[Bug tree-optimization/43431] Diagnostic message is not clear for vectorization profitability analysis

["irar at il dot ibm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #1 from irar at il dot ibm dot com  2010-03-28 09:41 -------
(In reply to comment #0)

> What does this message mean?
> "vector iteration cost = 2056 is divisible by scalar iteration cost = 4 by a
> factor greater than or equal to the vectorization factor = 4 ."
> Is the vectorization not profitable?
> Why?

The cost of one vector iteration is 2056.
The cost of one scalar iteration is 4.
2056/4 = 514 
514 > 4 (= vectorization factor)
The vectorization is not profitable.

We want to vectorize only if one vector iteration cost is lower than one scalar
iteration cost multiplied by vectorization factor.

(Vector cost is so high here, because of the j,i access. We should vectorize
the outer loop, but we fail because of some unsupported features: unknown inner
loop bound, need for versioning (for alias) in outer loop.)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43431