Addition and multiplication are associative in real arithmetic - sums and products may be computed in any order. But in the presence of roundoff, the order of evaluation affects the computed answer.
Avoid the –xreduction parallelization option. Oracle Solaris Studio optimizes reductions in a way that is not deterministic.
Avoid Fortran's DOT and MATMUL operations. These intrinsics in Fortran 90 and later are implemented by different methods on different platforms and will round differently. If parallelization is also enabled, the results might not be deterministic due to reduction optimizations. Dot product and matrix multiplication operations can be coded in portable Fortran such as that available in the LAPACK library from the Netlib software repository.