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| <title>__constant</title> |
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| <meta name="keywords" content="__constant" /> |
| </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="__constant"></a> |
| <h1>__constant</h1> |
| <p> |
| Address Space Qualifier. |
| </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"> |
| __constant |
| constant |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a id="description"></a> |
| <h2>Description</h2> |
| <p> |
| The <code class="function">__constant</code> or <code class="function">constant</code> address space |
| name is used to describe variables allocated in global memory and which are accessed |
| inside a kernel(s) as read-only variables. These read-only variables can be accessed |
| by all (global) work-items of the kernel during its execution. Pointers to the |
| <code class="function">__constant</code> address space are allowed as arguments to functions |
| (including kernel functions) and for variables declared inside functions. |
| </p> |
| <p> |
| All string literal storage shall be in the <code class="function">__constant</code> address space. |
| </p> |
| <p> |
| NOTE: Each argument to a kernel that is a pointer to the <code class="function">__constant</code> |
| address space is counted separately towards the maximum number of such arguments, |
| defined as <code class="constant">CL_DEVICE_MAX_CONSTANT_ARGS</code> are described in the |
| table for <a class="citerefentry" href="clGetDeviceInfo.html"><span class="citerefentry"><span class="refentrytitle">clGetDeviceInfo</span></span></a>. |
| </p> |
| <p> |
| Variables in the program scope must be declared in the |
| <code class="function">__constant</code> address space. Variables |
| in the outermost scope of kernel functions can be |
| declared in the <code class="function">__constant</code> address space. |
| These variables are required to be initialized and the |
| values used to initialize these variables must |
| be a compile time constant. Writing to such a variable |
| results in a compile-time error. |
| </p> |
| <p> |
| Implementations are not required to aggregate these declarations into the fewest |
| number of constant arguments. This behavior is implementation defined. |
| </p> |
| <p> |
| Thus portable code must conservatively assume that each variable declared inside |
| a function or in program scope allocated in the <code class="function">__constant</code> |
| address space counts as a separate constant argument. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="notes"></a> |
| <h2>Notes</h2> |
| <h4><a id="id-1.6.2"></a>General information about address space qualifiers</h4> |
| <p> |
| OpenCL implements the following disjoint named address |
| spaces: <code class="function">__global</code>, |
| <code class="function">__local</code>, <code class="function">__constant</code>, and |
| <code class="function">__private</code>. The address space qualifier may be used in variable |
| declarations to specify the region of memory that is used to allocate the object. The |
| C syntax for type qualifiers is extended in OpenCL |
| to include an address space name as a valid type qualifier. |
| If the type of an object is qualified by an address |
| space name, the object is allocated in the |
| specified address name; otherwise, the object is |
| allocated in the generic address space. |
| </p> |
| <p> |
| The address space names without the __ prefix i.e. <code class="function">global</code>, |
| <code class="function">local</code>, <code class="function">constant</code> |
| and <code class="function">private</code> |
| may be substituted for the corresponding address |
| space names with the __ prefix. |
| </p> |
| <p> |
| The address space name for arguments to a function |
| in a program, or local variables of a function |
| is <code class="function">__private</code>. All function |
| arguments shall be in the <code class="function">__private</code> address space. |
| The address space for a variable at program scope |
| or a static variable inside a function can either |
| be <code class="function">__global</code> or |
| <code class="function">__constant</code>, but defaults to |
| <code class="function">__global</code> if not specified. |
| </p> |
| <p> |
| OpenCL 2.0 adds support for an unnamed <code class="function">generic</code> |
| address space. Pointers that are declared |
| without pointing to a named address space point to the |
| <code class="function">generic</code> address space. Before |
| referring to the region pointed to, the pointer |
| must be associated with a named address space. |
| Functions may be written with arguments and return values that point to the |
| <code class="function">generic</code> address space. |
| </p> |
| <p> |
| kernel function arguments declared to be a pointer or |
| an array of a type must point to one of |
| the named address spaces <code class="function">__global</code>, |
| <code class="function">__local</code> or <code class="function">__constant</code>. |
| </p> |
| <p> |
| The named address spaces are a subset of the generic |
| address space except for the <code class="function">constant</code> |
| address space. |
| </p> |
| <p> |
| A pointer to address space A can only be assigned to |
| a pointer to the same address space A or a |
| pointer to the <code class="function">generic</code> address |
| space. Casting a pointer to address space A to a pointer to |
| address space B is illegal if A and B are named |
| address spaces and A is not the same as B. |
| </p> |
| <p> |
| The <code class="function">__global</code>, <code class="function">__constant</code>, |
| <code class="function">__local</code>, |
| <code class="function">__private</code>, <code class="function">__generic</code>, |
| <code class="function">global</code>, |
| <code class="function">constant</code>, <code class="function">local</code>, and |
| <code class="function">private</code> names are reserved for use as address space |
| qualifiers and shall not be used otherwise. |
| The <code class="function">__generic</code> and <code class="function">generic</code> |
| names are reserved for future use. |
| </p> |
| <p> |
| The size of pointers to different address spaces |
| may differ. It is not correct to assume |
| that, for example, <code class="code">sizeof(__global int *)</code> |
| always equals <code class="code">sizeof(__local int *)</code>. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="example1"></a> |
| <h3> |
| Example |
| </h3> |
| <p> |
| General qualifier examples follow: |
| </p> |
| <div class="informaltable"> |
| <table class="informaltable" border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| // declares a pointer p in the private address space that |
| // points to an object in address space global |
| global int *p; |
| |
| void foo (...) |
| { |
| // declares an array of 4 floats in the private address space |
| float x[4]; |
| ... |
| } |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| Example: |
| </p> |
| <div class="informaltable"> |
| <table class="informaltable" border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| private int f() { ... } // should generate an error |
| local int *f() { ... } // allowed |
| local int * private f() { ... }; // should generate an error. |
| </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=34" target="OpenCL Spec">OpenCL Specification</a> |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="seealso"></a> |
| <h2>Also see</h2> |
| <p> |
| <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a>, |
| <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a>, |
| <a class="citerefentry" href="private.html"><span class="citerefentry"><span class="refentrytitle">__private</span></span></a>, |
| <a class="citerefentry" href="genericAddressSpace.html"><span class="citerefentry"><span class="refentrytitle">genericAddressSpace</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. |
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