[ffmpeg] ld: warning: could not create compact unwind

[daschuer]

When I try to link to the ffmpeg libraries I get the following linker warnings:

2021-08-18T22:06:41.3473950Z ld: warning: could not create compact unwind for _ff_cfhd_init_vlcs: stack subq instruction is too different from dwarf stack size
2021-08-18T22:06:41.3475230Z ld: warning: could not create compact unwind for _ff_rl_init_vlc: stack subq instruction is too different from dwarf stack size

The linking itself succeeds and so far it works as well.
How can I get rid of the warnings?

ffmpeg is build with these options

-- Building Options: --enable-pic --disable-doc --enable-debug --enable-runtime-cpudetect --enable-asm --enable-x86asm --enable-gpl --disable-ffmpeg --disable-ffplay --disable-ffprobe --enable-avcodec --enable-avdevice --enable-avformat --enable-avfilter --enable-postproc --enable-swresample --enable-swscale --disable-libass --disable-avisynth --disable-bzlib --disable-libdav1d --disable-libfdk-aac --disable-libfontconfig --disable-libfreetype --disable-libfribidi --disable-iconv --disable-libilbc --disable-lzma --disable-libmp3lame --disable-libmodplug --disable-cuda --disable-nvenc --disable-nvdec  --disable-cuvid --disable-ffnvcodec --disable-opencl --disable-opengl --disable-libopenh264 --disable-libopenjpeg --disable-openssl --disable-libopus --disable-sdl2 --disable-libsnappy --disable-libsoxr --disable-libspeex --disable-libssh --disable-libtensorflow --disable-libtesseract --disable-libtheora --disable-libvorbis --disable-libvpx --disable-libwebp --disable-libx264 --disable-libx265 --disable-libxml2 --disable-zlib --disable-vdpau --extra-cflags="-isysroot /Applications/Xcode_12.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk" --extra-ldflags="-isysroot /Applications/Xcode_12.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk"  --enable-cross-compile --target-os=darwin --arch=x86_64 --extra-ldflags=-arch --extra-ldflags=x86_64 --extra-cflags=-arch --extra-cflags=x86_64 --extra-cxxflags=-arch --extra-cxxflags=x86_64 --pkg-config-flags=--static

Environment

• OS: osx
• Compiler: AppleClang 12.0.0.12000032

To Reproduce
Steps to reproduce the behavior:

1. ./vcpkg install ffmpeg
2. link mixxx application

Here is the workflow run building Mixxx
https://github.com/mixxxdj/mixxx/pull/4225/checks?check_run_id=3365982171

And here the Run building vcpkg
https://github.com/mixxxdj/vcpkg/runs/3332915409

Expected behavior
No warning should be logged

I would be great if someone has an idea how to fix the issue or debug it more.
Thank you.

[Be-ing]

Are you sure ffmpeg was built as 64 bit? https://stackoverflow.com/questions/39598302/ld-warning-stack-subl-instruction-is-too-different-from-dwarf-stack-size-on-os

[daschuer]

At least the --arch=x86_64 option is set. See above.

[Be-ing]

I am not sure it actually does get set:

--arch=x86_64 --extra-ldflags=-arch --extra-ldflags=x86_64 --extra-cflags=-arch --extra-cflags=x86_64 --extra-cxxflags=-arch --extra-cxxflags=x86_64

--extra-ldflags is set twice, first to -arch then to x86_64. The same happens for --extra-cflags and --extra-cxxflags. Perhaps only the first one has an effect.

[NancyLi1013]

Hi @daschuer

Thanks for posting this issue.

Do you encounter this issue on your local machine or on the CI mixxxdj/mixxx#4225?

Could you please try to pass the compiler option to rebuild?

LDFLAGS="-Wl,-no_compact_unwind"

[daschuer]

This linker flag removes the warning. See https://github.com/daschuer/mixxx/runs/3399675678

I encounter the issue on the GitHub CI. I have unfortunately no access to a local macOS machine.

Can you give some insights?

• Why the linker only complains about these two ffmpeg functions? Are other warnings suppressed or are only these two affected?
• For my feeling the linker flag is a workaround since it affect all functions, right?
• It seems to cause issues with the exception stack see: https://developer.apple.com/documentation/xcode/addressing-language-exception-crashes
• Crawling in GitHub I have found other solutions where the warning is silenced with LDFLAGS="-Wl,-w"
• What is the recommended Fix in case of this warning by the supplier?
• Is there something official where the "no_compact_unwind" flag is explained?

[daschuer]

By the way, when I do not link to ffmpeg the warning disappears. What could be the cause? It there something special with it?

[JackBoosY]

@daschuer Sorry but I didn'f found the error messages you mentioned in https://github.com/daschuer/mixxx/runs/3399675678, can you please extract it and provide me a log file?

Also, can you get the config logs and install logs in your ci system?

Thanks.

[daschuer]

https://github.com/daschuer/mixxx/runs/3399675678 is the run with LDFLAGS="-Wl,-no_compact_unwind"

The original error can be found here:
https://github.com/mixxxdj/mixxx/pull/4225/checks?check_run_id=3403021507

The logs from the vcpkg build run can be downloaded as artifact here:
https://github.com/mixxxdj/vcpkg/runs/3332915409

Thank you for having a look.

[daschuer]

Maybe quotes are missing when passing the arch parameters to ./configure

--extra-ldflags=-arch --extra-ldflags=x86_64

May become

--extra-ldflags="-arch x86_64"

Just a guess

[daschuer]

vcpkg/ports/ffmpeg/portfile.cmake

Line 588 in dd3d6df

set(OPTIONS_CROSS " --enable-cross-compile --target-os=darwin --arch=x86_64 --extra-ldflags=-arch --extra-ldflags=x86_64 --extra-cflags=-arch --extra-cflags=x86_64 --extra-cxxflags=-arch --extra-cxxflags=x86_64")

[JackBoosY]

@daschuer

[daschuer]

You need to open the "build" section fist in order to make the search feature work.
Alternative you may look at the whole log using the "..." Menu.

[daschuer]

I have used single quotes for --extra-ldflags='-arch x86_64' but this does not make a difference.

[JackBoosY]

@mcmtroffaes Do you know why?

[mcmtroffaes]

I've seen this warning too on ci, but I don't know the cause. It seems to have no impact on the executables as far as I've tested. I'm sorry I can't be more helpful at this instance.

[daschuer]

By default ffmpeg is build with gcc. I have patched the build.sh.in to use clang but the warning remains, so it is not a compiler bug.
If I disable the dfhd encoder and decoder, one warning disappears. So it seems to be an issue with the function itself.
Both function have in common that they allocate big arrays on the stack. I guess that this causes the linker to fail applying compact unwind.

I will report the issue upstream, maybe they can suggest a workaround.

[daschuer]

https://trac.ffmpeg.org/ticket/9399

[JackBoosY]

Close this PR until upstream fixes this.