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 > >
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