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From: jlg@lanl-a.UUCP
Newsgroups: net.physics
Subject: Fortran vs. C
Message-ID: <10309@lanl-a.UUCP>
Date: Thu, 12-Jul-84 13:07:15 EDT
Article-I.D.: lanl-a.10309
Posted: Thu Jul 12 13:07:15 1984
Date-Received: Sat, 14-Jul-84 01:01:54 EDT
Organization: Los Alamos National Laboratory
Lines: 18

>C explicitly does NOT rearrange most arithmetic operations
>(exceptions are supposedly commutative operators).

That's not the problem.  C rearranges 'supposedly' associative operations,
even when the expression has been parenthesized to prevent it.  For example:

a = b + (c + d);

The C compiler may (and frequently does) do (b+c)+d or (b+d)+c instead of
b+(c+d).  Since floating point addition is NOT associative, the two
rearrangements may give incorrect answers.  The problem is magnified when
there is a large expression to be compiled with several non-associative
adds present -- often the result from C is completely unpredictable.  This
is probably one of the reasons that ALL C arithmetic is double precision,
i.e. so that bad instruction scheduling won't be noticed as often.

				J.L.Giles
				...inhp4!cmcl2!lanl-a!jlg