Unfortunately, I think this is a fundamental problem!

Time-critical, by its very nature, tends to be non-portable.

'C' gives you absolutely no guarantees at all about what specific machine code may be generated for any particular source lines.
You may be able to "tune" your source code to a specific version of a specific compiler, but to get this to work across different compilers is quite unlikely, I think.


You can never be sure of that - see above.

The best you can do is to inspect the generated code, which largely defeats the object - you probably might as well have written it in assembler in the first place!


I think you're just going to have to re-write your assembler into a "least common subset" of the 2 assemblers.

And don't forget: the two compilers may well have different internal data representations, different calling conventions, etc, etc...