On Thu, Oct 23, 2014 at 11:51 PM, Robinson, Paul < [email protected]> wrote:
> See the existing commentary for TargetInfo::isCLZForZeroUndef. ARM, > AArch64, Mips, PPC all have this as defined for zero input, see overrides > in Targets.cpp. > > This is well-trod ground, and merely corrects a deficiency in the X86 info. > I guess if there is precedent for this sort of thing then go ahead. For some reason I was interpreting the patch as adding the isCLZForZeroUndef interface. > > > Andrea has a patch coming that does exactly the simplification you're > suggesting, but driven by the flag passed in by Clang rather than a-priori > knowledge about instruction behavior. > I don't see the point of requiring clang to inform the optimizer. No information is lost by the transformation. -- Sean Silva > > > I'm not willing to take on the chore of reassigning responsibility for > which component knows exactly which arcana about particular instructions. > I could see adding a note to the description of the builtins. > > --paulr > > > > *From:* Sean Silva [mailto:[email protected]] > *Sent:* Thursday, October 23, 2014 7:03 PM > *To:* Robinson, Paul > *Cc:* [email protected] > *Subject:* Re: [PATCH][X86] __builtin_ctz/clz sometimed defined for zero > input > > > > > > If I understand correctly, this patch is trying to change the meaning of > __builtin_ctz (et al.) under some extremely specific conditions. I don't > think that is the right direction since it will cause surprising undefined > behavior bugs across platforms. The intrinsic is documented to have > undefined behavior in the 0 case (everywhere I looked, including our > internal docs); a user that relies on the 0 case has a bug. It would be > nice to add a UBSan check for this undefined behavior though to help users > fix their code. > > > > It would be better to just ensure that we always generate optimal code in > the presence of a manual guard for the 0 case. For example, in the > middle-end we could fold a manual 0 guard followed by @llvm.ctlz.*(X, true) > into @llvm.ctlz.*(X, false). > > > > -- Sean Silva > > > > On Thu, Oct 23, 2014 at 4:40 PM, Robinson, Paul < > [email protected]> wrote: > > In general, count-zeros instructions are undefined for a zero input value. > However the X86 TZCNT and LZCNT instructions do return the bit-width on a > zero input, so make Clang tell LLVM so. > One quirk is that these instructions aren't necessarily both defined, so > also create a separate predicate so we can do the right thing for all CPUs. > --paulr > > > _______________________________________________ > cfe-commits mailing list > [email protected] > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits > > >
_______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
