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author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>SIMD alignment and fftw_malloc - FFTW 3.3.3</title>
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57
58 <h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
59
60 <p>SIMD, which stands for &ldquo;Single Instruction Multiple Data,&rdquo; is a set of
61 special operations supported by some processors to perform a single
62 operation on several numbers (usually 2 or 4) simultaneously. SIMD
63 floating-point instructions are available on several popular CPUs:
64 SSE/SSE2/AVX on recent x86/x86-64 processors, AltiVec (single precision)
65 on some PowerPCs (Apple G4 and higher), NEON on some ARM models, and MIPS Paired Single
66 (currently only in FFTW 3.2.x). FFTW can be compiled to support the
67 SIMD instructions on any of these systems.
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>
69
70 <p>A program linking to an FFTW library compiled with SIMD support can
71 obtain a nonnegligible speedup for most complex and r2c/c2r
72 transforms. In order to obtain this speedup, however, the arrays of
73 complex (or real) data passed to FFTW must be specially aligned in
74 memory (typically 16-byte aligned), and often this alignment is more
75 stringent than that provided by the usual <code>malloc</code> (etc.)
76 allocation routines.
77
78 <p><a name="index-portability-109"></a>In order to guarantee proper alignment for SIMD, therefore, in case
79 your program is ever linked against a SIMD-using FFTW, we recommend
80 allocating your transform data with <code>fftw_malloc</code> and
81 de-allocating it with <code>fftw_free</code>.
82 <a name="index-fftw_005fmalloc-110"></a><a name="index-fftw_005ffree-111"></a>These have exactly the same interface and behavior as
83 <code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
84 that the returned pointer has the necessary alignment (by calling
85 <code>memalign</code> or its equivalent on your OS).
86
87 <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
88 allocate your data in any way that you like, from <code>malloc</code> to
89 <code>new</code> (in C++) to a fixed-size array declaration. If the array
90 happens not to be properly aligned, FFTW will not use the SIMD
91 extensions.
92 <a name="index-C_002b_002b-112"></a>
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
94 complex arrays, we provide two convenient wrapper routines
95 <code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
96 equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
97 <code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
98 respectively (or their equivalents in other precisions).
99
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