Guard uses of __builtin_clz and __builtin_clzll

[allanjude]

Prevents undefined reference to `__clzdi2' on FreeBSD sparc64 compiled with gcc 4.2

Fallback to the software implementation by making the preprocessor guard better

[allanjude]

I see that this only works in clang, and does not fail in gcc on FreeBSD because of a compatibility macro.

Will need to revisit this

[Cyan4973]

Maybe using __GNUC_MINOR__ ?

Also, is there a difference to do between __builtin_clz and __builtin_clzll, the 32-bits version being more "commonly" supported ?

[cemeyer]

We could provide a similar compatibility macro in Zstd, right Allan?

As it is, the lack of (something like) this revision in Zstandard makes backporting new versions to FreeBSD more painful than necessary. (Some of our architectures do not provide __builtin_clz or __builtin_clzll.)

Or even something dumber like a configure-style HAVE_BUILTIN_CLZ / HAVE_BUILTIN_CLZLL we can enable/disable based on platform defines in one location?

[Cyan4973]

note that __has_builtin is clang specific, it will not work on gcc.

[allanjude]

Yeah, the problem with the FreeBSD compatibility macro, is that is just returns false, which would be a large pessimization on modern GCC that does have it.

[cemeyer]

Sure. We could do HAVE_BUILTIN_CLZ(LL) instead of the specific checks for GCC versions. There is precedent in the tree, e.g., DYNAMIC_BMI2.

[terrelln]

Yeah, we can definitely make a macro for the test in compiler.h so updates only have to change one place.

[tsoome]

There is also an problem that even in (recent) gcc, the __has_builtin() itself may be missing; meaning that lib/common/compiler.h should have something like:

#ifdef __has_builtin
#define HAS_BUILTIN(x) __has_builtin(x)
#else
#define HAS_BUILTIN(x) 0
#endif

and then you can use HAS_BUILTIN() ...

[cemeyer]

@tsoome Yeah, that is basically the compatibility macro that just returns false that @allanjude is describing.

Instead we are suggesting specific HAVE_BUILTIN_FOO macros for the feature in compiler.h, that can be override by distro/architectures in one place. Then code that uses __builtin_clz or whatever is guarded by one of those macros.