<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 12 (filtered medium)">
<title>Message</title>
<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:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
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;
font-family:"Calibri","sans-serif";
color:windowtext;}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:"Calibri","sans-serif";
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page Section1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.Section1
{page:Section1;}
-->
</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=Section1>
<p class=MsoNormal><span style='color:#1F497D'>Hi Marc,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>I’m a bit confused. Isn’t
the standard compare (i.e. the one for a language like C) an ordered one? I
tried converting some C code to LLVM C++ API code with the online demo, and it uses
FCMP_OEQ…<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Cheers,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Nicolas<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<div>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>
<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"'>
llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu] <b>On Behalf
Of </b>Marc B. Reynolds<br>
<b>Sent:</b> Tuesday, 27 May, 2008 14:07<br>
<b>To:</b> 'LLVM Developers Mailing List'<br>
<b>Subject:</b> Re: [LLVMdev] Float compare-for-equality and select
optimizationopportunity<o:p></o:p></span></p>
</div>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<div>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:blue'>Both ZF and PF will be set if unordered, so the code below is IEEE
correct...you want to generate 'fcmp ueq' instead of 'fcmp oqe'</span><span
style='font-size:12.0pt;font-family:"Times New Roman","serif"'><o:p></o:p></span></p>
</div>
<blockquote style='margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt'>
<p class=MsoNormal>This is the resulting x86 assembly code:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span lang=NL-BE
style='font-size:8.0pt;font-family:"Courier New";color:gray'>ucomiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span lang=NL-BE
style='font-size:8.0pt;font-family:"Courier New";color:gray'>sete
al <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>setnp
dl <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>test
dl,al <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>mov
edx,edi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmovne
edx,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmovne
ecx,esi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmovne
esi,edi</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>While I’m pleasantly surprised that my branch does get
turned into several select operations as intended (cmov - conditional move
– in x86), I’m confused why it uses the ucomiss instruction
(unordered compare and set flags). I only used IRBuilder::CreateFCmpOEQ. It
also appears to invert the conditional, for no clear reason. I think it could
be rewritten as follows:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>comiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>mov
edx,edi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
edx,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
ecx,esi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
esi,edi</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Compared to the original C syntax code this looks pretty
straightforward. Curiously, when I replace the compare-for-equality with
something like a less-than, it does generate such compact code (using <span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>comiss</span> and <span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmova</span>). And
the not-equal case looks like this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>ucomiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>mov
esi,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
esi,edx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmovne
ecx,eax <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
edx,eax</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So this generates compact code but with an unordered
compare.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Anyway, it looks like the compare-for-equality case in
particular is missing an optimization opportunity. It’s no big deal to me
but I thought someone here might be interested…<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Cheers,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Nicolas Capens<o:p></o:p></p>
</blockquote>
</div>
</body>
</html>