third_party/ffmpeg/README.chromium

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:
       http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/mingw/
  2) Grab Chromium's copy of FFmpeg:
       http://src.chromium.org/viewvc/chrome/trunk/deps/third_party/ffmpeg/
  3) Follow the instructions to build and install.
  4) Go to your install location and copy the following into the Chromium tree:
       /path/to/install/include/libavcodec
       /path/to/install/include/libavformat
       /path/to/install/include/libavutil

The project contains some hand-written DEF files used to generate import
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 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

     For Mac, replace the ".so" in the files above with ".dylib".

  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.