x264, a project under the Videolan organizational umbrella, is one of the most popular video compression libraries, used worldwide for applications such as web video, television broadcast, and Blu-ray creation. It outclasses practically all commercial implementations both speed and compression-wise.

x264 has many assembly functions that would be useful in ffmpeg's H.264 decoder.  Porting such functions can improve H.264 decoding performance in all open source media players.  For many of these functions, porting is somewhat nontrivial; for example, ffh264 often uses variable stride in places that x264 uses constant stride.

IMPORTANT: if you want to work on an x264 task, it is required that you come to #x264dev on irc://irc.freenode.net.  We will help answer your questions, guide you through whatever issues you have, and do code reviews.  In order to successfully complete an x264 task, the result must be of sufficiently high quality to be committed to the official x264 repository.   To ensure this, we will review your code and give you comments on how to improve it.  A task is only completed when these steps are done.

In short: do not take an x264 task, work on it without telling anyone, and then dump the code on us.  You'll waste your time, our time, and won't get credit for the task.