Compilation warnings ATOMIC_VAR_INIT

[Jozef Chutka]

While compiling ffmpeg (commit 1368b5a) with emscripten 3.1.17, there are two warnings, I thinks maintainers might want to look into:

fftools/ffmpeg.c:339:41: warning: macro 'ATOMIC_VAR_INIT' has been marked as deprecated [-Wdeprecated-pragma]
static atomic_int transcode_init_done = ATOMIC_VAR_INIT(0);
                                        ^
/home/XYZ/ffmpeg-wasm/modules/emsdk/upstream/lib/clang/15.0.0/include/stdatomic.h:50:41: note: macro marked 'deprecated' here
#pragma clang deprecated(ATOMIC_VAR_INIT)
                                        ^

To my knowledge, this macro was a part of early draft design for C11 atomic types. It is not needed in C11, and is deprecated in C17 and removed in C23.

The other warning is also interesting:

fftools/ffmpeg_filter.c:898:35: warning: floating-point comparison is always true; constant cannot be represented exactly in type 'float' [-Wliteral-range]
        if (audio_drift_threshold != 0.1)
            ~~~~~~~~~~~~~~~~~~~~~ ^  ~~~

It turns out that 0.1 is one of the numbers that it is impossible to encode in binary floating point. And according to the message the condition is as good as having if(true) in place. Consider using epsilon:

if (abs(audio_drift_threshold - 0.1) < epsilon)

SO helped me with understanding the details ​https://stackoverflow.com/questions/73203857/ffmpeg-compilation-warnings-atomic-var-init

[Carl Eugen Hoyos]

ATOMIC_VAR_INIT() is part of C11 and FFmpeg explicitely requests C11 with -std=c11
So while this ticket is missing all necessary information to reproduce (like at least the used configure line), I do not immediately see an issue.

[Balling]

ATOMIC_VAR_INIT() is not needed by C11 and deprecated by C17, stop reading drafts and download normal standard on libgen. But since when do we use C11? I thought it was C90 (so all "declaration before code" applies) with only some parts C99, but should be frowned upon.

​https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents

See that C11 has one TC, where they modify constands for version, public page: ​https://www.iso.org/obp/ui/#iso:std:iso-iec:9899:ed-3:v1:cor:1:v1:en

Deprecation may not be obvious, since that is just two words removal in C17 standard... ​https://en.cppreference.com/mwiki/index.php?title=c/atomic/ATOMIC_VAR_INIT&diff=97502&oldid=77535

[Jozef Chutka]

Attaching emscripten configure and build commands. Its based on ​https://github.com/ffmpegwasm/ffmpeg.wasm-core :

emconfigure
  ./configure 
  --target-os=none
  --arch=x86_32
  --enable-cross-compile
  --enable-gpl
  --enable-version3
  --enable-zlib
  --enable-libaom
  --enable-libx264
  --enable-libx265
  --enable-libvpx
  --enable-libmp3lame
  --enable-libtheora
  --enable-libvorbis
  --enable-libopus
  --enable-libwebp
  --enable-librubberband
  --disable-x86asm
  --disable-inline-asm
  --disable-stripping
  --disable-programs
  --disable-doc
  --disable-debug
  --disable-runtime-cpudetect
  --disable-autodetect
  --extra-cflags="-O3 -I$BUILD_DIR/include -s USE_PTHREADS=1 -msimd128"
  --extra-cxxflags="-O3 -I$BUILD_DIR/include -s USE_PTHREADS=1 -msimd128"
  --extra-ldflags="-O3 -I$BUILD_DIR/include -s USE_PTHREADS=1 -msimd128"
  --pkg-config-flags="--static"
  --nm="llvm-nm"
  --ar=emar
  --ranlib=emranlib
  --cc=emcc
  --cxx=em++
  --objcc=emcc
  --dep-cc=emcc

emcc
  -O3 -I$BUILD_DIR/include -s USE_PTHREADS=1 -msimd128
  -I. -I./fftools -I$BUILD_DIR/include
  -Llibavcodec -Llibavdevice -Llibavfilter -Llibavformat -Llibavresample -Llibavutil -Llibpostproc -Llibswscale -Llibswresample -Lrubberband -Lsamplerate -Lflite -L$BUILD_DIR/lib
  -Wno-deprecated-declarations -Wno-pointer-sign -Wno-implicit-int-float-conversion -Wno-switch -Wno-parentheses -Qunused-arguments
  -lavdevice -lavfilter -lavformat -lavcodec -lswresample -lswscale -lavutil -lpostproc -lm -laom -lx264 -lx265 -lvpx -lmp3lame -lvorbis -lvorbisenc -lvorbisfile -logg -ltheora -ltheoraenc -ltheoradec -lz -lopus -lwebp -lwebpmux -lrubberband -lsamplerate
  fftools/cmdutils.c fftools/ffmpeg.c fftools/ffmpeg_filter.c fftools/ffmpeg_hw.c fftools/ffmpeg_mux.c fftools/ffmpeg_opt.c fftools/objpool.c fftools/opt_common.c fftools/sync_queue.c fftools/thread_queue.c
  -lworkerfs.js
  -s USE_SDL=2
  -s INVOKE_RUN=0
  -s EXIT_RUNTIME=1
  -s MODULARIZE=1
  -s EXPORT_NAME="createFFmpeg"
  -s EXPORTED_FUNCTIONS="[_main, ___wasm_init_memory_flag]"
  -s EXPORTED_RUNTIME_METHODS="[callMain, FS, WORKERFS]"
  -s INITIAL_MEMORY=128mb
  -s ALLOW_MEMORY_GROWTH=1
  -s MAXIMUM_MEMORY=4gb
  -s ENVIRONMENT=worker
  -s PROXY_TO_PTHREAD=1
  -pthread
  -o $WASM_DIR/ffmpeg.js

With this configuration I observe the reported issues (ATOMIC_VAR_INIT and audio_drift_threshold != 0.1). Please advise if you see something wrong in my commands

[Balling]

So this happened with ​https://github.com/llvm/llvm-project/commit/0d459444e5105d689e6388612784b89a93054944

Yet they say DR 485 was closed, but it was not: ​https://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm

It is still in review. Very shady.

[Balling]

​https://en.cppreference.com/mwiki/index.php?title=c/atomic/ATOMIC_VAR_INIT&diff=140123&oldid=138682

So, it were to be removed in C23, 100%.

[Balling]

One warning fixed in ​https://github.com/FFmpeg/FFmpeg/commit/686096739b129c7e3ea26be29c875e0887e58c49, the other please somehow force emscripten to use older C standard. Please verify.

[Jozef Chutka]

Thanks for the fix.

In regards to C standard, there are a few numbers mentioned in this thread, which is the right one to be used?

[Balling]

It is C90 with parts of C99. So it should be c99, I imagine.

[mkver]

No, the atomics are only part of C11. FFmpeg has not been C90 for a long time; currently it is "C99 excluding variable-length arrays, with C11 atomics very much encouraged". (There is a fallback in case atomics are unavailable, but you should really not use it.)

[Balling]

ATOMIC_VAR_INIT() is not needed by C11 though, but it says that with deleting two words.

[Carl Eugen Hoyos]

I cannot reproduce this with emscripten 3.1.22