[Bug c/66970] Add __has_builtin() macro

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970

felix <felix.von.s at posteo dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |felix.von.s at posteo dot de

--- Comment #7 from felix <felix.von.s at posteo dot de> ---
Created attachment 42188
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42188&action=edit
Preliminary patch

Based on the already-existing __has_attribute() support, I've developed a patch
implementing this feature. Attached. 

After some superficial testing (./cc1 -E), it seems to be working correctly. I
don't really have the resources to run regression tests, so I didn't (although
I think regressions would be unlikely). I also didn't write any new tests or
documentation. (I noticed __has_attribute also doesn't seem to have much of
either.)

I made the feature closely mimic The Other Compiler's behaviour: only function
built-ins are recognised. This includes generic functions like
__builtin_add_overflow and C library functions specially handled by the
front-end like printf (unless -fno-builtin is used), but excludes types like
__builtin_va_list and function-like constructs (implemented as keywords) like
__builtin_offsetof and __builtin_types_compatible_p. This is not so much of a
problem for them, since they also provide __has_feature(), __has_extension()
and __is_identifier() macros. The former has been requested in PR 60512 and
rejected; it may be reasonable to revisit the issue. 

However, if more such preprocessor, er, built-ins are desired, it may start
becoming unwieldy to implement them the way I did in this patch; it involved
some amount of copy-pasting.