On Wed, Jun 11, 2008 at 07:49:12AM -0700, H.J. Lu wrote:
> > 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.
Why should the 32-bit ABI influence x86-64 ABI decisions?
There are clear advantages of passing __m128 named arguments in registers
(shorter/faster code both on the caller and callee side) and there
are advantages of passing __m128 unnamed arguments on the stack
(for va_arg to work, they need to be on the stack, and if they
are passed in registers, the callee would need to push them
to the stack).
Jakub
