133 lines
6.3 KiB
HTML
133 lines
6.3 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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<title>SIMD alignment and fftw_malloc (FFTW 3.3.10)</title>
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<span id="SIMD-alignment-and-fftw_005fmalloc"></span><div class="header">
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<p>
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Next: <a href="Multi_002ddimensional-Array-Format.html" accesskey="n" rel="next">Multi-dimensional Array Format</a>, Previous: <a href="Other-Important-Topics.html" accesskey="p" rel="prev">Other Important Topics</a>, Up: <a href="Other-Important-Topics.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="SIMD-alignment-and-fftw_005fmalloc-1"></span><h3 class="section">3.1 SIMD alignment and fftw_malloc</h3>
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<p>SIMD, which stands for “Single Instruction Multiple Data,” is a set of
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special operations supported by some processors to perform a single
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operation on several numbers (usually 2 or 4) simultaneously. SIMD
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floating-point instructions are available on several popular CPUs:
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SSE/SSE2/AVX/AVX2/AVX512/KCVI on some x86/x86-64 processors, AltiVec and
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VSX on some POWER/PowerPCs, NEON on some ARM models. FFTW can be
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compiled to support the SIMD instructions on any of these systems.
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<span id="index-SIMD-1"></span>
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<span id="index-SSE"></span>
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<span id="index-SSE2"></span>
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<span id="index-AVX"></span>
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<span id="index-AVX2"></span>
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<span id="index-AVX512"></span>
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<span id="index-AltiVec"></span>
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<span id="index-VSX"></span>
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<span id="index-precision-2"></span>
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</p>
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<p>A program linking to an FFTW library compiled with SIMD support can
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obtain a nonnegligible speedup for most complex and r2c/c2r
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transforms. In order to obtain this speedup, however, the arrays of
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complex (or real) data passed to FFTW must be specially aligned in
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memory (typically 16-byte aligned), and often this alignment is more
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stringent than that provided by the usual <code>malloc</code> (etc.)
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allocation routines.
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</p>
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<span id="index-portability"></span>
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<p>In order to guarantee proper alignment for SIMD, therefore, in case
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your program is ever linked against a SIMD-using FFTW, we recommend
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allocating your transform data with <code>fftw_malloc</code> and
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de-allocating it with <code>fftw_free</code>.
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<span id="index-fftw_005fmalloc-1"></span>
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<span id="index-fftw_005ffree-1"></span>
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These have exactly the same interface and behavior as
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<code>malloc</code>/<code>free</code>, except that for a SIMD FFTW they ensure
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that the returned pointer has the necessary alignment (by calling
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<code>memalign</code> or its equivalent on your OS).
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</p>
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<p>You are not <em>required</em> to use <code>fftw_malloc</code>. You can
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allocate your data in any way that you like, from <code>malloc</code> to
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<code>new</code> (in C++) to a fixed-size array declaration. If the array
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happens not to be properly aligned, FFTW will not use the SIMD
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extensions.
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<span id="index-C_002b_002b-1"></span>
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</p>
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<span id="index-fftw_005falloc_005freal"></span>
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<span id="index-fftw_005falloc_005fcomplex-1"></span>
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<p>Since <code>fftw_malloc</code> only ever needs to be used for real and
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complex arrays, we provide two convenient wrapper routines
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<code>fftw_alloc_real(N)</code> and <code>fftw_alloc_complex(N)</code> that are
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equivalent to <code>(double*)fftw_malloc(sizeof(double) * N)</code> and
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<code>(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)</code>,
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respectively (or their equivalents in other precisions).
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Multi_002ddimensional-Array-Format.html" accesskey="n" rel="next">Multi-dimensional Array Format</a>, Previous: <a href="Other-Important-Topics.html" accesskey="p" rel="prev">Other Important Topics</a>, Up: <a href="Other-Important-Topics.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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