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

[Jan Hubicka]

> 
> I don't understand why you want to pass __m256 and 256-bit vector values
> to anonymous arguments in registers.  The only thing the vararg functions
> would do with it would be save it somewhere on the stack.
> Given the x86_64 ABI, you can't expect calling an implicitly
> prototyped or non-vararg prototyped function which is actually
> defined as vararg function (as %rax wouldn't be properly initialized),

Unprototyped functions calls all get rax set. If calle is variadic,
things still work.  Sure, for __m256 we can also declare prototypes for
variadic functions mandatory and simply pass things on stack.

Honza
> which means you need a prototype for all vararg functions and
> at that point the caller can just do the job for the callee and push stuff
> on the stack.  Then vararg prologue doesn't need to save %ymm* registers
> at all and va_arg will handle __m256 just fine.
> 
> 	Jakub