[Bug tree-optimization/51499] vectorizer missing simple case

["fb.programming at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51499

--- Comment #13 from fb.programming at gmail dot com 2011-12-12 14:20:58 UTC ---
(In reply to comment #9)

> So, you are suggesting to remove the need in flag_associative_math for fp for
> cases when a reduction computation is already unrolled by the vectorization
> factor. Sounds reasonable to me.

Yes I think that's it, basically only require flag_associative_math if
the order of summation or products is changed by the vectorizer. That is
quite important I think, as most of the time
 -ffast-math / -funsafe-math-optimizations / -fassociative-math
might not be acceptable for many projects.

However, I don't fully understand Richard Guenther's example. Yes his
example requires -fassociative-math to be vectorized, however, my example
would translate to something like

  sum1 += a[i];
  sum2 += a[i+1];

and now it doesn't matter if it's executed this way or the other way
around

  sum2 += a[i+1];
  sum1 += a[i];

Second issue is just to double check the profitability calculation
as it wrongly decided:

  7: not vectorized: vectorization not profitable.