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264 lines
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264 lines
12 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Vector Extensions (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Vector Extensions (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Vector Extensions (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Offsetof.html#Offsetof" rel="next" title="Offsetof">
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<link href="Return-Address.html#Return-Address" rel="prev" title="Return Address">
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</style>
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</head>
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<body lang="en">
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<a name="Vector-Extensions"></a>
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<div class="header">
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<p>
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Next: <a href="Offsetof.html#Offsetof" accesskey="n" rel="next">Offsetof</a>, Previous: <a href="Return-Address.html#Return-Address" accesskey="p" rel="prev">Return Address</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Using-Vector-Instructions-through-Built_002din-Functions"></a>
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<h3 class="section">6.50 Using Vector Instructions through Built-in Functions</h3>
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<p>On some targets, the instruction set contains SIMD vector instructions which
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operate on multiple values contained in one large register at the same time.
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For example, on the x86 the MMX, 3DNow! and SSE extensions can be used
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this way.
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</p>
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<p>The first step in using these extensions is to provide the necessary data
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types. This should be done using an appropriate <code>typedef</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int v4si __attribute__ ((vector_size (16)));
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</pre></div>
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<p>The <code>int</code> type specifies the base type, while the attribute specifies
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the vector size for the variable, measured in bytes. For example, the
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declaration above causes the compiler to set the mode for the <code>v4si</code>
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type to be 16 bytes wide and divided into <code>int</code> sized units. For
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a 32-bit <code>int</code> this means a vector of 4 units of 4 bytes, and the
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corresponding mode of <code>foo</code> is <acronym>V4SI</acronym>.
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</p>
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<p>The <code>vector_size</code> attribute is only applicable to integral and
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float scalars, although arrays, pointers, and function return values
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are allowed in conjunction with this construct. Only sizes that are
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a power of two are currently allowed.
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</p>
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<p>All the basic integer types can be used as base types, both as signed
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and as unsigned: <code>char</code>, <code>short</code>, <code>int</code>, <code>long</code>,
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<code>long long</code>. In addition, <code>float</code> and <code>double</code> can be
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used to build floating-point vector types.
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</p>
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<p>Specifying a combination that is not valid for the current architecture
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causes GCC to synthesize the instructions using a narrower mode.
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For example, if you specify a variable of type <code>V4SI</code> and your
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architecture does not allow for this specific SIMD type, GCC
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produces code that uses 4 <code>SIs</code>.
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</p>
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<p>The types defined in this manner can be used with a subset of normal C
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operations. Currently, GCC allows using the following operators
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on these types: <code>+, -, *, /, unary minus, ^, |, &, ~, %</code>.
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</p>
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<p>The operations behave like C++ <code>valarrays</code>. Addition is defined as
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the addition of the corresponding elements of the operands. For
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example, in the code below, each of the 4 elements in <var>a</var> is
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added to the corresponding 4 elements in <var>b</var> and the resulting
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vector is stored in <var>c</var>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int v4si __attribute__ ((vector_size (16)));
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v4si a, b, c;
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c = a + b;
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</pre></div>
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<p>Subtraction, multiplication, division, and the logical operations
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operate in a similar manner. Likewise, the result of using the unary
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minus or complement operators on a vector type is a vector whose
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elements are the negative or complemented values of the corresponding
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elements in the operand.
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</p>
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<p>It is possible to use shifting operators <code><<</code>, <code>>></code> on
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integer-type vectors. The operation is defined as following: <code>{a0,
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a1, …, an} >> {b0, b1, …, bn} == {a0 >> b0, a1 >> b1,
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…, an >> bn}</code>. Vector operands must have the same number of
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elements.
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</p>
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<p>For convenience, it is allowed to use a binary vector operation
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where one operand is a scalar. In that case the compiler transforms
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the scalar operand into a vector where each element is the scalar from
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the operation. The transformation happens only if the scalar could be
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safely converted to the vector-element type.
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Consider the following code.
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int v4si __attribute__ ((vector_size (16)));
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v4si a, b, c;
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long l;
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a = b + 1; /* a = b + {1,1,1,1}; */
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a = 2 * b; /* a = {2,2,2,2} * b; */
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a = l + a; /* Error, cannot convert long to int. */
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</pre></div>
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<p>Vectors can be subscripted as if the vector were an array with
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the same number of elements and base type. Out of bound accesses
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invoke undefined behavior at run time. Warnings for out of bound
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accesses for vector subscription can be enabled with
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<samp>-Warray-bounds</samp>.
