No-named-argument variadic functions

[Joseph Myers]

C2x allows variable-argument functions declared with (...) as parameters - 
no named arguments - as in C++.  It *also* allows such functions to access 
their parameters, unlike C++, by relaxing the requirements on va_start so 
it no longer needs to be passed the name of the last named parameter.

My assumption is that such functions should thus use the ABI for 
variable-argument functions, to the extent that's different from that for 
other functions.  The main implementation issue I see is that GCC's 
internal representation for function types can't actually distinguish the 
(...) type from an unprototyped function - C++ functions with (...) 
arguments are treated by the middle end and back ends as unprototyped.  
(This probably works sufficiently well in ABI terms when the function 
can't actually use its arguments.  Back ends may well call what they think 
are unprototyped functions in a way compatible with variadic callees 
anyway, for compatibility with pre-standard C code that calls e.g. printf 
without a prototype, even though standard C has never allowed calling 
variable-argument functions without a prototype.)

So there are a few questions here for implementing this C2x feature:

1. How should (...) be represented differently from unprototyped functions 
so that stdarg_p and prototype_p handle it properly?  Should I add a new 
language-independent type flag (there are plenty spare) to use for this?

2. Does anyone see any likely ABI or back end issues from allowing 
single-argument calls to __builtin_va_start to access the arguments to 
such a function?  (I'd propose to redefine va_start in stdarg.h to use a 
single-argument call, discarding any subsequent arguments, only for C2x.)

3. Should the C++ front end be changed to mark (...) functions in whatever 
way is chosen for question 1 above, so that they start using the 
appropriate ABI (and, in particular, calls between C and C++, where a C 
implementation of such a function might use the arguments, work properly)?  
Or would there be problems with compatibility with existing callers or 
callees assuming the unprototyped function ABI?

-- 
Joseph S. Myers
joseph@codesourcery.com