Remove SECP256K1_GNUC_PREREQ? #1868

We have a SECP256K1_GNUC_PREREQ macro (defined in terms of __GNUC__ and __GNUC_MINOR__ for checking gcc versions):

https://github.com/bitcoin-core/secp256k1/blob/0f4a7e6bf9d971addb2b851df7cd2777fc62b212/include/secp256k1.h#L104-L111

Current usage in the codebase:

 ❯ git grep _GNUC_
examples/examples_util.h:#elif defined(__GNUC__)
include/secp256k1.h:# if !defined(SECP256K1_GNUC_PREREQ)
include/secp256k1.h:#  if defined(__GNUC__)&&defined(__GNUC_MINOR__)
include/secp256k1.h:#   define SECP256K1_GNUC_PREREQ(_maj,_min) \
include/secp256k1.h: ((__GNUC__<<16)+__GNUC_MINOR__>=((_maj)<<16)+(_min))
include/secp256k1.h:#   define SECP256K1_GNUC_PREREQ(_maj,_min) 0
include/secp256k1.h:#        elif !defined(_WIN32) && defined (__GNUC__) && (__GNUC__ >= 4)
include/secp256k1.h:# if defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
include/secp256k1.h:# if !defined(SECP256K1_BUILD) && defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
src/checkmem.h:#  elif defined(__GNUC__) && (__GNUC__ >= 15)
src/checkmem.h:#  elif defined(__GNUC__) && (__GNUC__ >= 15)
src/int128_native.h:SECP256K1_GNUC_EXT typedef unsigned __int128 uint128_t;
src/int128_native.h:SECP256K1_GNUC_EXT typedef __int128 int128_t;
src/util.h:#  if SECP256K1_GNUC_PREREQ(2,7)
src/util.h:#if SECP256K1_GNUC_PREREQ(3, 0)
src/util.h:#  if SECP256K1_GNUC_PREREQ(3,0)
src/util.h:#if defined(__GNUC__)
src/util.h:# define SECP256K1_GNUC_EXT __extension__
src/util.h:# define SECP256K1_GNUC_EXT
src/util.h:#elif defined(__GNUC__)
src/util.h:#if (__has_builtin(__builtin_ctz) || SECP256K1_GNUC_PREREQ(3,4))
src/util.h:#if (__has_builtin(__builtin_ctzl) || SECP256K1_GNUC_PREREQ(3,4))
src/util.h:#if (__has_builtin(__builtin_ctzl) || SECP256K1_GNUC_PREREQ(3,4))
src/util.h:#if (__has_builtin(__builtin_ctzll) || SECP256K1_GNUC_PREREQ(3,4))
src/util_local_visibility.h:#if !defined(_WIN32) && defined(__GNUC__) && (__GNUC__ >= 4)

• It's currently used only for ancient GCC versions (see above). The only check against a contemporary GCC version is in checkmem.h but here we don't even use the macro. (#1865 would add a (4, 2) check but I consider this also ancient). If you have an ancient compiler, it's reasonable to except that you're on your own.
• Contributors apparently don't remember its existence (see the checkmem.h case, and also util_local_visibility.h and even secp256k1.h itself).
• It's misleading because __GNUC__ is defined also in Clang. Clang sets __GNUC__ to 4 and __GNUC_MINOR__ to 2 (mimicking GCC 4.2) by default for compatibility, regardless of the actual Clang version. Modern Clang (8+) supports -fgnuc-version=major.minor.patch to override these values, but that's of no use to us.

I suggest removing the macro and just checking defined(__GNUC__) instead (when it comes to ancient versions), or something like (__GNUC__) && (__GNUC__ >= 15), or __has_builtin when appropriate (introduced in GCC 10).

I agree with all three points mentioned above.

Wasn't __inline__ available even earlier than GCC 2.7: https://github.com/bitcoin-core/secp256k1/blob/0f4a7e6bf9d971addb2b851df7cd2777fc62b212/src/util.h#L49-L50 ?

I suggest removing the macro and just checking defined(__GNUC__) instead (when it comes to ancient versions), or something like (__GNUC__) && (__GNUC__ >= 15), or __has_builtin when appropriate (introduced in GCC 10).

I wonder whether this would allow dropping this line:https://github.com/bitcoin-core/secp256k1/blob/0f4a7e6bf9d971addb2b851df7cd2777fc62b212/src/util.h#L10 ?

Wasn't __inline__ available even earlier than GCC 2.7:

I could imagine that, whoever wrote this code, didn't look for earlier versions than 2.7. (On godbolt, they have a gcc 1.27 but no 2.xx versions :D).

I wonder whether this would allow dropping this line:

I don't know but this would be nice.