[PATCH][X86] __builtin_ctz/clz sometimed defined for zero input

[Sean Silva]

On Thu, Oct 23, 2014 at 11:51 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> 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:chisophugis at gmail.com]
> *Sent:* Thursday, October 23, 2014 7:03 PM
> *To:* Robinson, Paul
> *Cc:* cfe-commits at cs.uiuc.edu
> *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 <
> Paul_Robinson at playstation.sony.com> 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
>
>
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