon.

Well, it seems a valid generalisation, so I have no problems in adding
the patterns.  Actually that's why I think we should also add %ebx.

>   . The current implementation is SVR4 ABI oriented AFAICT, and ebx is
>     not a possible candidate because it is callee-saved. We're not sure
>     about the status of non-elf targets.

Ah wait, that's true even for "absolute" code.  I had a quick look at
the GCC code and it seems to always treat %ebx as callee-saved.  So
I'm happy with leaving it out, but you could add a comment saying so
to prevent us from having this same discussion in about two months
;-).

Mark