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roberto.alfieri:pub:vectorization [13/06/2017 19:45] roberto.alfieri |
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====== Vectorization ====== | ====== Vectorization ====== | ||
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+ | [[ http://www.training.prace-ri.eu/uploads/tx_pracetmo/intel_mic_optimization.pdf | Vectorization and Code Optimization ]] | ||
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Processor peak performance includes the speed-up provided by the vector instructions, | Processor peak performance includes the speed-up provided by the vector instructions, | ||
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{{:roberto.alfieri:pub:waytovectorize.png?200|}} | {{:roberto.alfieri:pub:waytovectorize.png?200|}} | ||
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+ | |||
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+ | Auto-vectorization is the easiest and more portable way to get vectorization. | ||
+ | |||
+ | The compiler recognize several vectiorization options. | ||
+ | |||
+ | Main vectorization options: | ||
+ | |||
+ | ^ ^ Intel compiler ^ | ||
+ | ^ KNL | -xMIC-AVX512 | | ||
+ | ^ BDW | -xCORE-AVX2 | | ||
+ | ^ Disable | -no-vec | | ||
+ | |||
+ | Not all loops can be vectorized: | ||
+ | |||
+ | Some examples: | ||
+ | |||
+ | * Loop with dependencies between iterations | ||
+ | |||
+ | <code> | ||
+ | for (i=1; i<MAX; i++) { | ||
+ | d[i] = e[i] – a[i-1]; | ||
+ | a[i] = b[i] + c[i]; | ||
+ | } | ||
+ | </code> | ||
+ | |||
+ | |||
+ | * Complex loops | ||
+ | |||
+ | * Function calls inside the loop: | ||
+ | |||
+ | for (int i = 0; i < N; i++) a[i] = foo(b[i]); | ||
+ | |||
+ | * Loops on data that are not aligned in memory | ||
+ | |