Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!hc!beta!unm-la!unmvax!nmtsun!hydrovax
From: hydrovax@nmtsun.nmt.edu (M. Warner Losh)
Newsgroups: comp.lang.c
Subject: Re: mini-flame re: Fortran arguments and `noalias'
Summary: ON VMS it isn't even used.
Keywords: pragmas, compiler hints
Message-ID: <1226@nmtsun.nmt.edu>
Date: 13 Jan 88 20:44:47 GMT
References: <485@cresswell.quintus.UUCP> <1453@cuuxb.ATT.COM> <9934@mimsy.UUCP> <10144@mimsy.UUCP>
Organization: NMT Hydrology program
Lines: 44

In article <10144@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> >In article <9934@mimsy.UUCP> I mentioned
> >>Some FORTRAN compilers may indeed use copy-in/copy-out ...
> >>but I think it is not mandated.
> >       ^^^^^
> Seriously, though, since value-result is *not* mandated, those
> writing Fortran code should not count on it.

On VMS btw, FORTRAN uses call by reference for ALL of it's arguements.
This presents many aliasing problems.  Consider the following code:

	PROGRAM FOO
	COMMON X
	X = 4
	CALL BAR (X)
	END

	SUBROUTINE BAR (Y)

	COMMON X

	Y = X * Y
	WRITE (*,*) X
	RETURN
	END

now then, X and Y are aliases for each other.  Yes, Quite a problem.  In
fact, I seem to remeber seeing something like this breaking the Optimizer
in an ancient version of VMS FORTRAN.  The point is, that if your compiler
uses call by reference (which I maintain that most do), then you will get
16 printed.  If it uses call by value-return (copy-in,copy-out), then you
should get 4.  The point is that not only should you not write code that
depends on one, or the other of these features, because it is non-portable.
You should avoid it because it produces LOTS AND LOTS of hard to find
bugs.



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