<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=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<!--[if !mso]><style>v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
</style><![endif]--><style><!--
/* Font Definitions */
@font-face
{font-family:Helvetica;
panose-1:2 11 6 4 2 2 2 2 2 4;}
@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;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",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;}
p
{mso-style-priority:99;
margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
p.MsoListParagraph, li.MsoListParagraph, div.MsoListParagraph
{mso-style-priority:34;
margin-top:0cm;
margin-right:0cm;
margin-bottom:0cm;
margin-left:36.0pt;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman",serif;}
span.EmailStyle18
{mso-style-type:personal;
font-family:"Calibri",sans-serif;
color:#1F497D;}
span.EmailStyle19
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 90.0pt 72.0pt 90.0pt;}
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:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">>
</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">The vectorization cost model is, generally speaking, concerned with instruction throughputs, and is used to estimate the relative speed of a vectorized loop vs. the scalar one
(it assumes that a proper amount of ILP is available, or created by interleaving, such that the throughputs matter much more than the latencies).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Note that the loop vectorizer also acts as an unroller, and considers other costs of register pressure and expanding reductions in doing so.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">The SLP vectorizer should also consider latency costs in latency-bound situations, rather considering only throughput costs, as observed in PR25108.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Ayal.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></a></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><a name="_____replyseparator"></a><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Hal Finkel via llvm-dev<br>
<b>Sent:</b> Tuesday, January 05, 2016 23:12<br>
<b>To:</b> Matthew Arsenault <Matthew.Arsenault@amd.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] TargetTransformInfo getOperationCost uses<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Hi Matt,<br>
<br>
So the problem here is that, as you imply, TTI has two cost models: One is used for vectorization (and has received a fair amount of attention), and one is used for inlining/unrolling, and has received less attention.<br>
<br>
The vectorization cost model is, generally speaking, concerned with instruction throughputs, and is used to estimate the relative speed of a vectorized loop vs. the scalar one (it assumes that a proper amount of ILP is available, or created by interleaving,
such that the throughputs matter much more than the latencies).<br>
<br>
The "user" code model, used by the inliner and unroller, is concerned with estimating something more-closely related to code size (although there is obviously a desired correlation with performance). It has primarily been customized to let the inliner/unroller,
on a target specific basis, understand that certain zext/sexts are free, etc.<br>
<br>
As you also imply. there has been a lot more specific target customization work on the vectorization cost model. To be honest, I don't think this situation is ideal (we could have one cost model that returns information on size, latency and throughput that
is used by all clients). Nevertheless, hopefully this makes things somewhat clearer.<br>
<br>
-Hal<o:p></o:p></span></p>
<div class="MsoNormal" align="center" style="text-align:center"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">
<hr size="2" width="100%" align="center" id="zwchr">
</span></div>
<blockquote style="border:none;border-left:solid #1010FF 1.5pt;padding:0cm 0cm 0cm 4.0pt;margin-left:3.75pt;margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-family:"Helvetica",sans-serif;color:black">From:
</span></b><span style="font-family:"Helvetica",sans-serif;color:black">"Matthew via llvm-dev Arsenault" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>To: </b>"llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>Sent: </b>Tuesday, January 5, 2016 10:25:46 AM<br>
<b>Subject: </b>[llvm-dev] TargetTransformInfo getOperationCost uses<o:p></o:p></span></p>
<div id="divtagdefaultwrapper">
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">Hi,<o:p></o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">I'm trying to implement the TTI hooks for AMDGPU to avoid unrolling loops for operations with huge expansions (i.e. integer division).<o:p></o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">The values that are ultimately reported by opt -cost-model -analyze (the actual cost model tests) seem to not matter for this. The huge cost I've assigned division doesn't
prevent the loop from being unrolled, because it isn't actually considered when loop unrolling.<o:p></o:p></span></p>
<div>
<p class="MsoNormal" style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
</div>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">The loop unrolled uses CodeMetrics, which via getUserCost ultimately uses TargetTransformInfoImplBase::getOperationCost(), which returns various fixed values (4 for division
(TCC_Expensive, but this isn't nearly expensive enough)).<o:p></o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">getOperationCost only uses the type and opcode to estimate, so it doesn't require a value. No target overrides this. The hooks that targets do really implement, like getArithmeticInstrCost,
use some information about the operands so require a value. These don't appear to be used at all by the cost model, and instead in specific places in some passes.<o:p></o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">Why don't any targets override getOperationCost or why aren't there any other hooks that change its behavior? Having these two parallel path for operation costs is confusing,
especially since I expected to be able to use the -cost-model output for testing all of the computed costs.<o:p></o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black"><o:p> </o:p></span></p>
<p style="background:white"><span style="font-family:"Calibri",sans-serif;color:black">-Matt<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-family:"Helvetica",sans-serif;color:black"><br>
_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></span></p>
</blockquote>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black"><br>
<br>
<br>
-- <o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Hal Finkel<br>
Assistant Computational Scientist<br>
Leadership Computing Facility<br>
Argonne National Laboratory<o:p></o:p></span></p>
</div>
</div>
</div>
<p>---------------------------------------------------------------------<br>
Intel Israel (74) Limited</p>
<p>This e-mail and any attachments may contain confidential material for<br>
the sole use of the intended recipient(s). Any review or distribution<br>
by others is strictly prohibited. If you are not the intended<br>
recipient, please contact the sender and delete all copies.</p></body>
</html>