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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents d0c2a83c1364
children
rev   line source
Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
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Chris@82 6 Copyright (C) 2003 Matteo Frigo.
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: SIMD alignment and fftw_malloc</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: SIMD alignment and fftw_malloc">
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Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="SIMD-alignment-and-fftw_005fmalloc"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 Next: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="n" rel="next">Multi-dimensional Array Format</a>, Previous: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="p" rel="prev">Other Important Topics</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="SIMD-alignment-and-fftw_005fmalloc-1"></a>
Chris@82 78 <h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
Chris@82 79
Chris@82 80 <p>SIMD, which stands for &ldquo;Single Instruction Multiple Data,&rdquo; is a set of
Chris@82 81 special operations supported by some processors to perform a single
Chris@82 82 operation on several numbers (usually 2 or 4) simultaneously. SIMD
Chris@82 83 floating-point instructions are available on several popular CPUs:
Chris@82 84 SSE/SSE2/AVX/AVX2/AVX512/KCVI on some x86/x86-64 processors, AltiVec and
Chris@82 85 VSX on some POWER/PowerPCs, NEON on some ARM models. FFTW can be
Chris@82 86 compiled to support the SIMD instructions on any of these systems.
Chris@82 87 <a name="index-SIMD-1"></a>
Chris@82 88 <a name="index-SSE"></a>
Chris@82 89 <a name="index-SSE2"></a>
Chris@82 90 <a name="index-AVX"></a>
Chris@82 91 <a name="index-AVX2"></a>
Chris@82 92 <a name="index-AVX512"></a>
Chris@82 93 <a name="index-AltiVec"></a>
Chris@82 94 <a name="index-VSX"></a>
Chris@82 95 <a name="index-precision-2"></a>
Chris@82 96 </p>
Chris@82 97
Chris@82 98 <p>A program linking to an FFTW library compiled with SIMD support can
Chris@82 99 obtain a nonnegligible speedup for most complex and r2c/c2r
Chris@82 100 transforms. In order to obtain this speedup, however, the arrays of
Chris@82 101 complex (or real) data passed to FFTW must be specially aligned in
Chris@82 102 memory (typically 16-byte aligned), and often this alignment is more
Chris@82 103 stringent than that provided by the usual <code>malloc</code> (etc.)
Chris@82 104 allocation routines.
Chris@82 105 </p>
Chris@82 106 <a name="index-portability"></a>
Chris@82 107 <p>In order to guarantee proper alignment for SIMD, therefore, in case
Chris@82 108 your program is ever linked against a SIMD-using FFTW, we recommend
Chris@82 109 allocating your transform data with <code>fftw_malloc</code> and
Chris@82 110 de-allocating it with <code>fftw_free</code>.
Chris@82 111 <a name="index-fftw_005fmalloc-1"></a>
Chris@82 112 <a name="index-fftw_005ffree-1"></a>
Chris@82 113 These have exactly the same interface and behavior as
Chris@82 114 <code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
Chris@82 115 that the returned pointer has the necessary alignment (by calling
Chris@82 116 <code>memalign</code> or its equivalent on your OS).
Chris@82 117 </p>
Chris@82 118 <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
Chris@82 119 allocate your data in any way that you like, from <code>malloc</code> to
Chris@82 120 <code>new</code> (in C++) to a fixed-size array declaration. If the array
Chris@82 121 happens not to be properly aligned, FFTW will not use the SIMD
Chris@82 122 extensions.
Chris@82 123 <a name="index-C_002b_002b-1"></a>
Chris@82 124 </p>
Chris@82 125 <a name="index-fftw_005falloc_005freal"></a>
Chris@82 126 <a name="index-fftw_005falloc_005fcomplex-1"></a>
Chris@82 127 <p>Since <code>fftw_malloc</code> only ever needs to be used for real and
Chris@82 128 complex arrays, we provide two convenient wrapper routines
Chris@82 129 <code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
Chris@82 130 equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
Chris@82 131 <code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
Chris@82 132 respectively (or their equivalents in other precisions).
Chris@82 133 </p>
Chris@82 134 <hr>
Chris@82 135 <div class="header">
Chris@82 136 <p>
Chris@82 137 Next: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="n" rel="next">Multi-dimensional Array Format</a>, Previous: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="p" rel="prev">Other Important Topics</a>, Up: <a href="Other-Important-Topics.html#Other-Important-Topics" accesskey="u" rel="up">Other Important Topics</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@82 138 </div>
Chris@82 139
Chris@82 140
Chris@82 141
Chris@82 142 </body>
Chris@82 143 </html>