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</p>
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<p>Vector comparison is supported with standard comparison
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operators: <code>==, !=, <, <=, >, >=</code>. Comparison operands can be
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vector expressions of integer-type or real-type. Comparison between
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integer-type vectors and real-type vectors are not supported. The
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result of the comparison is a vector of the same width and number of
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elements as the comparison operands with a signed integral element
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type.
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</p>
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<p>Vectors are compared element-wise producing 0 when comparison is false
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and -1 (constant of the appropriate type where all bits are set)
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otherwise. Consider the following example.
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int v4si __attribute__ ((vector_size (16)));
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v4si a = {1,2,3,4};
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v4si b = {3,2,1,4};
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v4si c;
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c = a > b; /* The result would be {0, 0,-1, 0} */
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c = a == b; /* The result would be {0,-1, 0,-1} */
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</pre></div>
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<p>In C++, the ternary operator <code>?:</code> is available. <code>a?b:c</code>, where
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<code>b</code> and <code>c</code> are vectors of the same type and <code>a</code> is an
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integer vector with the same number of elements of the same size as <code>b</code>
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and <code>c</code>, computes all three arguments and creates a vector
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<code>{a[0]?b[0]:c[0], a[1]?b[1]:c[1], …}</code>. Note that unlike in
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OpenCL, <code>a</code> is thus interpreted as <code>a != 0</code> and not <code>a < 0</code>.
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As in the case of binary operations, this syntax is also accepted when
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one of <code>b</code> or <code>c</code> is a scalar that is then transformed into a
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vector. If both <code>b</code> and <code>c</code> are scalars and the type of
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<code>true?b:c</code> has the same size as the element type of <code>a</code>, then
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<code>b</code> and <code>c</code> are converted to a vector type whose elements have
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this type and with the same number of elements as <code>a</code>.
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</p>
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<p>In C++, the logic operators <code>!, &&, ||</code> are available for vectors.
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<code>!v</code> is equivalent to <code>v == 0</code>, <code>a && b</code> is equivalent to
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<code>a!=0 & b!=0</code> and <code>a || b</code> is equivalent to <code>a!=0 | b!=0</code>.
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For mixed operations between a scalar <code>s</code> and a vector <code>v</code>,
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<code>s && v</code> is equivalent to <code>s?v!=0:0</code> (the evaluation is
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short-circuit) and <code>v && s</code> is equivalent to <code>v!=0 & (s?-1:0)</code>.
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</p>
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<a name="index-_005f_005fbuiltin_005fshuffle"></a>
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<p>Vector shuffling is available using functions
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<code>__builtin_shuffle (vec, mask)</code> and
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<code>__builtin_shuffle (vec0, vec1, mask)</code>.
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Both functions construct a permutation of elements from one or two
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vectors and return a vector of the same type as the input vector(s).
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The <var>mask</var> is an integral vector with the same width (<var>W</var>)
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and element count (<var>N</var>) as the output vector.
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</p>
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<p>The elements of the input vectors are numbered in memory ordering of
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<var>vec0</var> beginning at 0 and <var>vec1</var> beginning at <var>N</var>. The
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elements of <var>mask</var> are considered modulo <var>N</var> in the single-operand
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case and modulo <em>2*<var>N</var></em> in the two-operand case.
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</p>
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<p>Consider the following example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">typedef int v4si __attribute__ ((vector_size (16)));
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v4si a = {1,2,3,4};
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v4si b = {5,6,7,8};
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v4si mask1 = {0,1,1,3};
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v4si mask2 = {0,4,2,5};
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v4si res;
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res = __builtin_shuffle (a, mask1); /* res is {1,2,2,4} */
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res = __builtin_shuffle (a, b, mask2); /* res is {1,5,3,6} */
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</pre></div>
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<p>Note that <code>__builtin_shuffle</code> is intentionally semantically
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compatible with the OpenCL <code>shuffle</code> and <code>shuffle2</code> functions.
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</p>
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<p>You can declare variables and use them in function calls and returns, as
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well as in assignments and some casts. You can specify a vector type as
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a return type for a function. Vector types can also be used as function
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arguments. It is possible to cast from one vector type to another,
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provided they are of the same size (in fact, you can also cast vectors
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to and from other datatypes of the same size).
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</p>
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<p>You cannot operate between vectors of different lengths or different
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signedness without a cast.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Offsetof.html#Offsetof" accesskey="n" rel="next">Offsetof</a>, Previous: <a href="Return-Address.html#Return-Address" accesskey="p" rel="prev">Return Address</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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