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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: SIMD alignment and fftw_malloc</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: SIMD alignment and fftw_malloc">
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="SIMD-alignment-and-fftw_005fmalloc"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 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@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="SIMD-alignment-and-fftw_005fmalloc-1"></a>
Chris@42 79 <h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
Chris@42 80
Chris@42 81 <p>SIMD, which stands for &ldquo;Single Instruction Multiple Data,&rdquo; is a set of
Chris@42 82 special operations supported by some processors to perform a single
Chris@42 83 operation on several numbers (usually 2 or 4) simultaneously. SIMD
Chris@42 84 floating-point instructions are available on several popular CPUs:
Chris@42 85 SSE/SSE2/AVX/AVX2/AVX512/KCVI on some x86/x86-64 processors, AltiVec and
Chris@42 86 VSX on some POWER/PowerPCs, NEON on some ARM models. FFTW can be
Chris@42 87 compiled to support the SIMD instructions on any of these systems.
Chris@42 88 <a name="index-SIMD-1"></a>
Chris@42 89 <a name="index-SSE"></a>
Chris@42 90 <a name="index-SSE2"></a>
Chris@42 91 <a name="index-AVX"></a>
Chris@42 92 <a name="index-AVX2"></a>
Chris@42 93 <a name="index-AVX512"></a>
Chris@42 94 <a name="index-AltiVec"></a>
Chris@42 95 <a name="index-VSX"></a>
Chris@42 96 <a name="index-precision-2"></a>
Chris@42 97 </p>
Chris@42 98
Chris@42 99 <p>A program linking to an FFTW library compiled with SIMD support can
Chris@42 100 obtain a nonnegligible speedup for most complex and r2c/c2r
Chris@42 101 transforms. In order to obtain this speedup, however, the arrays of
Chris@42 102 complex (or real) data passed to FFTW must be specially aligned in
Chris@42 103 memory (typically 16-byte aligned), and often this alignment is more
Chris@42 104 stringent than that provided by the usual <code>malloc</code> (etc.)
Chris@42 105 allocation routines.
Chris@42 106 </p>
Chris@42 107 <a name="index-portability"></a>
Chris@42 108 <p>In order to guarantee proper alignment for SIMD, therefore, in case
Chris@42 109 your program is ever linked against a SIMD-using FFTW, we recommend
Chris@42 110 allocating your transform data with <code>fftw_malloc</code> and
Chris@42 111 de-allocating it with <code>fftw_free</code>.
Chris@42 112 <a name="index-fftw_005fmalloc-1"></a>
Chris@42 113 <a name="index-fftw_005ffree-1"></a>
Chris@42 114 These have exactly the same interface and behavior as
Chris@42 115 <code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
Chris@42 116 that the returned pointer has the necessary alignment (by calling
Chris@42 117 <code>memalign</code> or its equivalent on your OS).
Chris@42 118 </p>
Chris@42 119 <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
Chris@42 120 allocate your data in any way that you like, from <code>malloc</code> to
Chris@42 121 <code>new</code> (in C++) to a fixed-size array declaration. If the array
Chris@42 122 happens not to be properly aligned, FFTW will not use the SIMD
Chris@42 123 extensions.
Chris@42 124 <a name="index-C_002b_002b-1"></a>
Chris@42 125 </p>
Chris@42 126 <a name="index-fftw_005falloc_005freal"></a>
Chris@42 127 <a name="index-fftw_005falloc_005fcomplex-1"></a>
Chris@42 128 <p>Since <code>fftw_malloc</code> only ever needs to be used for real and
Chris@42 129 complex arrays, we provide two convenient wrapper routines
Chris@42 130 <code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
Chris@42 131 equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
Chris@42 132 <code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
Chris@42 133 respectively (or their equivalents in other precisions).
Chris@42 134 </p>
Chris@42 135 <hr>
Chris@42 136 <div class="header">
Chris@42 137 <p>
Chris@42 138 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@42 139 </div>
Chris@42 140
Chris@42 141
Chris@42 142
Chris@42 143 </body>
Chris@42 144 </html>