Accessing non-existing varargs argument

[Oleg Endo]

On Mon, 2014-12-15 at 11:39 +0000, Richard Earnshaw wrote:
> On 12/12/14 23:50, Florian Weimer wrote:
> > On 12/12/2014 08:02 PM, Jeff Law wrote:
> > 
> >> It'd probably work for the specific cases Florian wants to tackle, but
> >> I'd consider it highly non-portable.
> > 
> > I agree, that's why I asked for something “GCC-portable”.
> > 
> 
> Don't go this way.  Seriously, you can't assume that a future platform
> ABI won't break if you do.  ABI's at this level really aren't controlled
> by GCC, which just has to conform to whatever the platform ABI requires.
>  We were very close in AArch64 to defining the ABI for variadics to push
> all variadic (from the ... onwards) arguments onto the stack - it leads
> to a very simple ABI from an implementation viewpoint and eliminates
> much of the grotty code around reconstructing the parameter list.

FYI, that's what the Renesas ABI does on SH.  Makes capturing and
forwarding va arg lists more efficient.

Cheers,
Oleg