Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!think!snorkelwacker!spdcc!esegue!johnl
From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.lang.misc
Subject: Re: NOT Educating FORTRAN programmers to use C
Message-ID: <1990Jan24.035548.13115@esegue.segue.boston.ma.us>
Date: 24 Jan 90 03:55:48 GMT
References: <12950@cbnewsc.ATT.COM> <14199@lambda.UUCP> <4540@scolex.sco.COM>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 29

In article <4540@scolex.sco.COM> seanf@sco.COM (Sean Fagan) writes:
>	x[i] = x[i] + foo();
>vs.
>	x[i] += foo();
>
>where one or more of x and i are global.  Oops.  If your FORTRAN copmiler
>optimizes that away, you quite probably, have some problems.

If you write code that depends on the order of evaluation of foo() and x[i],
you certainly have problems.  Quoting again from F77, secton 6.6, Evaluation
of Expressions, "The execution of a function reference in a statement may not
alter the value of any other entity within the statement in which the function
appears.  The execution of a function reference in a statement may not alter
the value of any entity in common that affects the value of any other function
reference in that statement."

In a later paragraph it also says that it need not evaluate part of all of
an expression if it can figure out the value some other way.  This explicitly
permits short circuit (C style) evaluation of logical expressions and 
implicitly allows arbitrarily agressive common subexpresion elimination.

People who express strong opinions about what a Fortran compiler should and
shouldn't do would be well advised to have at least a passing familiarity
with what the standard says Fortran really is.  I express no opinions on F90
yet, I'm still chewing through my copy of the draft.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650
johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl
"Now, we are all jelly doughnuts."