RFR: 8312591: GCC 6 build failure after JDK-8280982

[Alexander Zvegintsev]

On Mon, 24 Jul 2023 20:24:23 GMT, Phil Race <prr at openjdk.org> wrote:

>> There is a simple build failure after [JDK-8280982](https://bugs.openjdk.org/browse/JDK-8280982) with older GCCs:
>> 
>> 
>> * For target support_native_java.desktop_libawt_xawt_screencast_pipewire.o:
>> In file included from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/spa/buffer/buffer.h:12:0,
>>                  from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/pipewire/stream.h:171,
>>                  from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libawt_xawt/awt/screencast_pipewire.h:36,
>>                  from /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libawt_xawt/awt/screencast_pipewire.c:33:
>> /home/buildbot/worker/build-jdkX-debian9/build/src/java.desktop/unix/native/libpipewire/include/spa/utils/defs.h:61:24: error: "__clang_major__" is not defined [-Werror=undef]
>>  #elif __GNUC__ >= 7 || __clang_major__ >= 10
>>                         ^~~~~~~~~~~~~~~
>> 
>> 
>> There is an obvious fix for this: we need to check for `defined(__GNUC__)` explicitly before touching `__clang_major__`.
>> 
>> (Yes, GCC 6 is old; but we would like to make sure it builds until we run into hard to resolve build issues. This allows modern JDKs to be built in legacy enterprise environments for e.g. portable builds.)
>> 
>> Additional testing:
>>  - [x] Linux GCC 6 fastdebug build (passes with HarfBuzz warnings, to be fixed separately)
>
> src/java.desktop/unix/native/libpipewire/include/spa/utils/defs.h line 62:
> 
>> 60: #  define SPA_FALLTHROUGH [[clang::fallthrough]];
>> 61: #elif defined(__GNUC__) && __GNUC__ >= 7
>> 62: #  define SPA_FALLTHROUGH __attribute__ ((fallthrough));
> 
> Upstream still has this issue/bug. 
> https://gitlab.freedesktop.org/pipewire/pipewire/-/blob/master/spa/include/spa/utils/defs.h
> Do they intend to support only GNUC 7 and up ? Not clear. Best to ask.

Ideally, this should be fixed upstream, but while we wait, we can try to fix the build failure in our source by adding definition of `__clang_major__` just before including the [pipewire/stream.h](https://github.com/openjdk/jdk/blob/master/src/java.desktop/unix/native/libawt_xawt/awt/screencast_pipewire.h#L36)


// some comment why we are doing this
#ifndef __clang_major__
#define __clang_major__ 0
#endif


It looks like that the `defs.h` is the only file that uses `__clang_major__`  in libxawt, so this define should be safe.


But the question of GCC < 7 support remains open.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14995#discussion_r1274858361