[ms] RFC: MASM support

Hi all,

I’m proposing to add MASM support to LLVM’s assembler capabilities, which should nearly complete LLVM’s support for cross-platform Windows compilation.

Goal: Match the functionality of Microsoft’s ml.exe and ml64.exe.

I’m currently trying to implement this as a set of extensions to the assembler; it will correctly handle assembly files containing both GNU and MASM syntax.

The features would be added to AsmParser and controlled via MCAsmInfo, and would be available through both llvm-mc and clang. Both tools would remain able to parse GNU-syntax assembler, but when passed the appropriate flags, would also handle MASM. My first thought is to trigger MASM support whenever we’re targeting Windows, but we could also make it a discrete function controlled by a different command-line flag.

We will also define a new driver (llvm-ml) that matches the command-line interface of ml.exe and/or ml64.exe. This will likely be similar to clang-cl and llvm-lib in building on top of a previous driver: either llvm-mc or clang.

Known obstacles:

  1. Support for "ifdef ": already completed, with a new tryParseRegister method added to all TargetAsmParsers.
  2. Syntax variations: MASM uses infix notation for many directives, and does not use “.”-prefixes on its directives.
  3. Macro functions: MASM includes macro functions, which can emit parameters and not just full instructions. (Probably tricky.)
  4. Richer macro language in general: some MASM files rely heavily on text-substitution for named symbols, as well as resolution of fields in STRUCTs.

3 & 4 might be best handled by adding a preprocessor stage, but the syntax is not similar to the C preprocessor… I’m going to try to augmenting existing macro support first.

Any obstacles I’ve missed, or critiques of the approaches, would be very welcome!


  • Eric Astor