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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">See the existing commentary for TargetInfo::isCLZForZeroUndef. ARM, AArch64, Mips, PPC all have this as defined for zero input, see overrides in Targets.cpp.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">This is well-trod ground, and merely corrects a deficiency in the X86 info.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Sean Silva [mailto:chisophugis@gmail.com]
<br>
<b>Sent:</b> Thursday, October 23, 2014 7:03 PM<br>
<b>To:</b> Robinson, Paul<br>
<b>Cc:</b> cfe-commits@cs.uiuc.edu<br>
<b>Subject:</b> Re: [PATCH][X86] __builtin_ctz/clz sometimed defined for zero input<o:p></o:p></span></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">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).<o:p></o:p></p>
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<p class="MsoNormal">-- Sean Silva<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Oct 23, 2014 at 4:40 PM, Robinson, Paul <<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank">Paul_Robinson@playstation.sony.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">In general, count-zeros instructions are undefined for a zero input value.<br>
However the X86 TZCNT and LZCNT instructions do return the bit-width on a<br>
zero input, so make Clang tell LLVM so.<br>
One quirk is that these instructions aren't necessarily both defined, so<br>
also create a separate predicate so we can do the right thing for all CPUs.<br>
--paulr<br>
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