<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Verdana;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Verdana",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="#0563C1" vlink="#954F72">
<div class="WordSection1">
<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:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"> // information.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"> bool IsCmpZero = RHS.isPosZero();<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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.<o:p></o:p></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)<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"> return nullptr;<o:p></o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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=">
http://llvm.org/docs/LangRef.html</a> describes the difference between “ordered” and “unordered” as pertaining to QNAN operands,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">with no mention of negative zero.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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(<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">; CHECK: uitofp<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">; CHECK: fcmp oeq<o:p></o:p></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) {<o:p></o:p></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<o:p></o:p></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<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"> ret i1 %cmp<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></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?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Verdana",sans-serif">- Arch D. Robison<o:p></o:p></span></p>
</div>
</body>
</html>