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<span id="SIMD-alignment-and-fftw_005fmalloc"></span><div class="header">
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<span id="SIMD-alignment-and-fftw_005fmalloc-1"></span><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.
<span id="index-SIMD-1"></span>
<span id="index-SSE"></span>
<span id="index-SSE2"></span>
<span id="index-AVX"></span>
<span id="index-AVX2"></span>
<span id="index-AVX512"></span>
<span id="index-AltiVec"></span>
<span id="index-VSX"></span>
<span id="index-precision-2"></span>
</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>
<span id="index-portability"></span>
<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>.
<span id="index-fftw_005fmalloc-1"></span>
<span id="index-fftw_005ffree-1"></span>
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.
<span id="index-C_002b_002b-1"></span>
</p>
<span id="index-fftw_005falloc_005freal"></span>
<span id="index-fftw_005falloc_005fcomplex-1"></span>
<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>
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