Yes, but in the context of this thread: undocumented instructions can change from generation to generation of a processor architecture, while documented instruc

Yes, but in the context of this thread: undocumented instructions can change from generation to generation of a processor architecture, while documented instructions are much more stable; and because of that, compilers will not generate undocumented instructions. When reading compiler output, the official documentation of the instructions (plus documentation of the calling conventions) should be enough.