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# Unstable SIMD module
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This module provides helper functionality to build code with SIMD instructions.
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Available since 0.42.0.
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**Note**: this module is unstable. It is only provided as a technology preview.
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Its API may change in arbitrary ways between releases or it might be removed
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from Meson altogether.
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## Usage
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This module is designed for the use case where you have an algorithm with one
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or more SIMD implementation and you choose which one to use at runtime.
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The module provides one method, `check`, which is used like this:
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rval = simd.check('mysimds',
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mmx : 'simd_mmx.c',
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sse : 'simd_sse.c',
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sse2 : 'simd_sse2.c',
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sse3 : 'simd_sse3.c',
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ssse3 : 'simd_ssse3.c',
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sse41 : 'simd_sse41.c',
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sse42 : 'simd_sse42.c',
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avx : 'simd_avx.c',
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avx2 : 'simd_avx2.c',
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neon : 'simd_neon.c',
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compiler : cc)
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Here the individual files contain the accelerated versions of the functions
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in question. The `compiler` keyword argument takes the compiler you are
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going to use to compile them. The function returns an array with two values.
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The first value is a bunch of libraries that contain the compiled code. Any
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SIMD code that the compiler can't compile (for example, Neon instructions on
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an x86 machine) are ignored. You should pass this value to the desired target
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using `link_with`. The second value is a `configuration_data` object that
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contains true for all the values that were supported. For example if the
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compiler did support sse2 instructions, then the object would have `HAVE_SSE2`
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set to 1.
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Generating code to detect the proper instruction set at runtime is
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straightforward. First you create a header with the configuration object and
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then a chooser function that looks like this:
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void (*fptr)(type_of_function_here) = NULL;
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#if HAVE_NEON
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if(fptr == NULL && neon_available()) {
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fptr = neon_accelerated_function;
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}
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#endif
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#if HAVE_AVX2
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if(fptr == NULL && avx2_available()) {
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fptr = avx_accelerated_function;
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}
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#endif
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...
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if(fptr == NULL) {
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fptr = default_function;
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}
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Each source file provides two functions, the `xxx_available` function to query
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whether the CPU currently in use supports the instruction set and
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`xxx_accelerated_function` that is the corresponding accelerated
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implementation.
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At the end of this function the function pointer points to the fastest
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available implementation and can be invoked to do the computation.
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