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### 3.1 SIMD alignment and fftw_malloc

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 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 `malloc` (etc.) allocation routines.

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 `fftw_malloc` and de-allocating it with `fftw_free`. These have exactly the same interface and behavior as `malloc`/`free`, except that for a SIMD FFTW they ensure that the returned pointer has the necessary alignment (by calling `memalign` or its equivalent on your OS).

You are not required to use `fftw_malloc`. You can allocate your data in any way that you like, from `malloc` to `new` (in C++) to a fixed-size array declaration. If the array happens not to be properly aligned, FFTW will not use the SIMD extensions. Since `fftw_malloc` only ever needs to be used for real and complex arrays, we provide two convenient wrapper routines `fftw_alloc_real(N)` and `fftw_alloc_complex(N)` that are equivalent to `(double*)fftw_malloc(sizeof(double) * N)` and `(fftw_complex*)fftw_malloc(sizeof(fftw_complex) * N)`, respectively (or their equivalents in other precisions).