Ever since Apple released their iPhone SDK, the FFmpeg mailing lists have seen a steady stream of error reports from users attempting to build FFmpeg for the iPhone, and eventually they got my attention.
The iPhone is built around an ARM1176 CPU, so the SDK includes an ARM cross-compiler and assembler. Most of the reported errors originate from the Apple assembler which appears to have trouble processing the assembler source files from FFmpeg.
The source files use the GNU assembler syntax, and the Apple assembler is based on an old GNU version, so one might reasonably expect it to work. What I had not realised was just how old a version Apple based their assembler on. The version they chose was 1.38.1, released in January 1991, 18 years ago. Features which have since been added to the GNU assembler, and there are many, have not been merged by Apple. As a result, many special directives and macro features used in FFmpeg are not recognised by the Apple assembler, and modifying the code to work with this assembler would render it unusable with modern GNU versions.
Why not replace the assembler in the SDK with a GNU version, one might ask. The answer is that this is not possible. The Apple system uses an object file format, Mach-O, not supported by the GNU tools. The chances of Apple updating their assembler to support the newer syntax appear slim, so our best hope is for the GNU binutils package to gain support for the Mach-O format. This will need a lot of work, and a working version cannot be expected for yet some time.
While this incompatibility persists, those wishing to run an optimised FFmpeg build on their iPhone will have to rely on patches to make it palatable to the Apple assembler. Supporting the Apple syntax directly in FFmpeg is unfortunately not feasible.