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

Auto-vectorization is the easiest and more portable way to get vectorization.

The compiler recognize several vectiorization options.

Main vectorization options:

Intel compiler
Disable -no-vec

Not all loops can be vectorized:

Some examples:

  • Loop with dependencies between iterations
for (i=1; i<MAX; i++) {
   d[i] = e[i] – a[i-1];
   a[i] = b[i] + c[i];
  • 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
roberto.alfieri/pub/vectorization.1497377451.txt.gz · Ultima modifica: 13/06/2017 20:10 da roberto.alfieri