<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:Calibri;
panose-1:2 15 5 2 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;}
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;}
--></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">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am using the LLVM JIT infrastructure (MCJIT). I wanted to see if there are any performance gains as the compiler can detect the target CPU at runtime. But, I didn’t see any improvement (I compile with –no-mmx and –no-sse).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I then tried an experiment, where I compiled the program with clang-3.3, with and without specifying the target cpu as “corei7”. I was shocked to see that the only difference in the two binaries were related to “Instruction Set Extensions”.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Further I tried the same experiment with gcc, and saw that the instructions were shuffled around in the binary. I expected this, because every CPU differs in some way or the other (has different buffer size for out-of-order execution, different
cache sizes, etc.).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For clang, I was passing the “-march=corei7” flag.<o:p></o:p></p>
<p class="MsoNormal">For gcc, I was passing the “-mcpu=corei7” flag.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Am I passing the correct flags?<o:p></o:p></p>
<p class="MsoNormal">Any help, comments or suggestions, would be helpful.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">Varun Agrawal<o:p></o:p></p>
<p class="MsoNormal">PhD Student<o:p></o:p></p>
<p class="MsoNormal">Computer Science, Stony Brook University<o:p></o:p></p>
<p class="MsoNormal"><a href="http://compas.cs.stonybrook.edu/~vagrawal/">http://compas.cs.stonybrook.edu/~vagrawal/</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</body>
</html>