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Re: [PATCH] Vector return on ARM

On Thursday 12 May 2005 01:57, Julian Brown wrote:
> Julian Brown wrote:
> > Andrew Pinski wrote:
> >> On May 11, 2005, at 8:13 PM, Julian Brown wrote:
> >>> Hi,
> >>>
> >>> This patch corrects returning vectors from functions on ARM, both
> >>> with the classic APCS and the new AAPCS. Vectors of word-size or less
> >>> are returned in r0, and larger vectors are returned in memory.
> >>> Previously, vector values were always returned in registers, which
> >>> sometimes caused wrong code (on arm-none-elf) and sometimes ICEd on
> >>> both arm-none-elf and arm-none-eabi.
> >>
> >> This is the same as the patch which Josh Conner posted?
> >> <>
> >
> > It's functionally identical, by the looks of it. Sorry for the noise!
> ...except for the wrong-code case I saw on arm-none-elf, I think. Josh's
> patch tries to retain backward compatibility with the legacy ABI, which
> is probably the right thing to do, but this apparently exposes a bug in
> epilogue generation which manifests at -O0 with, e.g.,
> g++.dg/ext/attribute-test-1.C. A four-register vector is returned from a
> constructor in r0-r3, but the epilogue clobbers r3. I'm not sure why,
> but I suspect it's transferring an extra register to preserve stack
> alignment, or something.

That sounds strange. There used to be a similar bug relating to thumb 
interworking, but I though I'd fixed that...

> My patch kills backward compatibility by returning only vectors <= 4
> *bytes* (not words) in registers, but generates correct code for the
> case mentioned above. Given that vector return in registers probably
> doesn't always work anyway at the moment, maybe this is a better plan.

I think I prefer Josh's patch.

If r3 is clobbered in the epilogue returning of complex double types is 
probably similarly broken, so it needs fixing anyway.
The patch is merely preserving existing behaviour. It's shouldn't introduce 
any new failures.

Patch is ok if Julian can confirm it doesn't break anything new.


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