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Current fftw source
| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Tue, 18 Oct 2016 13:40:26 +0100 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.5/doc/html/SIMD-alignment-and-fftw_005fmalloc.html Tue Oct 18 13:40:26 2016 +0100 @@ -0,0 +1,144 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This manual is for FFTW +(version 3.3.5, 30 July 2016). + +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. --> +<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> +<head> +<title>FFTW 3.3.5: SIMD alignment and fftw_malloc</title> + +<meta name="description" content="FFTW 3.3.5: SIMD alignment and fftw_malloc"> +<meta name="keywords" content="FFTW 3.3.5: SIMD alignment and fftw_malloc"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Other-Important-Topics.html#Other-Important-Topics" rel="up" title="Other Important Topics"> +<link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="next" title="Multi-dimensional Array Format"> +<link href="Other-Important-Topics.html#Other-Important-Topics" rel="prev" title="Other Important Topics"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="SIMD-alignment-and-fftw_005fmalloc"></a> +<div class="header"> +<p> +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> [<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> +</div> +<hr> +<a name="SIMD-alignment-and-fftw_005fmalloc-1"></a> +<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/AVX2/AVX512/KCVI on some x86/x86-64 processors, AltiVec and +VSX on some POWER/PowerPCs, NEON on some ARM models. FFTW can be +compiled to support the SIMD instructions on any of these systems. +<a name="index-SIMD-1"></a> +<a name="index-SSE"></a> +<a name="index-SSE2"></a> +<a name="index-AVX"></a> +<a name="index-AVX2"></a> +<a name="index-AVX512"></a> +<a name="index-AltiVec"></a> +<a name="index-VSX"></a> +<a name="index-precision-2"></a> +</p> + +<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"></a> +<p>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-1"></a> +<a name="index-fftw_005ffree-1"></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> +<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-1"></a> +</p> +<a name="index-fftw_005falloc_005freal"></a> +<a name="index-fftw_005falloc_005fcomplex-1"></a> +<p>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). +</p> +<hr> +<div class="header"> +<p> +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> [<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> +</div> + + + +</body> +</html>