Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!cmcl2!lanl!jlg
From: jlg@lanl.gov (Jim Giles)
Newsgroups: comp.lang.misc
Subject: Re: C vs. FORTRAN aliasing
Message-ID: <7224@lanl.gov>
Date: 28 Nov 90 21:15:27 GMT
References: <15704.274a7509@levels.sait.edu.au>
Organization: Los Alamos Natl Lab, Los Alamos, N.M.
Lines: 24

From article <15704.274a7509@levels.sait.edu.au>, by ccml@levels.sait.edu.au:
< In article <RANG.90Nov18160113@nexus.cs.wisc.edu>, rang@cs.wisc.edu (Anton Rang) writes:
<> [...]
<>      SUBROUTINE BLAH(X,Y)
<>      INTEGER X,Y
<>      X = 3
<>      Y = 2
<>      IF (A(X) .EQ. 0) ...
<> [...]
<>   A compiler for FORTRAN can optimize the test to be A(3) since it
<> knows that the assignment to Y can never change X.  A compiler for C,
<> in the absence of interprocedural aliasing information, cannot do the
<> same.
< Assuming that there was no 'EQUIVALENCE (a,b)'  in the routine
< which does the 'CALL BLAH(a,b)'!

Yes, that is the assumption being made.  It is a valid assumption.
An EQUIVALENCE (a,b) in the context you mention is _illegal_, so
the compiler is perfectly free to assume it didn't happen.  Now, it
_is_ true that most Fortran implementations don't actually test to
make sure that the illegal aliasing didn't occur - but _many_ available
Fortran analysis tools can discover these for you.

J. Giles