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<body class="article">
<div id="header">
<h1>cl_intel_required_subgroup_size</h1>
<div id="toc">
  <div id="toctitle">Table of Contents</div>
  <noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_name_strings">Name Strings</h2>
<div class="sectionbody">
<div class="paragraph"><p><span class="monospaced">cl_intel_required_subgroup_size</span></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_contact">Contact</h2>
<div class="sectionbody">
<div class="paragraph"><p>Ben Ashbaugh, Intel (ben <em>dot</em> ashbaugh <em>at</em> intel <em>dot</em> com)</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_contributors">Contributors</h2>
<div class="sectionbody">
<div class="paragraph"><p>Ben Ashbaugh, Intel</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_notice">Notice</h2>
<div class="sectionbody">
<div class="paragraph"><p>Copyright (c) 2018 Intel Corporation.  All rights reserved.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_status">Status</h2>
<div class="sectionbody">
<div class="paragraph"><p>Final Draft</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_version">Version</h2>
<div class="sectionbody">
<div class="paragraph"><p>Built On: 2018-11-16<br>
Revision: 2</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_dependencies">Dependencies</h2>
<div class="sectionbody">
<div class="paragraph"><p>Support for OpenCL 2.1, <span class="monospaced">cl_khr_subgroups, or `cl_intel_subgroups</span> is required.
This extension is written against revision 23 of the OpenCL 2.1 API specification, against revision 30 of the OpenCL 2.0 OpenCL C specification, against version 31 of the OpenCL 2.0 Extensions specification, and against version 3 of the <span class="monospaced">cl_intel_subgroups</span> specification.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_overview">Overview</h2>
<div class="sectionbody">
<div class="paragraph"><p>The goal of this extension is to allow programmers to optionally specify the required subgroup size for a kernel function.
This information is important for the correctness of many subgroup algorithms, and in some cases may be used by the compiler to generate more optimal code.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_new_api_functions">New API Functions</h2>
<div class="sectionbody">
<div class="paragraph"><p>None.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_new_api_enums">New API Enums</h2>
<div class="sectionbody">
<div class="paragraph"><p>Accepted as the <em>param_name</em> parameter of <strong>clGetDeviceInfo</strong>:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>CL_DEVICE_SUB_GROUP_SIZES_INTEL                 <span style="color: #993399">0x4108</span></tt></pre></div></div>
<div class="paragraph"><p>Accepted as the <em>param_name</em> parameter of <strong>clGetKernelWorkGroupInfo</strong>:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>CL_KERNEL_SPILL_MEM_SIZE_INTEL                  <span style="color: #993399">0x4109</span></tt></pre></div></div>
<div class="paragraph"><p>Accepted as the <em>param_name</em> parameter of <strong>clGetKernelSubGroupInfo</strong> and/or
<strong>clGetKernelSubGroupInfoKHR</strong>:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>CL_KERNEL_COMPILE_SUB_GROUP_SIZE_INTEL          <span style="color: #993399">0x410A</span></tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_new_opencl_c_optional_attribute_qualifiers">New OpenCL C Optional Attribute Qualifiers</h2>
<div class="sectionbody">
<div class="paragraph"><p>Optional __kernel qualifier:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #000000">__attribute__</span></span><span style="color: #990000">((</span><span style="font-weight: bold"><span style="color: #000000">intel_reqd_sub_group_size</span></span><span style="color: #990000">(&lt;</span><span style="color: #009900">int</span><span style="color: #990000">&gt;)))</span></tt></pre></div></div>
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<div class="sect1">
<h2 id="_modifications_to_the_opencl_api_specification">Modifications to the OpenCL API Specification</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_additions_to_table_4_3_opencl_device_queries">Additions to Table 4.3 - "OpenCL Device Queries"</h3>
<table class="tableblock frame-all grid-all"
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<col style="width:30%;">
<col style="width:20%;">
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<th class="tableblock halign-left valign-top" > <strong>cl_device_info</strong> </th>
<th class="tableblock halign-left valign-top" > Return Type </th>
<th class="tableblock halign-left valign-top" > Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><strong>CL_DEVICE_SUB_GROUP_SIZES_INTEL</strong></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">size_t[]</span></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Returns the set of subgroup sizes supported by the device.</p></td>
</tr>
</tbody>
</table>
</div>
<div class="sect2">
<h3 id="_additions_to_table_5_21_clgetkernelworkgroupinfo_parameter_queries">Additions to Table 5.21 - "clGetKernelWorkGroupInfo parameter queries":</h3>
<table class="tableblock frame-all grid-all"
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<col style="width:34%;">
<col style="width:33%;">
<col style="width:33%;">
<thead>
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<th class="tableblock halign-left valign-top" > <strong>cl_kernel_work_group_info</strong> </th>
<th class="tableblock halign-left valign-top" > Return Type </th>
<th class="tableblock halign-left valign-top" > Info. returned in <em>param_value</em></th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><strong>CL_KERNEL_SPILL_MEM_SIZE_INTEL</strong></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">cl_ulong</span></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Returns the amount of spill memory used by a kernel.
