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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="idm1"></a> |
| <div class="titlepage"></div> |
| <div xmlns="" class="refnamediv"> |
| <a xmlns="http://www.w3.org/1999/xhtml" id="Function Qualifiers"></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<span class="emphasis"><em>n</em></span>>))) |
| __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>))) |
| </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> |
| 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="constant">__attribute__((vec_type_hint(<type<span class="emphasis"><em>n</em></span>>)))</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. |
| <code class="constant">vec_type_hint (<type<span class="emphasis"><em>n</em></span>>)</code> |
| shall be one of the built-in scalar or vector data type described in |
| tables 6.1 and 6.2. |
| If |
| <code class="constant">vec_type_hint (<type<span class="emphasis"><em>n</em></span>>)</code> |
| is not specified, the default value is <span class="type">int</span>. |
| </p> |
| <p> |
| The |
| <code class="constant">__attribute__((vec_type_hint(int)))</code> |
| is the default type. |
| </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> is declared with |
| <code class="constant">__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 |
| <code class="constant">__attribute__(( vec_type_hint (double2))) __kernel</code>s |
| 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="constant">__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="constant">__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="constant">__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. The optional |
| <code class="constant">__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>, |
| if specified, must be (1, 1, 1) if the kernel is executed via |
| <a class="citerefentry" href="clEnqueueTask.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueTask</span></span></a>. |
| </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> |
| </div> |
| <div class="refsect1"> |
| <a id="notes"></a> |
| <h2>Notes</h2> |
| <p> |
| Implicit in autovectorization is the assumption that |
| any libraries called from the |
| __kernel 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> |
| </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="http://www.khronos.org/registry/cl/specs/opencl-1.0.pdf#page=154" target="OpenCL Spec">OpenCL Specification</a> |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="seealso"></a> |
| <h2>Also see</h2> |
| <p> |
| <a class="citerefentry" href="clEnqueueNDRangeKernel.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueNDRangeKernel</span></span></a> |
| <a class="citerefentry" href="clEnqueueTask.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueTask</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 © 2007-2009 The Khronos Group Inc. |
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