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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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