Dealing with compilers that pretend to be GCC

[Ludovic Courtès]

Hi,

Chris Lattner <clattner@apple.com> skribis:

> On Jan 31, 2012, at 5:15 AM, Ludovic Courtès wrote:
>
>>> 
>>> Interestingly enough:
>>> $ cat q.c
>>> __has_builtin
>>> $ clang -E q.c
>>> <segfault>
>> 
>> Yes, that’s what I was asking.
>> 
>> It makes me think that the old CPP predicates (info "(gcc) Obsolete
>> Features") would be more appropriate than compiler magic, with the
>> caveat that they’re presumably not widely supported.
>
> They are similar in spirit.  The major difference between the two is that it is easy for __has_feature and friends gracefully degrade when a compiler doesn't support them, because you can do:
>
> #ifndef __has_builtin
> #define __has_builtin(x) 0
> #endif
>
> in your code, but you can't do anything like this for #assertion.  That and assertions don't have any coverage over the features that you're interested in, and the grammar doesn't have a good way to handle multiple cases like features/attributes/extensions/etc.

Rather than assertions, one could use predicates:

  #if #has_builtin(foo) && #has_attribute(bar)
  ...
  #endif

The difference being that (1) predicates were designed specifically for
this purpose, and (2) there’s no magic involved.

Thanks,
Ludo’.