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Diffstat (limited to 'third_party/ffmpeg/README.chromium')
| -rw-r--r-- | third_party/ffmpeg/README.chromium | 191 |
1 files changed, 186 insertions, 5 deletions
diff --git a/third_party/ffmpeg/README.chromium b/third_party/ffmpeg/README.chromium index 9548c1d..5184cf5 100644 --- a/third_party/ffmpeg/README.chromium +++ b/third_party/ffmpeg/README.chromium @@ -1,5 +1,10 @@ -This contains FFmpeg's public header files from the output of a "make install" -command. The header files are from Chromium's copy of FFmpeg. +This file describes how to produce the FFmpeg include directory, and how to +create the ffmpeg.gyp file and related configurations. + +-- FFmpeg headers in the 'include' directory. + +The include directory contains FFmpeg's public header files from the output of +a "make install" command. The header files are from Chromium's copy of FFmpeg. Steps to reproduce: 1) If on Windows, refer to our MinGW/MSYS environment setup: @@ -13,11 +18,187 @@ Steps to reproduce: /path/to/install/include/libavutil The project contains some hand-written DEF files used to generate import -libraries to permit dynamically loading FFmpeg. The libaries are linked in -using /DELAYLOAD to avoid having the DLLs present at run-time. +libraries to permit dynamically loading FFmpeg. On Windows, the libraries are +linked in using /DELAYLOAD to avoid having the DLLs present at run-time. On +POSIX systems, dlopen() is used to achieve a similar effect. We don't use the import libraries generated from building FFmpeg because they -export every method by ordinal, which makes binary compatability with different +export every method by ordinal, which makes binary compatibility with different builds of FFmpeg difficult if not impossible. Furthermore, it is much easier to update a DEF file instead of rebuilding FFmpeg to generate new import libraries. + + +-- Recreating the ffmpeg.gyp file and populating the config directory. +The ffmpeg.gyp file is meant to be used in place of FFmpeg's + + ./configure && make + +steps. The file was created by inspecting the build log from above. +The FFmpeg build is relatively straightforward. All files are built with +the same CFLAGS. The config.h and version.h files are the only files generated +by ./configure that are included elsewhere. They require a small bit of +post-processing. + +Other than the configure step, FFmpeg just compiles its .c files, assembles a +few more using yasm, and that's it. Exact instructions for reproducing +ffmpeg.gyp are in the "Detailed Directions" section. + +Here is a list of gotchas that have shown up. + 1) FFmpeg requires special configure (--disable-optimizations) in order + to be built with -O0 successfully due to some of the hand-written + assembler using ebp. -O0 implies -fno-omit-frame-pointer which breaks + this. This will produce compiler errors like: + libavcodec/cabac.h:527: error: can't find a register in class + 'GENERAL_REGS' while reloading 'asm' + cabac.h:527: error: 'asm' operand has impossible constraints + + 2) On ia32, FFmpeg cannot be built with -fPIC, again due to assembly + issues. There may be a workaround, but the performance impact is + unknown. + + 3) Sometimes, with -O0, invalid code will be exposed because dead-branch + pruning is disabled in gcc. This can manifest itself as strange link + issues or compile issues. Be careful to read all warnings in this case. + + 4) Since config.h is generated via ./configure, the generated file will + be sensitive to the configuration of the machine it was produced on. + In particular, yasm does not seem to always be detected if + cross-compiling for 32-bit on a 64-bit machine. Since yasm is built in + tree, make sure to force things with --enable-yasm. + + 5) Similar to issue #4, ./configure may detect the presence of SDL and + adjust config.h accordingly. This is harmless because all the SDL + related code has been disabled in our configuration. + + 6) On ia32, we want to be able to compile with WITHOUT -fomit-frame-pointer + (so breakpad can function). To do this, we need to disable the use of the + EBP register, otherwise some of FFmpeg's inline assembly will cause + compilation errors similar to gotcha #1. For more details, see the file + comment in the munge_config_optimizations.sh. This script will fix up + the generated config.h to be building without -fomit-frame-pointer. + + +Detailed Directions: + 1) Get a clean version of the patched