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| <title>Function Qualifiers</title> |
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| <meta name="keywords" content="Function Qualifiers" /> |
| </head> |
| <body> |
| <div class="refentry"> |
| <a id="id-1"></a> |
| <div class="titlepage"></div> |
| <div xmlns="" class="refnamediv"> |
| <a xmlns="http://www.w3.org/1999/xhtml" id="FunctionQualifiers"></a> |
| <h1>Function Qualifiers</h1> |
| <p> |
| Qualifiers for kernel functions. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="synopsis"></a> |
| <h3> |
| </h3> |
| <div class="informaltable"> |
| <table class="informaltable" border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __kernel |
| kernel |
| |
| __attribute__((vec_type_hint(<type>))) |
| __attribute__((work_group_size_hint(<span class="emphasis"><em>X</em></span>, <span class="emphasis"><em>Y</em></span>, <span class="emphasis"><em>Z</em></span>))) |
| __attribute__((reqd_work_group_size(<span class="emphasis"><em>X</em></span>, <span class="emphasis"><em>Y</em></span>, <span class="emphasis"><em>Z</em></span>))) |
| __attribute__((nosvm)) |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a id="description"></a> |
| <h2>Description</h2> |
| <p> |
| The <code class="function">__kernel</code> (or <code class="function">kernel</code>) qualifier declares a |
| function to be a kernel that can be executed by an application on an OpenCL device(s). |
| The following rules apply to functions that are declared with this qualifier: |
| </p> |
| <div class="itemizedlist"> |
| <ul class="itemizedlist" style="list-style-type: bullet; "> |
| <li class="listitem" style="list-style-type: disc"> |
| <p> |
| It can be executed on the device only |
| </p> |
| </li> |
| <li class="listitem" style="list-style-type: disc"> |
| <p> |
| It can be called by the host |
| </p> |
| </li> |
| <li class="listitem" style="list-style-type: disc"> |
| <p> |
| It is just a regular function call if a <code class="function">__kernel</code> function |
| is called by another kernel function. |
| </p> |
| </li> |
| </ul> |
| </div> |
| <p> |
| Kernel functions with variables declared inside the function with the |
| <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a> |
| or <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">local</span></span></a> |
| qualifier can be called by the host using appropriate APIs such as |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a>. |
| </p> |
| <p> |
| The <code class="function">__kernel</code> and <code class="function">kernel</code> names are reserved |
| for use as functions qualifiers and shall not be used otherwise. |
| </p> |
| <h4><a id="id-1.5.6"></a>Optional Attribute Qualifiers</h4> |
| <p> |
| The <code class="function">__kernel</code> qualifier can be used with the keyword <a class="citerefentry" href="attribute.html"><span class="citerefentry"><span class="refentrytitle">__attribute__</span></span></a> to |
| declare additional information about the kernel function as described below. |
| </p> |
| <p> |
| The optional <code class="code">__attribute__((vec_type_hint(<type>)))</code> |
| is a hint to the compiler and is intended to be a representation of the computational |
| <span class="emphasis"><em>width</em></span> of the <code class="function">__kernel</code>, and should serve as the |
| basis for calculating processor bandwidth utilization when the compiler is looking to |
| autovectorize the code. In the <code class="code">__attribute__((vec_type_hint(<type>)))</code> |
| qualifier <type> is one of the built-in vector types or the constituent scalar |
| element types. If <code class="code">vec_type_hint (<type>)</code> is not specified, the |
| kernel is assumed to have the <code class="code">__attribute__((vec_type_hint(int)))</code> qualifier. |
| </p> |
| <p> |
| Implicit in autovectorization is the assumption that any libraries called from |
| the <code class="function">__kernel</code> must be recompilable at run time to handle cases |
| where the compiler decides to merge or separate workitems. This probably means that |
| such libraries can never be hard coded binaries or that hard coded binaries must be |
| accompanied either by source or some retargetable intermediate representation. This |
| may be a code security question for some. |
| </p> |
| <p> |
| For example, where the developer specified a width of <span class="type">float4</span>, the compiler |
| should assume that the computation usually uses up 4 lanes of a float vector, and would |
| decide to merge work-items or possibly even separate one work-item into many threads |
| to better match the hardware capabilities. A conforming implementation is not required |
| to autovectorize code, but shall support the hint. A compiler may autovectorize, even |
| if no hint is provided. If an implementation merges <code class="constant">N</code> work-items |
| into one thread, it is responsible for correctly handling cases where the number of |
