GCC inline assembly looks incredibly cursed. Back in the day, the Borland tool suite (Turbo/Borland C++, Turbo Pascal) had inline assembly that looked more like the D compiler example above.
GCC does, however, know what to do with a .s file, so you can write your assembly routines outside your C(++) source and just compile them in like a C module, which is what I did last time I was hardcore slinging x86 opcodes.
It's the attempt to tell the host language what you're doing with the arguments that makes a real mess. Module scope is roughly the same as putting it in a separate file, i.e. less horrible. C++ has better string literal escaping options which would help.
It's very easy to get the constraints wrong and have the aggregate still "work", until unrelated changes months later perturb the register allocation slightly such that it no longer runs as hoped.
It's documented and usable but working with it is never a very good time.
GCC does, however, know what to do with a .s file, so you can write your assembly routines outside your C(++) source and just compile them in like a C module, which is what I did last time I was hardcore slinging x86 opcodes.