FFmpeg/gas-preprocessor

gas-preprocessor

This is a script for parsing GAS (GNU Assembler) files, expanding the macros and similar directives into a linear sequence of instructions, to allow building such files with assemblers that don't support the GAS directives.

Currently, this tool is primarily useful for building GAS files with MSVC, for ARM and ARM64. The tool can be used for building the ARM/AArch64 assembly in projects like FFmpeg, dav1d, x264 and openh264.

Additionally, for ARM assembly, the tool can rewrite some instructions that only can be assembled in ARM mode, into the corresponding sequences of instructions that can be assembled in Thumb mode. This can allow assembling sources that have been written without support for Thumb mode for Windows (where Thumb mode is the default).

Earlier use cases

Originally, the tool was made for building things for Apple platforms; ARM and PowerPC assembly. Apple used to ship an ancient version of GAS, which used a different macroing syntax than the modern GNU assembler.

Since Apple have replaced the ancient version of GAS it with LLVM/Clang based tooling, and LLVM's assembler support has improved to handle most relevant directives, gas-preprocessor is no longer needed for Apple platforms. Since Xcode 9.3 (LLVM 5.0), those tools support everything needed for building FFmpeg's ARM assembly without gas-preprocessor. (FFmpeg's AArch64 assembly never required gas-preprocessor on Apple platforms.)

The main remaining platform where gas-preprocessor is needed, is for MSVC tooling, to make GAS assembly files buildable with the MS armasm/armasm64 assemblers.

Usage

The projects listed above automatically try to use gas-preprocessor when configuring for a toolchain and architecture where needed; it doesn't need to be manually specified.