Comment by gpderetta

I believe that MSVC inline asm allows referencing variables in the asm as it can parse and understand the asm (at least before they got rid of inline asm completely for 64 bit code).

AFAIK GCC does not attempt to parse the asm by design, as it is meant to be used for code that the compiler might not understand, so you have to describe input, outputs and side effects with annotations.