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