====== Vectorization ======

[[ http://www.training.prace-ri.eu/uploads/tx_pracetmo/intel_mic_optimization.pdf | Vectorization and Code Optimization ]]


Processor peak performance includes the speed-up provided by the vector instructions, 
but in order  to exploit it you need  specific programming techniques.



{{:roberto.alfieri:pub:vectortrend.png?200|}}





{{:roberto.alfieri:pub:waytovectorize.png?200|}}



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