On April 28, 2025 5:12:13 PM PDT, Andrew Cooper <andrew.coop...@citrix.com> wrote: >On 28/04/2025 10:38 pm, H. Peter Anvin wrote: >> On April 28, 2025 9:14:45 AM PDT, Linus Torvalds >> <torva...@linux-foundation.org> wrote: >>> On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mi...@kernel.org> wrote: >>>> And once we remove 486, I think we can do the optimization below to >>>> just assume the output doesn't get clobbered by BS*L in the zero-case, >>>> right? >>> We probably can't, because who knows what "Pentium" CPU's are out there. >>> >>> Or even if Pentium really does get it right. I doubt we have any >>> developers with an original Pentium around. >>> >>> So just leave the "we don't know what the CPU result is for zero" >>> unless we get some kind of official confirmation. >>> >>> Linus >> If anyone knows for sure, it is probably Christian Ludloff. However, there >> was a *huge* tightening of the formal ISA when the i686 was introduced >> (family=6) and I really believe this was part of it. >> >> I also really don't trust that family=5 really means conforms to >> undocumented P5 behavior, e.g. for Quark. > >https://www.sandpile.org/x86/flags.htm > >That's a lot of "can't even characterise the result" in the P5. > >Looking at P4 column, that is clearly what the latest SDM has >retroactively declared to be architectural. > >~Andrew
Yes, but it wasn't about flags here. Now, question: can we just use __builtin_*() for these? I think gcc should always generate inline code for these on x86.
