Auto vectorization

1-- How you define the profitability of auto-vectorization phase? Is
it just the speed up? If we do not get any speed up over scalar code
then there is no need to do auto-parallelization.

Seems to be right. Since vector instructions typically do more work
than scalar ones, vector code is usually more *power* hungry. However,
if you achieve a considerable speed-up, the overall *energy*
consumption (power * time) can be less than that of scalar
code. (That's why vector instructions are so popular in embedded DSP
architectures.)

2 are the phases or features in a compiler ( especially in the
GCC) that control the quality of auto-vectorization?

Vectorization is (profitably) applicable to fairly specific code. Loop
restructuring transformations can massage code to a form more amenable
to vectorization.

If you are interested, I can send you a pdf of my PhD thesis on
programming and compiling for embedded SIMD architectures, which has a
survey chapter on automatic vectorization techniques.

Cheers,
Anton.