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<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Intel documents _lzcnt_u32/64 as returning operand size for 0 input. Ditto tzcnt. Here's the lzcnt reference:</span><o:p></o:p></p>
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<a href="https://software.intel.com/sites/products/documentation/doclib/iss/2013/compiler/cpp-lin/GUID-67C6440B-D49E-4D7C-98A2-667E47ED63CC.htm" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">https://software.intel.com/sites/products/documentation/doclib/iss/2013/compiler/cpp-lin/GUID-67C6440B-D49E-4D7C-98A2-667E47ED63CC.htm</span></a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in">These seem reasonable to support when compiling and targeting a microarch with support for the instructions.<o:p></o:p></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">The Intel documentation does not suggest any such restriction. Currently the only Clang support is in Intrin.h, which we can live with.<o:p></o:p></span></p>
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<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Microsoft documentation claims that __lzcnt* returns the operand size for 0 input, while noting "If you run code that uses this intrinsic on hardware that does not support the lzcnt
instruction, the results are unpredictable."</span><o:p></o:p></p>
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<a href="http://msdn.microsoft.com/en-us/library/vstudio/bb384809(v=vs.100).aspx" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">http://msdn.microsoft.com/en-us/library/vstudio/bb384809(v=vs.100).aspx</span></a><o:p></o:p></p>
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While if we absolutely must due to compatibility, I would push hard to not support these functions unless targeting such hardware.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in">I find this particularly important because in the grander scheme of things *very little* x86 hardware in the world has lzcnt, and even less tzcnt. =[ Weak APIs like this seem really problematic there.<o:p></o:p></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">Hardware that Sony cares about does support these instructions, so, yeah, we want this. The existing lzcntintrin.h requires __LZCNT__ defined, which subsets
this into the hardware we care about, so…<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""> Chandler Carruth [mailto:chandlerc@google.com]
<br>
<b>Sent:</b> Monday, October 27, 2014 9:38 PM<br>
<b>To:</b> Robinson, Paul<br>
<b>Cc:</b> Richard Smith; Sean Silva; 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">On Mon, Oct 27, 2014 at 9:16 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>
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I can't find any documentation *at all* for lzcntintrin.h, so I can find no suggestion that its __lzcnt* functions would have defined behavior on an input of 0.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a name="14954fb3d0a35a65__MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> </span></a><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I think it's not the exact header so much as the functions themselves that we should care about.</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Intel documents _lzcnt_u32/64 as returning operand size for 0 input. Ditto tzcnt. Here's the lzcnt
reference:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a href="https://software.intel.com/sites/products/documentation/doclib/iss/2013/compiler/cpp-lin/GUID-67C6440B-D49E-4D7C-98A2-667E47ED63CC.htm" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">https://software.intel.com/sites/products/documentation/doclib/iss/2013/compiler/cpp-lin/GUID-67C6440B-D49E-4D7C-98A2-667E47ED63CC.htm</span></a><o:p></o:p></p>
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<p class="MsoNormal">These seem reasonable to support when compiling and targeting a microarch with support for the instructions.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Microsoft documentation claims that __lzcnt* returns the operand size for 0 input, while noting "If
you run code that uses this intrinsic on hardware that does not support the lzcnt instruction, the results are unpredictable."</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a href="http://msdn.microsoft.com/en-us/library/vstudio/bb384809(v=vs.100).aspx" target="_blank"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">http://msdn.microsoft.com/en-us/library/vstudio/bb384809(v=vs.100).aspx</span></a><o:p></o:p></p>
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While if we absolutely must due to compatibility, I would push hard to not support these functions unless targeting such hardware.<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 find this particularly important because in the grander scheme of things *very little* x86 hardware in the world has lzcnt, and even less tzcnt. =[ Weak APIs like this seem really problematic there.<o:p></o:p></p>
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