Unused GCC builtins

[Florian Weimer]

* Jakub Jelinek:

> On Wed, Jan 24, 2018 at 03:04:55PM +0100, Manuel Rigger wrote:
>> In a second step, we also considered internal builtins and found that the
>> vararg handling builtins (__builtin_va_start, __builtin_va_end,
>> __builtin_va_arg, and __builtin_va_copy) are relied upon by many projects,
>> even though they are undocumented in GCC's builtins API. Could they be
>> added to the documentation?
>
> Why?  What is documented is va_start/va_end/va_arg/va_copy, that is
> what people should use, the builtins are just internal implementation of
> those macros.

And these builtins differ from the math builtins because <stdarg.h> is
provided by GCC, but <math.h> is not, and there are many different
implementations.