------- Comment #8 from irar at il dot ibm dot com 2009-01-05 13:58 ------- To handle unknown alignment of data, the vectorizer creates a prolog loop to peel a statically unknown number of scalar iterations (0<=n<VF). This loop is followed by a vectorized loop (with the remaining, multiple of VF, number of iterations), and an epilog scalar loop that completes the iterations that were not executed (0<=n<VF). Therefore, the created scalar loops have unknown number of iterations, which prevents their unrolling (while the original scalar loop is unrolled). Vectorizer cost model does not take possible unrolling into account.
Another cost model problem is that the calculation of scalar outside cost for this case is performed not for the original scalar version, but includes run-time guards. Which seems to be wrong in case that the original loop bound is known. I am going to submit a patch to fix that. Ira -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37194
