not vectorized: can't determine dependence

[Tim Prince]

Apparently, gfortran considers arrays unacceptably aliased when they are 
in the same COMMON, or 2 slices of an array, or even when they start at 
the same variable index:

       mid= n/2
       ...
           a(:mid-1)= a(:mid-1)+b(:mid-1)*c(:mid-1)
           a(mid:n)= a(mid:n)+b(mid:n)*d(mid:n)


This example is from the Levine, Callahan, Dongarra VECTOR benchmark, 
simplified to make it vectorizable. The first assignment is vectorized, 
but the second throws the "can't determine dependence." (recent 4.3 for 
x86-64)
The restriction against vectorizing when using COMMON is a big 
disadvantage for semi-legacy applications.  For example, several 
Livermore Fortran Kernels are not vectorized on account of using 
neighboring arrays from the same labeled COMMON.
ifort takes advantage of COMMON blocks in vectorization by analyzing the 
relative alignments.