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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Yes, the second version of my patch provides new builtins that we can define however we please, and leaves the originals alone.<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>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></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""> cfe-commits-bounces@cs.uiuc.edu [mailto:cfe-commits-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Richard Smith<br>
<b>Sent:</b> Monday, October 27, 2014 5:20 PM<br>
<b>To:</b> Chandler Carruth<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"><o:p> </o:p></p>
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<p class="MsoNormal">On Sun, Oct 26, 2014 at 10:24 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com" target="_blank">chandlerc@google.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Sorry for the late replies, but as the last person to work on the CLZ / CTZ stuff, I have some small opinions here. =]<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Sun, Oct 26, 2014 at 9:52 PM, David Majnemer <<a href="mailto:david.majnemer@gmail.com" target="_blank">david.majnemer@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">I think Paul's patch is fine.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I firmly disagree.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">Having `__builtin_ctz` asking the target if `isCLZForZeroUndef` seems busted and this patch seems to fix that just fine.<o:p></o:p></p>
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Yes, that is busted and we should fix that aspect regardless. But changing the behavior on x86 seems actively harmful to portability.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">So, let's look at the history.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I went and taught LLVM to produce optimal code in cases where undef-at-zero was acceptable and only instructions with undef-at-zero were available. In order to generate reasonable code for these scenarios, Clang and LLVM had to respect
the contract specified for the GCC __builtin_clz and __builtin_ctz. That spec[1] *very* clearly states that if the input is zero the result is undefined.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">1: <a href="https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html" target="_blank">https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html</a><o:p></o:p></p>
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<p class="MsoNormal">However, after this Bob changed ARM to not follow this spec citing a radar only in r149086. I objected at the time, and continue to feel this is the *wrong* approach for ARM. We should not be deviating from the clear, documented, and widely
followed spec here. However, I had no real basis for arguing about the behavior of ARM at the time, and I doubt that I have much say in the behavior there even now. =] That said, on x86, I am firmly opposed to deviating from the clear and unambiguous spec.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">If folks want a builtin to access unambiguously defined-at-zero CLZ or CTZ behavior, we should define *new* builtins which have a clear spec here. However, I'm unpersuaded that this is better or more portable. The alternative code seems
quite palatable: (x == 0 ? 32 : __builtin_clz(x))<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">If LLVM fails to turn this into the maximally efficient code on some architecture, then much as others have suggested, we should enhance LLVM.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I agree with Chandler on all points here.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">__builtin_clz and __builtin_ctz is clearly and unambiguously defined by GCC's specification for them, and we should not be providing additional definedness guarantees. If we want a defined-for-zero-input form of these intrinsics, they
should have a different name (or an additional argument, or be written as "x == 0 ? 32 : __builtin_clz(x)" or similar).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">It is a major feature of Clang that we try not to lock people in by providing incompatible extensions.<o:p></o:p></p>
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