On October 23, 2020 2:11:19 PM PDT, Linus Torvalds <torva...@linux-foundation.org> wrote: >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
Unconditional "A" is definitely wrong, no argument there. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
