Add and use SDL_FALLTHROUGH for fallthroughs

[InfoTeddy]

Case fallthrough warnings can be suppressed using the __fallthrough__ compiler attribute. Unfortunately, not all compilers have this attribute, or even have __has_attribute to check if they have the __fallthrough__ attribute. [[fallthrough]] is also available in C++17 and the next C2x, but not everyone uses C++17 or C2x.

So define the SDL_FALLTHROUGH macro to deal with those problems - if we are using C++17 or C2x, it expands to [[fallthrough]]; else if the compiler has __has_attribute and has the __fallthrough__ attribute, then it expands to __attribute__((__fallthrough__)); else it expands to an empty statement, with a /* fallthrough */ comment (it's a do {} while (0) statement, because users of this macro need to use a semicolon, because [[fallthrough]] and __attribute__((__fallthrough__)) require a semicolon).

Applications using SDL are also free to use this macro (because it is defined in begin_code.h).

All existing /* fallthrough */ comments have been replaced with this macro. Some of them were unnecessary because they were the last case in a switch; using SDL_FALLTHROUGH in those cases would result in a compile error on compilers that support __fallthrough__, for having a __attribute__((__fallthrough__)) statement that didn't immediately precede a case label.

[sezero]

This causes multitudes of warning from old clang, e.g. clang-3.4.2:

src/audio/SDL_wave.c:1718:9: warning: declaration does not declare anything [-Wmissing-declarations]
        SDL_FALLTHROUGH;
        ^~~~~~~~~~~~~~~
include/begin_code.h:179:25: note: expanded from macro 'SDL_FALLTHROUGH'
#define SDL_FALLTHROUGH __attribute__((__fallthrough__))
                        ^~~~~~~~~~~~~
src/audio/SDL_wave.c:1857:9: warning: declaration does not declare anything [-Wmissing-declarations]
        SDL_FALLTHROUGH;
        ^~~~~~~~~~~~~~~
include/begin_code.h:179:25: note: expanded from macro 'SDL_FALLTHROUGH'
#define SDL_FALLTHROUGH __attribute__((__fallthrough__))
                        ^~~~~~~~~~~~~
2 warnings generated.

[many many others...]

[sezero]

Also see:
https://boringssl.googlesource.com/boringssl/+/fbebe833b180a1a0685803c59bba38916826dd2b%5E%21/
https://lore.kernel.org/linux-btrfs/b49378ba-486f-1354-ef9e-fa8151eeffe7@suse.com/
https://stackoverflow.com/questions/45349079/how-to-use-attribute-fallthrough-correctly-in-gcc

[slouken]

Do you have a fix?

[sezero]

Do you have a fix?

unfortunately I don't

[slouken]

Okay, I reverted it for now. @InfoTeddy, can you look and see the best way to fix this?

[sezero]

FWIW: clang documentation shows that the GNU syntax, i.e. __attribute__((__fallthrough__)) is supported as of clang-10.0. The warnings will thus show with clang-9.x and older.
https://releases.llvm.org/9.0.0/tools/clang/docs/AttributeReference.html#fallthrough
https://releases.llvm.org/10.0.0/tools/clang/docs/AttributeReference.html#fallthrough

[InfoTeddy]

Should be as simple as expanding to __attribute__((__fallthrough__)) only if Clang is 10 or later and if GCC is 7 or later, otherwise do {} while (0) /* fallthrough */.

[slouken]

Do you want to create a new pull request for that?

[InfoTeddy]

Sure, I can do that.

[InfoTeddy]

Done: #4944