tree. This should be here: + + src/third_party/ffmpeg/source/patched-ffmpeg-mt + + 2) Run the configure in a directory out of the tree with the arguments you + want. To see what was used before, find the config.h for the platform + of interest in: + + src/third_party/ffmpeg/source/config/[branding]/[platform]/[variant] + + The value of the FFMPEG_CONFIGURATION macro should have the configure + commandline that generated the file. + + Note that if you are trying to build a 32-bit FFmpeg for linux on a + 64-bit box, the extra flags you want to pass to ./configure are + + --arch=i686 --extra-cflags=-m32 --extra-ldflags=-m32 + + Also, as noted in gotcha #4, explicitly setting --enable-yasm is + a good idea. + + 3) Copy the newly generated config.h and version.h into the correct platform + location: + + src/third_party/ffmpeg/source/config/[branding]/[platform]/[variant] + + Make sure to double-check that config.h and version.h are the only files + of interest. By that, I mean check that the other generated files are + makefiles, documentation, .pc files, or something else that is not + relevant to our build. + + TODO(ajwong): Check if we can modify version.h to tag our builds. + + 3b) If on ia32, handle gotcha #6 by munging the geneated config.h file to + disable use of EBP. Call the munge_config_optimizations.sh script on + the config.h for each ia32 variant. + + ** This script is not idempotent. Don't run it twice ** + + Remember, this is only necessary for ia32 config.h files. Running this + on config.h files for other platforms (in particular, for x64) will + likely result in unecessarily slow code, or compile failures. + + 4) Next, capture all the output from a build of libavcodec.so and + libavformat.so. We will use the build log as a reference for making + the ffmpeg.gyp file. + + make libavcodec/libavcodec.so libavformat/libavformat.so \ + > ffmpeg_build_log 2> ffmpeg_build_err + + 5) Check ffmpeg_build_err to see if there are any significant + anomalies. FFmpeg source generates a lot of compiler warnings; it + is safe to ignore those. + + 6) Examine all non-gcc commands to see if we're missing anything + interesting: + + grep -v '^gcc' ffmpeg_build_log + + There should be yasm commands for assembling two yasm files, but nothing + else. Include those yasm files in the sources list for gyp. That means + + grep -v '^gcc\|^yasm' + + should generate nothing beyond "cd" and "ln" commands. + + 7) Verify that the all the gcc commands have the same compiler flags. + Do that with the following "one-liner": + + grep - '^gcc' ffmpeg_build_log | + grep -v ' -MM ' | + grep -v ' -shared ' | + sed -e 's/ -o .*$//' | + sort | uniq -c + + This should find all gcc commands, exclude the dependency generation + lines, the link lines, and strip the output/input file names leaving + just the compiler flags + invocation. You should only see one "line" + of output. If there is more than one, figure out if the differences + in compiler flags are significant, and then use your best judgment. + + Look at gotcha #2 in for notes about the -fPIC flag in particular. + + 8) Examine the output from step 7 and update the compiler flags in + ffmpeg.gyp. For easier cut/paste, append the following to the previous + command line to isolate each flag on its own line and add + single-quotes: + + tr -s ' ' | tr ' ' '\n' | sed -e "s/\(.*\)/'\1',/" | sort -u + + 9) Next, examine the link flags to see if anything interesting appears. + + grep ' -shared ' ffmpeg_build_log | + tr ' ' '\n' | + grep -Ev '^[^-].*' | + grep -v rpath | + grep -Ev '^-L' | + sort -u + + This should find all link lines, move each flag to its own line, + remove any argument that isn't a flag, remove all the rpaths (not + useful for us anyways), and remove all the -L lines (also not useful + for us). + + The most interesting will likely be the -Wl,.* lines. Update the + ldflags section in ffmpeg.gyp accordingly. + + 10) Lastly, Find all the build .c files and update the sources line (this is + very similar to step 7): + + grep -E '^gcc' ffmpeg_build_log | + grep -v ' -MM ' | + grep -v ' -shared ' | + sed -e "s|.* -o .* \(.*\)$|'source/patched-ffmpeg-mt/\1',|" | + sort + + 11) Attempt to build. :) + +*12) Update the the sources! clause to exclude files that should only be built + for Chromium. For this, you basically need to do the steps above once + with the configure options for Chrome, then once with the options for + Chromium and diff the list of .c and .asm source files. |