On Fri, Oct 23, 2020 at 2:00 PM <h...@zytor.com> wrote: > > There is no same reason to mess around with hacks when we are talking about > dx:ax, though.
Sure there is. "A" doesn't actually mean %edx:%eax like you seem to think. It actually means %eax OR %edx, and then if given a 64-bit value, it will use the combination (with %edx being the high bits). So using "A" unconditionally doesn't work - it gives random behavior for 32-bit (or smaller) types. Or you'd have to cast the value to always be 64-bit, and have the extra code generation. IOW, an unconditional "A" is wrong. And the alternative is to just duplicate things, and go back to the explicit size testing, but honestly, I really think that's much worse than relying on a documented feature of "register asm()" that gcc _documents_ is for this kind of inline asm use. So the "don't do pointless conditional duplication" is certainly a very sane reason for the code. Linus