Re: RFC: Extend x86-64 psABI for 256bit AVX register

[Jan Hubicka <jh@suse.cz>]

> 
> ymm0 and xmm0 are the same register. xmm0 is the lower 128bit
> of xmm0. I am not sure if we need separate XMM registers from
> YMM registers.


Yes, I know that xmm0 is lower part of ymm0.  I still think we ought to
be able to support varargs that do save ymm0 registers only when ymm
values are passed same way as we touch SSE only when SSE values are
passed via EAX hint.
This way we will be able to support e.g. printf that has YMM printing %
construct but don't need YMM enabled hardware when those are not used.

This is why I think extending EAX to contain information about amount of
XMM values to save and in addition YMM values to save is sane.  Then old
non-YMM aware varargs prologues will crash when YMM values are passed,
but all other combinations will work.
> 
> >
> > I personally don't have much preferences over 1. or 2.. 1. seems
> > relatively easy to implement too, or is packaging two 128bit values to
> > single 256bit difficult in va_arg expansion?
> >
> 
> Access to 256bit register as lower and upper 128bits needs 2
> instructions. For store
> 
> vmovaps   %xmm7, -143(%rax)
> vextractf128 $1, %ymm7, -15(%rax)
> 
> For load
> 
> vmovaps  -143(%rax),%xmm7
> vinsert128 $1, -15(%rax),%ymm7,%ymm7
> 
> If we go beyond 256bit, we need more instructions to access
> the full register. For 512bit, it will be split into lower 128bit,
> middle 128bit and upper 256bit. 1024bit will have 4 parts.
> 
> For #2, only one instruction will be needed for 256bit and
> beyond.

Yes, but we will still save half of stack space.  Well, I don't have
much preferences here.  If it seems saner to simply save whole thing
saving lower part twice, I am fine with that.

Honza
> 
> Thanks.
> 
> 
> -- 
> H.J.