On 29/04/2025 3:00 am, H. Peter Anvin wrote: > 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.
Yes it does generate inline code. https://godbolt.org/z/M45oo5rqT GCC does it branchlessly, but cannot optimise based on context. Clang can optimise based on context, except the 0 case it seems. Moving to -march=i686 causes both GCC and Clang to switch to CMOV and create branchless code, but is still GCC still can't optimise out the CMOV based on context. ~Andrew
