<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 14 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Wingdings;
panose-1:5 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:Wingdings;
panose-1:5 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 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:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p.MsoListParagraph, li.MsoListParagraph, div.MsoListParagraph
{mso-style-priority:34;
margin-top:0in;
margin-right:0in;
margin-bottom:0in;
margin-left:.5in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri","sans-serif";}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri","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;}
/* List Definitions */
@list l0
{mso-list-id:1014458703;
mso-list-type:hybrid;
mso-list-template-ids:1940174660 136322754 67698691 67698693 67698689 67698691 67698693 67698689 67698691 67698693;}
@list l0:level1
{mso-level-start-at:2;
mso-level-number-format:bullet;
mso-level-text:\F06E;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:.75in;
text-indent:-.25in;
font-family:Wingdings;
mso-fareast-font-family:Calibri;
mso-bidi-font-family:"Times New Roman";}
@list l0:level2
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:1.25in;
text-indent:-.25in;
font-family:"Courier New";}
@list l0:level3
{mso-level-number-format:bullet;
mso-level-text:\F0A7;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:1.75in;
text-indent:-.25in;
font-family:Wingdings;}
@list l0:level4
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:2.25in;
text-indent:-.25in;
font-family:Symbol;}
@list l0:level5
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:2.75in;
text-indent:-.25in;
font-family:"Courier New";}
@list l0:level6
{mso-level-number-format:bullet;
mso-level-text:\F0A7;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:3.25in;
text-indent:-.25in;
font-family:Wingdings;}
@list l0:level7
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:3.75in;
text-indent:-.25in;
font-family:Symbol;}
@list l0:level8
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:4.25in;
text-indent:-.25in;
font-family:"Courier New";}
@list l0:level9
{mso-level-number-format:bullet;
mso-level-text:\F0A7;
mso-level-tab-stop:none;
mso-level-number-position:left;
margin-left:4.75in;
text-indent:-.25in;
font-family:Wingdings;}
ol
{margin-bottom:0in;}
ul
{margin-bottom:0in;}
--></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="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal">Consider the following C code. When it is compiled using clang with ‘ –O’, the assert fires. Gcc seems to do the opposite.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas">#include <assert.h>
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas">int main() <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas">{
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas"> unsigned a; // uninitialized!
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas"> unsigned b = a; <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas"> assert(b == a); // shouldn’t this be always true? <o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas"> return 0;
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.5in"><span style="font-family:Consolas">}
<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Would you consider this a LLVM bug? I think GVN is responsible for this. I don’t know if it falls within the realm of “undefined” or “implementation specific” behavior but there are probably examples of code where one can “correctly” skip
initialization. The Briggs-Torczon sparse set data structure comes to mind (where you are not comparing two uninitialized variables but an uninitialized variable with an initialized variable—LLVM seems to do the right thing in that case, though).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="margin-left:.75in;text-indent:-.25in;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="font-family:Wingdings"><span style="mso-list:Ignore">n<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>Riyaz<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</body>
</html>