| global or local work-items in any dimension modulo <code class="constant">N</code> is not zero. |
| </p> |
| <p> |
| If for example, a <code class="function">__kernel</code> function is declared with |
| <code class="code">__attribute__(( vec_type_hint (float4)))</code> (meaning that most |
| operations in the <code class="function">__kernel</code> are explicitly vectorized using |
| <span class="type">float4</span>) and the kernel is running using Intel® Advanced Vector |
| Instructions (Intel® AVX) which implements a 8-float-wide vector unit, the |
| autovectorizer might choose to merge two work-items to one thread, running a second |
| work-item in the high half of the 256-bit AVX register. |
| </p> |
| <p> |
| As another example, a Power4 machine has two scalar double precision floating-point |
| units with an 6-cycle deep pipe. An autovectorizer for the Power4 machine might choose |
| to interleave six kernels declared with the <code class="code">__attribute__(( vec_type_hint |
| (double2)))</code> qualifier into one hardware thread, to ensure that there is |
| always 12-way parallelism available to saturate the FPUs. It might also choose to |
| merge 4 or 8 work-items (or some other number) if it concludes that these are better |
| choices, due to resource utilization concerns or some preference for divisibility by 2. |
| </p> |
| <p> |
| The optional <code class="code">__attribute__((work_group_size_hint(<span class="emphasis"><em>X</em></span>, |
| <span class="emphasis"><em>Y</em></span>, <span class="emphasis"><em>Z</em></span>)))</code> is a hint to the |
| compiler and is intended to specify the work-group size that may be used i.e. value |
| most likely to be specified by the <code class="varname">local_work_size</code> argument to |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a>. |
| For example the <code class="code">__attribute__((work_group_size_hint(1, 1, 1)))</code> is |
| a hint to the compiler that the kernel will most likely be executed with a work-group |
| size of 1. |
| </p> |
| <p> |
| The optional <code class="code">__attribute__((reqd_work_group_size(<span class="emphasis"><em>X</em></span>, |
| <span class="emphasis"><em>Y</em></span>, <span class="emphasis"><em>Z</em></span>)))</code> is the work-group |
| size that must be used as the <code class="varname">local_work_size</code> argument to |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a>. This |
| allows the compiler to optimize the generated code appropriately for this kernel. |
| </p> |
| <p> |
| If <code class="varname">Z</code> is one, the <code class="varname">work_dim</code> argument to |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a> |
| can be 2 or 3. If <code class="varname">Y</code> and <code class="varname">Z</code> |
| are one, the <code class="varname">work_dim</code> argument to |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a> |
| can be 1, 2 or 3. |
| </p> |
| <p> |
| The optional <code class="code">__attribute__((nosvm))</code> |
| qualifier can be used with a pointer variable to |
| inform the compiler that the pointer does not |
| refer to a shared virtual memory region. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="example1"></a> |
| <h3> |
| Example |
| </h3> |
| <div class="informaltable"> |
| <table class="informaltable" border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| // autovectorize assuming float4 as the |
| // basic computation width |
| __kernel __attribute__((vec_type_hint(float4))) |
| void foo( __global float4 *p ) { .... } |
| |
| // autovectorize assuming double as the |
| // basic computation width |
| __kernel __attribute__((vec_type_hint(double))) |
| void foo( __global float4 *p ){ .... } |
| |
| // autovectorize assuming int (default) |
| // as the basic computation width |
| __kernel |
| void foo( __global float4 *p ){ .... } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a id="specification"></a> |
| <h2>Specification</h2> |
| <p> |
| <img src="pdficon_small1.gif" /> |
| |
| <a href="https://www.khronos.org/registry/cl/specs/opencl-2.1-openclc.pdf#page=47" target="OpenCL Spec">OpenCL Specification</a> |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="seealso"></a> |
| <h2>Also see</h2> |
| <p> |
| <a class="citerefentry" href="attribute.html"><span class="citerefentry"><span class="refentrytitle">__attribute__</span></span></a>, |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a>, |
| <a class="citerefentry" href="qualifiers.html"><span class="citerefentry"><span class="refentrytitle">qualifiers</span></span></a> |
| </p> |
| </div> |
| <div xmlns="" class="refsect3" lang="en" xml:lang="en"><a xmlns="http://www.w3.org/1999/xhtml" id="Copyright"></a><h4 xmlns="http://www.w3.org/1999/xhtml"></h4><img xmlns="http://www.w3.org/1999/xhtml" src="KhronosLogo.jpg" /><p xmlns="http://www.w3.org/1999/xhtml"></p>Copyright © 2007-2015 The Khronos Group Inc. |
| Permission is hereby granted, free of charge, to any person obtaining a |
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