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(version 3.2.2, 12 July 2009).
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(version 3.3, 26 July 2011).
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Copyright (C) 2003 Matteo Frigo.
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<h4 class="subsection">3.1.1 SIMD alignment and fftw_malloc</h4>
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<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 (single/double precision) on Pentium III and higher and on
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AMD64, AltiVec (single precision) on some PowerPCs (Apple G4 and
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higher), and MIPS Paired Single. FFTW can be compiled to support the
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SSE/SSE2/AVX on recent x86/x86-64 processors, AltiVec (single precision) on some PowerPCs (Apple G4 and
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higher), and MIPS Paired Single (currently only in FFTW 3.2.x). FFTW can be compiled to support the
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SIMD instructions on any of these systems.
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<a name="index-SIMD-102"></a><a name="index-SSE-103"></a><a name="index-SSE2-104"></a><a name="index-AltiVec-105"></a><a name="index-MIPS-PS-106"></a><a name="index-precision-107"></a>
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A program linking to an FFTW library compiled with SIMD support can
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<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>
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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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stringent than that provided by the usual <code>malloc</code> (etc.)
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allocation routines.
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<p><a name="index-portability-108"></a>In order to guarantee proper alignment for SIMD, therefore, in case
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<p><a name="index-portability-109"></a>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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<a name="index-fftw_005fmalloc-109"></a><a name="index-fftw_005ffree-110"></a>These have exactly the same interface and behavior as
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<a name="index-fftw_005fmalloc-110"></a><a name="index-fftw_005ffree-111"></a>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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<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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<a name="index-C_002b_002b-111"></a>
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<a name="index-C_002b_002b-112"></a>
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<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
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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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