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author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
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Chris@19 1 <html lang="en">
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Chris@19 3 <title>SIMD alignment and fftw_malloc - FFTW 3.3.4</title>
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Chris@19 13 This manual is for FFTW
Chris@19 14 (version 3.3.4, 20 September 2013).
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Chris@19 16 Copyright (C) 2003 Matteo Frigo.
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Chris@19 49 <a name="SIMD-alignment-and-fftw_malloc"></a>
Chris@19 50 <a name="SIMD-alignment-and-fftw_005fmalloc"></a>
Chris@19 51 <p>
Chris@19 52 Next:&nbsp;<a rel="next" accesskey="n" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>,
Chris@19 53 Previous:&nbsp;<a rel="previous" accesskey="p" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>,
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Chris@19 55 <hr>
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Chris@19 57
Chris@19 58 <h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
Chris@19 59
Chris@19 60 <p>SIMD, which stands for &ldquo;Single Instruction Multiple Data,&rdquo; is a set of
Chris@19 61 special operations supported by some processors to perform a single
Chris@19 62 operation on several numbers (usually 2 or 4) simultaneously. SIMD
Chris@19 63 floating-point instructions are available on several popular CPUs:
Chris@19 64 SSE/SSE2/AVX on recent x86/x86-64 processors, AltiVec (single precision)
Chris@19 65 on some PowerPCs (Apple G4 and higher), NEON on some ARM models, and MIPS Paired Single
Chris@19 66 (currently only in FFTW 3.2.x). FFTW can be compiled to support the
Chris@19 67 SIMD instructions on any of these systems.
Chris@19 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>
Chris@19 69
Chris@19 70 <p>A program linking to an FFTW library compiled with SIMD support can
Chris@19 71 obtain a nonnegligible speedup for most complex and r2c/c2r
Chris@19 72 transforms. In order to obtain this speedup, however, the arrays of
Chris@19 73 complex (or real) data passed to FFTW must be specially aligned in
Chris@19 74 memory (typically 16-byte aligned), and often this alignment is more
Chris@19 75 stringent than that provided by the usual <code>malloc</code> (etc.)
Chris@19 76 allocation routines.
Chris@19 77
Chris@19 78 <p><a name="index-portability-109"></a>In order to guarantee proper alignment for SIMD, therefore, in case
Chris@19 79 your program is ever linked against a SIMD-using FFTW, we recommend
Chris@19 80 allocating your transform data with <code>fftw_malloc</code> and
Chris@19 81 de-allocating it with <code>fftw_free</code>.
Chris@19 82 <a name="index-fftw_005fmalloc-110"></a><a name="index-fftw_005ffree-111"></a>These have exactly the same interface and behavior as
Chris@19 83 <code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
Chris@19 84 that the returned pointer has the necessary alignment (by calling
Chris@19 85 <code>memalign</code> or its equivalent on your OS).
Chris@19 86
Chris@19 87 <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
Chris@19 88 allocate your data in any way that you like, from <code>malloc</code> to
Chris@19 89 <code>new</code> (in C++) to a fixed-size array declaration. If the array
Chris@19 90 happens not to be properly aligned, FFTW will not use the SIMD
Chris@19 91 extensions.
Chris@19 92 <a name="index-C_002b_002b-112"></a>
Chris@19 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
Chris@19 94 complex arrays, we provide two convenient wrapper routines
Chris@19 95 <code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
Chris@19 96 equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
Chris@19 97 <code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
Chris@19 98 respectively (or their equivalents in other precisions).
Chris@19 99
Chris@19 100 <!-- -->
Chris@19 101 </body></html>
Chris@19 102