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Re: PING: [RFA/i386] 2 more patterns in i386_analyze_stack_align
- From: Mark Kettenis <mark dot kettenis at xs4all dot nl>
- To: brobecker at adacore dot com
- Cc: gdb-patches at sourceware dot org
- Date: Fri, 5 Jan 2007 12:04:59 +0100 (CET)
- Subject: Re: PING: [RFA/i386] 2 more patterns in i386_analyze_stack_align
- References: <20061220104945.GB27642@adacore.com> <20061231060844.GP3640@adacore.com> <200612311215.kBVCF75Z010607@brahms.sibelius.xs4all.nl> <20070105064916.GS17211@adacore.com>
> 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