Can I write (x86) assembly language which will build with both GCC and MSVC?

Question

I have a project which is entirely written in C. The same C files can be compiled using either GCC for Linux or MSVC for Windows. For performance reasons, I need to re-write some of the code as x86 assembly language.

Is it possible to write this assembly language as a source file which will build with both the GCC and MSVC toolchains? Alternatively, if I write an assembly source file for one toolchain, is there a tool to convert it to work with the other?

Or, am I stuck either maintaining two copies of the assembly source code, or using a third-party assembler such as NASM ?

Answer 1

I see two problems:

• masm and gas have different syntax. gas can be configured to use Intel syntax with the .syntax intel,noprefix directive, but even then small differences remain (such as, different directives). A possible approach is to preprocess your assembly source with the C preprocessor, using macros for all directives that differ between the two. This also has the advantage of providing a unified comment syntax.

  However, just using a portable third party assembler like nasm is likely to be less of a hassle.

• Linux and Windows have different calling conventions. A possible solution for x86-32 is to stick to a well-supported calling convention like stdcall . You can tell gcc what calling convention to use when calling a function using function attributes . For example, this would declare foo to use the stdcall calling convention:

   extern int foo(int x, int y) __attribute__((stdcall)); 

  You can do the same thing in MSVC with __declspec , solving this issue.

  On x86-64, a similar solution is likely possible, but I'm not exactly sure what attributes you have to set.

  You can of course also use the same cpp-approach as for the first problem to generate slightly different function prologues and epilogues depending on what calling convention you need. However, this might be less maintainable.