Path: utzoo!utgpu!water!watmath!clyde!ima!haddock!karl
From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: "logical" xor
Message-ID: <2237@haddock.ISC.COM>
Date: 13 Jan 88 00:46:53 GMT
References: <2946@zeus.TEK.COM> <195@fxgrp.UUCP> <2800@killer.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 23

Fact 1: "^" is a bitwise XOR operator.  When restricted to normalized boolean
operands, it is equivalent to logical XOR.

Fact 2: "!=" compares two integers.  When restricted to normalized boolean
operands, it is equivalent to logical XOR.

Fact 3: "^^" is a hypothetical logical XOR operator.  It would return 0 if its
operands are both zero or both nonzero, and 1 otherwise.  Unlike the "&&" and
"||" operators, there is no short-circuit possible; both operands are
evaluated.


Conclusion 1: In most applications (and in all applications written by those
of us who distinguish between booleans and integers), "^^" is unnecessary;
one or both of the alternatives will suffice.

Conclusion 2: If the compiler knows that both operands are normalized
booleans, then "x^y" and "x!=y" can generate the same code.  Depending on the
hardware, the best code might be the machine's equivalent of "x^y", "x!=y", or
"x ? !y : y".  Would any compiler-writers out there care to comment on
whether they've implemented this optimization?

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint