<div dir="ltr">It seems like -0 should be fine.<div><br></div><div><br></div><div>-- Sean Silva</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Jul 6, 2015 at 2:13 PM, Robison, Arch <span dir="ltr"><<a href="mailto:arch.robison@intel.com" target="_blank">arch.robison@intel.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">





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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">In InstCombineCompares.cpp, routine InstCombiner::FoldFCmp_IntToFP_Cst, there are these lines:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  // Comparisons with zero are a special case where we know we won't lose<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  // information.<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  bool IsCmpZero = RHS.isPosZero();<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  // If the conversion would lose info, don't hack on this.<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  if ((int)InputSize > MantissaWidth && !IsCmpZero)<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">    return nullptr;<u></u><u></u></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></b></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">Why check for positive zero instead of checking for any kind of zero?  My reading of IEEE 754-2008 is that floating-point comparison operations cannot distinguish a negative
 zero from a positive zero.  Further supporting this is that fact that <a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_LangRef.html&d=AwMFAg&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=ZaWNNVon_US-YjLoXztqD5ve4xvIIhIXYdnk3Xqwz8w&s=xonYObEo1TXUyjK3RYC4Pg3kAaSvaYy_Lnvaiq5G1N0&e=" target="_blank">
http://llvm.org/docs/LangRef.html</a> describes the difference between “ordered” and “unordered” as pertaining to QNAN operands,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">with no mention of negative zero.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">I tried fixing the issue, but then the following test in cast-int-fcmp-eq-0.ll fails:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">; CHECK-LABEL: @i32_cast_cmp_oeq_int_n0_uitofp(<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">; CHECK: uitofp<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">; CHECK: fcmp oeq<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">define i1 @i32_cast_cmp_oeq_int_n0_uitofp(i32 %i) {<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  %f = uitofp i32 %i to float<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  %cmp = fcmp oeq float %f, -0.0<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">  ret i1 %cmp<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">Is this test really justified, or is it just reinforcing an oversight?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">- Arch D. Robison<u></u><u></u></span></p>
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