/sys$common/syshlp/helplib.hlb FORTRAN, /ASSUME, DUMMY_ALIASES  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Controls whether dummy (formal) arguments that share memory
  locations with other dummy arguments or COMMON block variables
  affect program results.

  This option is provided because many important opportunities for
  optimizing are lost when the compiler is forced to assume that this
  type of memory sharing occurs in a source program.

  If DUMMY_ALIASES is specified, program semantics require frequent
  recomputations on expressions involving dummy arguments or COMMON
  block variables.  It assures correct results but inhibits some
  vectorization and some scalar optimizations, thus slowing
  performance.

  With the default, NODUMMY_ALIASES, program semantics allow
  optimizations on expressions involving dummy arguments and COMMON
  block variables.  It results in higher performance but can cause
  programs to fail if they depend on such aliases.  It should only be
  used for source programs that strictly obey FORTRAN-77 rules for
  overlapping memory references in dummy arguments.

  If you compile a program containing dummy aliasing with
  NODUMMY_ALIASES in effect, the run-time behavior of the program
  will be unpredictable.  The run-time behavior will depend on the
  exact optimizations that are performed.  In some cases, normal
  results will occur; however, in other cases, results will differ
  because the values used in computations involving the offending
  aliases will differ.