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diff src/fftw-3.3.3/doc/html/SIMD-alignment-and-fftw_005fmalloc.html @ 10:37bf6b4a2645
Add FFTW3
| author | Chris Cannam |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/SIMD-alignment-and-fftw_005fmalloc.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,102 @@ +<html lang="en"> +<head> +<title>SIMD alignment and fftw_malloc - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Other-Important-Topics.html#Other-Important-Topics" title="Other Important Topics"> +<link rel="prev" href="Other-Important-Topics.html#Other-Important-Topics" title="Other Important Topics"> +<link rel="next" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + + Permission is granted to make and distribute verbatim copies of + this manual provided the copyright notice and this permission + notice are preserved on all copies. + + Permission is granted to copy and distribute modified versions of + this manual under the conditions for verbatim copying, provided + that the entire resulting derived work is distributed under the + terms of a permission notice identical to this one. + + Permission is granted to copy and distribute translations of this + manual into another language, under the above conditions for + modified versions, except that this permission notice may be + stated in a translation approved by the Free Software Foundation. + --> +<meta http-equiv="Content-Style-Type" content="text/css"> +<style type="text/css"><!-- + pre.display { font-family:inherit } + pre.format { font-family:inherit } + pre.smalldisplay { font-family:inherit; font-size:smaller } + pre.smallformat { font-family:inherit; font-size:smaller } + pre.smallexample { font-size:smaller } + pre.smalllisp { font-size:smaller } + span.sc { font-variant:small-caps } + span.roman { font-family:serif; font-weight:normal; } + span.sansserif { font-family:sans-serif; font-weight:normal; } +--></style> +</head> +<body> +<div class="node"> +<a name="SIMD-alignment-and-fftw_malloc"></a> +<a name="SIMD-alignment-and-fftw_005fmalloc"></a> +<p> +Next: <a rel="next" accesskey="n" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>, +Previous: <a rel="previous" accesskey="p" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a>, +Up: <a rel="up" accesskey="u" href="Other-Important-Topics.html#Other-Important-Topics">Other Important Topics</a> +<hr> +</div> + +<h3 class="section">3.1 SIMD alignment and fftw_malloc</h3> + +<p>SIMD, which stands for “Single Instruction Multiple Data,” is a set of +special operations supported by some processors to perform a single +operation on several numbers (usually 2 or 4) simultaneously. SIMD +floating-point instructions are available on several popular CPUs: +SSE/SSE2/AVX on recent x86/x86-64 processors, AltiVec (single precision) +on some PowerPCs (Apple G4 and higher), NEON on some ARM models, and MIPS Paired Single +(currently only in FFTW 3.2.x). FFTW can be compiled to support the +SIMD instructions on any of these systems. +<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> + + <p>A program linking to an FFTW library compiled with SIMD support can +obtain a nonnegligible speedup for most complex and r2c/c2r +transforms. In order to obtain this speedup, however, the arrays of +complex (or real) data passed to FFTW must be specially aligned in +memory (typically 16-byte aligned), and often this alignment is more +stringent than that provided by the usual <code>malloc</code> (etc.) +allocation routines. + + <p><a name="index-portability-109"></a>In order to guarantee proper alignment for SIMD, therefore, in case +your program is ever linked against a SIMD-using FFTW, we recommend +allocating your transform data with <code>fftw_malloc</code> and +de-allocating it with <code>fftw_free</code>. +<a name="index-fftw_005fmalloc-110"></a><a name="index-fftw_005ffree-111"></a>These have exactly the same interface and behavior as +<code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure +that the returned pointer has the necessary alignment (by calling +<code>memalign</code> or its equivalent on your OS). + + <p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can +allocate your data in any way that you like, from <code>malloc</code> to +<code>new</code> (in C++) to a fixed-size array declaration. If the array +happens not to be properly aligned, FFTW will not use the SIMD +extensions. +<a name="index-C_002b_002b-112"></a> +<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 +complex arrays, we provide two convenient wrapper routines +<code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are +equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and +<code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>, +respectively (or their equivalents in other precisions). + +<!-- --> + </body></html> +