> 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).
For record I would also preffer passing all named AVX arguments in registers. x86-64 ABI was designed for performance not for backward compatibility, so it should be consistent with original idea and I think that ABIs are divergent anough so this won't cause too much of extra confussion. But I am happy with both solutions. Honza > > Jakub