Is `__GNUC__` defined in dmd when using importC?

[felixfxu]

On Friday, 10 October 2025 at 09:02:33 UTC, Dejan Lekic wrote:
> Reading the https://dlang.org/spec/importc.html will help you 
> solve the mystery. This section in particular: 
> https://dlang.org/spec/importc.html#preprocessor

yes, thanks, now it is clear.

Actually my default c pre-processor is my locally compiled cpp 
(version 16):
>which cpp
/usr/local/bin/cpp

and it's used to pre-process the .h files, so then __GNUC__ is 16.

But that introduce another question: dmd is not a complete gcc 
replacement I guess, but in the headers, all `#ifdef __GNUC__` is 
`true`, it will have problems for code like this:

```h
SDL_FORCE_INLINE int SDL_MostSignificantBitIndex32(Uint32 x)
{
#if defined(__GNUC__) && (__GNUC__ >= 4 || (__GNUC__ == 3 && 
__GNUC_MINOR__ >= 4))
     /* Count Leading Zeroes builtin in GCC.
      * 
http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
      */
     if (x == 0) {
         return -1;
     }
     return 31 - __builtin_clz(x);
#elif
     ...
```
`__builtin_clz` will then be undefined in dmd compiling.