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This makes the code much simpler (especially for adding support for other instruction set extensions), avoids needing inline assembly for this feature, and generally is more of the canonical way to do this. The CPU feature detection was added in 493fcde50a84cb23854335bcb0e55c6f383d55db, using HWCAP_CPUID. The argument for using that, was that HWCAP_CPUID was added much earlier in the kernel (in Linux v4.11), while the HWCAP flags for individual features always come later. This allows detecting support for new CPU extensions before the kernel exposes information about them via hwcap flags. However in practice, there's probably quite little advantage in this. E.g. HWCAP2_I8MM was added in Linux v5.10 - long after HWCAP_CPUID, but there's probably very little practical cases where one would run a kernel older than that on a CPU that supports those instructions. Additionally, we provide our own definitions of the flag values to check (as they are fixed constants anyway), with names not conflicting with the ones from system headers. This reduces the number of ifdefs needed, and allows detecting those features even if building with userland headers that are lacking the definitions of those flags. Also, slightly older versions of QEMU, e.g. 6.2 in Ubuntu 22.04, do expose support for these features via HWCAP flags, but the emulated cpuid registers are missing the bits for exposing e.g. I8MM. (This issue is fixed in later versions of QEMU though.) Signed-off-by: Martin Storsjö <martin@martin.st>
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides means to alter decoded audio and video through a directed graph of connected filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.