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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Jun 10, 2008 at 05:48:57PM +0200, Jan Hubicka wrote:
> > On Tue, Jun 10, 2008 at 8:11 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > > On Tue, Jun 10, 2008 at 04:50:14PM +0200, Jan Hubicka wrote:
> > >>  1) make __m256 passed on stack on variadic functions and in registers
> > >>  otherwse. Then we don't need to worry about varargs changes at all.
> > >>  This will break unprototyped calls.
> > >>  2) extend rax to pass info about if __m256 registers are present and
> > >>  upper half needs to be saved.
> > >>  This will break passing __m256 arguments to functions with prologues
> > >>  compiled with legacy compiler that will do wild jump.  All other cases
> > >>  should work
> > >>  3) Save upper halves whenever we want to save SSE registers.  This will
> > >>  break calling variadic functions compiled with __m256 support in.
> > >>
> > >> I guess either 1) or 2) is fine for me, as I told earlier, I am not big
> > >> fan of 3).  I guess 1) is easier and probably make more sense?
> > >
> > > I vote for 1), though I think it should be passed on stack only for ...
> > > args.  E.g. for
> > > void foo (__m256 x, ...);
> > > void bar (__m256 x, __m256 y, __m256 z)
> > > {
> > >  foo (x, y, z);
> > > }
> > > the first argument would be passed in %ymm0, while the unnamed arguments
> > > y and z would be pushed to stack.
> > >
> > 
> > I agree. We will add testcases for whatever psABI extension we have
> > chosen.
> 
> I guess we all agree on passing variadic arguments on stack (that is
> only those belonging on ...) and rest in registers.  It seems easiest in
> regard to future register set extensions too.  Only negative thing is
> that calls to variadic functions will become bit longer, but I guess it
> is not big deal. (the fact that register passing conventions are shorter
> and variadic functions tends to be called many times was also original
> motivation to support register passing on pretty much everything for
> varargs in psABI)
> 

There is no precedent for passing named parameters in registers but
unnamed parameters on the stack.  On IA32 for the __m128 types, we
pass ALL __m128 parameters on the stack for varargs functions, not
just the unnamed ones. I think we should do the same for x86-64.


H.J.