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3 <title>SIMD alignment and fftw_malloc - FFTW 3.3.3</title>
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5 <meta name="description" content="FFTW 3.3.3">
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9 <link rel="prev" href="Other-Important-Topics.html#Other-Important-Topics" title="Other Important Topics">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="SIMD-alignment-and-fftw_malloc"></a>
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50 <a name="SIMD-alignment-and-fftw_005fmalloc"></a>
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51 <p>
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52 Next: <a rel="next" accesskey="n" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>,
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53 Previous: <a rel="previous" accesskey="p" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>,
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54 Up: <a rel="up" accesskey="u" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>
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55 <hr>
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56 </div>
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57
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58 <h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
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59
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60 <p>SIMD, which stands for “Single Instruction Multiple Data,” is a set of
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61 special operations supported by some processors to perform a single
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62 operation on several numbers (usually 2 or 4) simultaneously. SIMD
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63 floating-point instructions are available on several popular CPUs:
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64 SSE/SSE2/AVX on recent x86/x86-64 processors, AltiVec (single precision)
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65 on some PowerPCs (Apple G4 and higher), NEON on some ARM models, and MIPS Paired Single
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66 (currently only in FFTW 3.2.x). FFTW can be compiled to support the
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67 SIMD instructions on any of these systems.
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68 <a name="index-SIMD-102"></a><a name="index-SSE-103"></a><a name="index-SSE2-104"></a><a name="index-AVX-105"></a><a name="index-AltiVec-106"></a><a name="index-MIPS-PS-107"></a><a name="index-precision-108"></a>
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69
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70 <p>A program linking to an FFTW library compiled with SIMD support can
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71 obtain a nonnegligible speedup for most complex and r2c/c2r
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72 transforms. In order to obtain this speedup, however, the arrays of
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73 complex (or real) data passed to FFTW must be specially aligned in
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74 memory (typically 16-byte aligned), and often this alignment is more
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75 stringent than that provided by the usual <code>malloc</code> (etc.)
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76 allocation routines.
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77
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78 <p><a name="index-portability-109"></a>In order to guarantee proper alignment for SIMD, therefore, in case
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79 your program is ever linked against a SIMD-using FFTW, we recommend
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80 allocating your transform data with <code>fftw_malloc</code> and
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81 de-allocating it with <code>fftw_free</code>.
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82 <a name="index-fftw_005fmalloc-110"></a><a name="index-fftw_005ffree-111"></a>These have exactly the same interface and behavior as
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83 <code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
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84 that the returned pointer has the necessary alignment (by calling
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85 <code>memalign</code> or its equivalent on your OS).
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86
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87 <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
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88 allocate your data in any way that you like, from <code>malloc</code> to
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89 <code>new</code> (in C++) to a fixed-size array declaration. If the array
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90 happens not to be properly aligned, FFTW will not use the SIMD
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91 extensions.
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92 <a name="index-C_002b_002b-112"></a>
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93 <a name="index-fftw_005falloc_005freal-113"></a><a name="index-fftw_005falloc_005fcomplex-114"></a>Since <code>fftw_malloc</code> only ever needs to be used for real and
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94 complex arrays, we provide two convenient wrapper routines
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95 <code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
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96 equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
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97 <code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
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98 respectively (or their equivalents in other precisions).
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99
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100 <!-- -->
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101 </body></html>
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102
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