sdk/1.1/docs/man/as_typen.xml

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<refentry>
    <refentryinfo>
        <keywordset>
            <keyword>Reinterpreting Types, as_typen(), as_type()</keyword>
        </keywordset>
    </refentryinfo>

    <refmeta>                                    
        <refentrytitle>Reinterpreting Types Using as_type() and as_typen()</refentrytitle>

        <refmiscinfo>
            <copyright>
                <year>2007-2010</year>
                <holder>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.</holder>
            </copyright>
        </refmiscinfo>
        <manvolnum>3</manvolnum>
    </refmeta>

<!-- ================================ SYNOPSIS -->

    <refnamediv id="Reinterpreting Types Using as_typen()">             
        <refname>Reinterpreting Types Using as_type() and as_type<emphasis>n</emphasis>()</refname>

        <refpurpose>
            Reinterpreting Types Using as_type() and as_type<varname>n</varname>()
        </refpurpose>
    </refnamediv>


    <!-- ALTERNATIVE SYNTAX SYNOPSIS (NON-FUNCTION) -->
    <refsect2 id="synopsis">
        <title></title>

        <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" />
                <tbody>
                    <row>
                        <entry>
as_type()
                        </entry>
                    </row>
                    <row>
                        <entry>
as_type<replaceable>n</replaceable>()
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect2>


<!-- ================================ DESCRIPTION  -->

    <refsect1 id="description"><title>Description</title>
        <para>
All data types described in
<citerefentry href="scalarDataTypes"><refentrytitle>Scalar Data Types</refentrytitle></citerefentry> and
<citerefentry href="vectorDataTypes"><refentrytitle>Vector Data Types</refentrytitle></citerefentry> (except <type>bool</type>, <type>half</type> (unless  the cl_khr_fp16 extension is supported), and <type>void</type>) may be also reinterpreted as another data type of the same size using the 
<function>as_type()</function>operator for scalar data types and the <function>as_type<replaceable>n</replaceable>()</function> operator for vector data types. When the operand and result type contain the same number of elements, the bits in the operand shall be returned directly without modification as the new type. The usual type promotion for function arguments shall not be performed.
        </para>

        <para>
For example, <code>as_float(0x3f800000)</code> returns <code>1.0f</code>, which is the value that the bit pattern <code>0x3f800000</code> has if viewed as an IEEE-754 single precision value.
        </para>

        <para>
When the operand and result type contain a different number of elements, the result shall be implementation-defined except if the operand is a 4-component vector and the result is a 3-component vector.  In this case, the bits in the operand shall be returned directly without
modification as the new type. That is, a conforming implementation shall explicitly define a behavior, but two conforming implementations need not have the same behavior when the number of elements in the result and operand types does not match. The implementation may define the result to contain all, some or none of the original bits in whatever order it chooses. It is an error to use the  <function>as_type()</function> or  <function>as_type<replaceable>n</replaceable>()</function> operators to reinterpret data to a type of a different number of bytes.
        </para>
    </refsect1>


<!-- ================================ NOTES  -->


    <refsect1 id="notes"><title>Notes</title>
        <para>
While the union is intended to reflect the organization of data in memory, the
<function>as_type()</function> and <function>as_type<replaceable>n</replaceable>()</function> constructs are intended to
reflect the organization of data in register. The
<function>as_type()</function> and <function>as_type<replaceable>n</replaceable>()</function> constructs
are intended to compile to no instructions on
devices that use a shared register file designed to
operate on both the operand and result types. 
        </para>
        <para>
Note that while differences in memory organization are expected
to largely be limited to those arising from endianness,
the register based representation may also differ due to
size of the element in register. (For example, an
architecture may load a char into a 32-bit register, or a
char vector into a SIMD vector register with fixed 32-bit
element size.) 
        </para>
        <para>
If the element count does not match, then
the implementation should pick a data representation that
most closely matches what would happen if an appropriate
result type operator was applied to a register containing
data of the source type. If the number of elements matches, then the
<function>as_type<replaceable>n</replaceable>()</function>
should faithfully reproduce the behavior expected from a
similar data type reinterpretation using memory/unions.
So, for example if an implementation stores all single
precision data as double in register, it should implement
as_int( float )
by first downconverting the double to single
precision and then (if necessary) moving the single
precision bits to a register suitable for operating on
integer data.
        </para>
        <para>
  If data stored in different address spaces do not have the same endianness, then the "dominant endianness" of the device should prevail.
        </para>
    </refsect1>


<!-- ================================ EXAMPLE  -->

    <refsect2 id="example1">
        <title>
            Example
        </title>

        <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" />
                <tbody>
                    <row>
                        <entry>
float f = 1.0f;
uint u = as_uint(f);  // Legal. Contains:  0x3f800000

float4 f = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
// Legal. Contains: (int4)
// (0x3f800000, 0x40000000, 0x40400000, 0x40800000)
int4 i = as_int4(f);

float4 f, g;
int4 is_less = f &lt; g;

// Legal. f[i] = f[i] &lt; g[i] ? f[i] : 0.0f
f = as_float4(as_int4(f) &amp; is_less);

int i;
// Legal. Result is implementation-defined.
short2 j = as_short2(i);

int4 i;
// Legal. Result is implementation-defined.
short8 j = as_short8(i);

float4 f;
//Error. result and operand have different size
double4 g = as_double4(f);   // Only if the cl_khr_fp64 extension is supported.

float4 f;
// Legal. g.xyz will have same values as f.xyz. g.w is undefined
float3 g = as_float3(f);

float3 f;
// Error. Result and operand have different sizes
float4 g = as_float4(f);

                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect2>


<!-- ================================ SPECIFICATION  -->
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    <refsect1 id="specification"><title>Specification</title>
        <para>
            <imageobject>
                <imagedata fileref="pdficon_small1.gif" format="gif" />
            </imageobject>
                                                 
            <olink uri="as_typen">OpenCL Specification</olink>
        </para>
    </refsect1>


<!-- ================================ ALSO SEE  -->

    <refsect1 id="seealso"><title>Also see</title>
        <para>
            <citerefentry href="scalarDataTypes"><refentrytitle>Scalar Data Types</refentrytitle></citerefentry>, 
            <citerefentry href="vectorDataTypes"><refentrytitle>Vector Data Types</refentrytitle></citerefentry>
        </para>
    </refsect1>



<!-- ============================== COPYRIGHT -->
<!-- Content included from copyright.inc.xsl -->

    <refsect3 id="Copyright"><title></title>
        <imageobject>
                <imagedata fileref="KhronosLogo.jpg" format="jpg" />
        </imageobject>
        <para />
    </refsect3>

</refentry>