The meaning of this value will vary from implementation-to-implementation, however a return value of 0 will always indicate that compiler was able to compile the kernel to fit into the device&#8217;s register file without spilling registers to memory.</p></td>
</tr>
</tbody>
</table>
</div>
<div class="sect2">
<h3 id="_additions_to_clgetkernelsubgroupinfo_parameter_queries">Additions to "clGetKernelSubGroupInfo parameter queries":</h3>
<div class="paragraph"><p>This is Table 5.22 - "<strong>clGetKernelSubGroupInfo</strong> parameter queries" in the OpenCL 2.1 API spec, in Section 9.17.2.1 for <strong>clGetKernelSubGroupInfoKHR</strong> in the OpenCL 2.0 Extensions spec, and in the section describing the changes to Section 5.9.3 for <strong>clGetKernelSubGroupInfoKHR</strong> in the <span class="monospaced">cl_intel_subgroups</span> spec:</p></div>
<table class="tableblock frame-all grid-all"
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">
<col style="width:25%;">
<col style="width:25%;">
<col style="width:25%;">
<col style="width:25%;">
<thead>
<tr>
<th class="tableblock halign-left valign-top" > <strong>cl_kernel_sub_group_info</strong> </th>
<th class="tableblock halign-left valign-top" > Input Type </th>
<th class="tableblock halign-left valign-top" > Return Type </th>
<th class="tableblock halign-left valign-top" > Info. returned in <em>param_value</em></th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><strong>CL_KERNEL_COMPILE_<br>
SUB_GROUP_SIZE_INTEL</strong></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">ignored</span></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><span class="monospaced">size_t</span></p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Returns the subgroup size specified by the <span class="monospaced">__attribute__(( intel_reqd_sub_group_size(&lt;int&gt;) ))</span> qualifier.
Refer to section 6.7.2.</p>
<p class="tableblock">If the subgroup size is not specified using the above attribute qualifier then 0 is returned.</p></td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_modifications_to_the_opencl_c_specification">Modifications to the OpenCL C Specification</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_additions_to_section_6_7_2_optional_attribute_qualifiers">Additions to Section 6.7.2 - "Optional Attribute Qualifiers"</h3>
<div class="paragraph"><p>The optional <span class="monospaced">__attribute__((intel_reqd_sub_group_size(&lt;int&gt;)))</span> can be used to indicate that the kernel must be compiled and executed with the specified subgroup size.
When this attribute is present, <strong>get_max_sub_group_size</strong>() is guaranteed to return the specified integer value.
This is important for the correctness of many subgroup algorithms, and in some cases may be used by the compiler to generate more optimal code.</p></div>
<div class="paragraph"><p>Note that there is no guarantee for the value of <strong>get_sub_group_size</strong>() even when this attribute is present, particularly when the work-group size is not evenly divisible by the required subgroup size.</p></div>
<div class="paragraph"><p>Note as well that some devices may support a limited number of subgroup sizes, and that some devices may not support all language constructs with all subgroup sizes.
This means that some kernels may fail compilation with one required subgroup size and succeed with another required subgroup size, even if both subgroup sizes are supported by the device.</p></div>
<div class="paragraph"><p>Finally, note that requiring one subgroup size (particularly, a larger subgroup size) may require more spill memory than another subgroup size, and may negatively impact application performance."</p></div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_issues">Issues</h2>
<div class="sectionbody">
<div class="paragraph"><p>None.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_revision_history">Revision History</h2>
<div class="sectionbody">
<table class="tableblock frame-all grid-rows"
style="
width:100%;
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<col style="width:4%;">
<col style="width:14%;">
<col style="width:14%;">
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<th class="tableblock halign-left valign-top" >Rev</th>
<th class="tableblock halign-left valign-top" >Date</th>
<th class="tableblock halign-left valign-top" >Author</th>
<th class="tableblock halign-left valign-top" >Changes</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top" ><p class="tableblock">1</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">2016-07-14</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Ben Ashbaugh</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock"><strong>First public revision.</strong></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top" ><p class="tableblock">2</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">2018-11-15</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Ben Ashbaugh</p></td>
<td class="tableblock halign-left valign-top" ><p class="tableblock">Conversion to asciidoc.</p></td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
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Last updated
 2018-11-16 10:11:35 PST
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