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| <title>Specifying Attribute For Unrolling Loops</title> |
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| <div class="refentry"> |
| <a id="id-1"></a> |
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| <div xmlns="" class="refnamediv"> |
| <a xmlns="http://www.w3.org/1999/xhtml" id="AttributesofBlocksandControl-Flow-Statements"></a> |
| <h1>Specifying Attribute For Unrolling Loops</h1> |
| <p> |
| Specifying Attribute For Unrolling Loops. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="synopsis"></a> |
| <h3> |
| </h3> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint)) |
| </td> |
| </tr> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint(n))) |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a id="description"></a> |
| <h2>Description</h2> |
| <p> |
| The <code class="code">__attribute__((opencl_unroll_hint))</code> |
| and <code class="code">__attribute__((opencl_unroll_hint(n)))</code> |
| attribute qualifiers can be used to specify that a loop |
| (<code class="code">for</code>, <code class="code">while</code> and <code class="code">do</code> loops) |
| can be unrolled. This attribute qualifier can be |
| used to specify full unrolling or partial unrolling |
| by a specified amount. This is a compiler hint |
| and the compiler may ignore this directive. |
| </p> |
| <p> |
| <code class="code">n</code> is the loop unrolling factor and must |
| be a positive integral compile time constant expression. |
| An unroll factor of 1 disables unrolling. If |
| <code class="code">n</code> is not specified, the compiler determines the |
| unrolling factor for the loop. |
| </p> |
| <p> |
| NOTE: The <code class="code">__attribute__((opencl_unroll_hint(n)))</code> |
| attribute qualifier must |
| appear immediately before the loop to be affected. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="example1"></a> |
| <h3> |
| Examples |
| </h3> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint(2))) |
| while (*s != 0) |
| *p++ = *s++; |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| This tells the compiler to unroll the above while loop by a factor of 2. |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint)) |
| for (int i=0; i<2; i++) |
| { |
| … |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| In the example above, the compiler will determine how much to unroll the loop. |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint(1))) |
| for (int i=0; i<32; i++) |
| { |
| … |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| The above is an example where the loop should not be unrolled. |
| </p> |
| <p> |
| Below are some examples of invalid usage of |
| <code class="code">__attribute__((opencl_unroll_hint(n)))</code>. |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint(-1))) |
| while (…) |
| { |
| … |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| The above example is an invalid usage of the loop unroll |
| factor as the loop unroll factor is negative. |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __attribute__((opencl_unroll_hint)) |
| if (…) |
| { |
| … |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| The above example is invalid because the unroll |
| attribute qualifier is used on a non-loop |
| construct. |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| kernel void |
| my_kernel( … ) |
| { |
| int x; |
| __attribute__((opencl_unroll_hint(x)) |
| for (int i=0; i<x; i++) |
| { |
| … |
| } |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| The above example is invalid because the loop unroll |
| factor is not a compile-time constant expression. |
| </p> |
| </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=61" 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="attributes-types.html"><span class="citerefentry"><span class="refentrytitle">Type Attributes</span></span></a>, |
| <a class="citerefentry" href="attributes-variables.html"><span class="citerefentry"><span class="refentrytitle">Variable Attributes</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 |
| copy of this software and/or associated documentation files (the |
| "Materials"), to deal in the Materials without restriction, including |
| without limitation the rights to use, copy, modify, merge, publish, |
| distribute, sublicense, and/or sell copies of the Materials, and to |
| permit persons to whom the Materials are furnished to do so, subject to |
| the condition that this copyright notice and permission notice shall be included |
| in all copies or substantial portions of the Materials. |
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