Checking for built-in functions from build systems

[Luca Bacci <luca.bacci982@gmail.com>]

Hello dear GCC developers,

I am working on an issue in Meson wrt GCC built-in functions, see
https://github.com/mesonbuild/meson/issues/10641.

As you already know, some built-ins are always inlined, while others are
inlined only at times (e.g. depending on the target architecture) and
resolve to a regular function call otherwise. The issue is about the
latter's, i.e. those that may emit regular function calls.

In fact, even though __has_builtin(func) evaluates to 1, func may not be
present in libraries (libgcc, libc, libm), causing linking errors if not
inlined.

Because some of the GCC built-in functions are not part of the C standard
(e.g. sincos), it's not worthwhile to require having implementations in
libc. In addition, should generic implementations be added to libgcc, those
would likely have sub-par performance or accuracy issues (being generic).

As such, I think that a build system should check if an implementation of a
conditionally-inlined built-in function is provided in the target libc.

I have a few questions:

1. Is inlining of built-ins dependant only on the target architecture and
command-line arguments?
2. If the answer to 1 is yes, could a __is_builtin_inlined (func) macro be
added to GCC? It should tell whether func is going to be substituted inline
for the given compiler invocation
3. Is it true that conditionally-inlined built-ins are exactly those that
come in the two variants of '__builtin_func' and 'func'?

Thanks,
Luca Bacci