Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!unmvax!pprg.unm.edu!hc!lanl!jlg
From: jlg@lanl.gov (Jim Giles)
Newsgroups: comp.lang.fortran
Subject: Re: Pointers to array sections
Message-ID: <13945@lanl.gov>
Date: 18 Jun 89 20:48:58 GMT
References: <750@stat.fsu.edu>
Distribution: usa
Organization: Los Alamos National Laboratory
Lines: 18

From article <750@stat.fsu.edu>, by guest@stat.fsu.edu (a guest account):
> [...]
> The ALIAS, IDENTIFY, POINTER, and SET RANGE options all overlap
> somewhat. (Quiz: which of the above are in the current draft
> standard?).  It is not clear that the most general mechanism (the
> extended pointers?) is the most reasonable or efficient.  [...]

No!  Neither characterization is clear.  What _is_ clear is that pointers
are lower-level mechanisms.  The lower-level construct allows the user to
simulate much of the other functionality (but not all - try to alias the
_diagonal_ of a two-d array with the pointer mechanism - can't be done).

The purpose of using higher-level mechanisms is for optimizations.  For
example, the RANGE mechanism allows someone to alias an array section
_and_nothing_else_!  The compiler may find the constrained semantics of
the construct useful in deciding dependencies and eliminating non-optimal
assumptions from the code.  Since a pointer can point to _anything_, the
compiler must generally assume that it _does_.