the logical xor operator!

[John F. Haugh II]

In article <1310 at ark.cs.vu.nl> maart at cs.vu.nl (Maarten Litmaath) writes:
>Of course the way to come around the deficiency is:
>
>	if (!(expr1) != !(expr2)) {
>		...
>	}
>
>but this only works if !0 always evaluates to the same value (viz. 1), that is,
>the optimizer is not allowed to f*ck around with the comparison.
>!0 is indeed a constant, isn't it?
>-- 
>I'd rather live in Russia             |Maarten Litmaath @ Free U Amsterdam:
>              than in South-Africa... |maart at cs.vu.nl, mcvax!botter!ark!maart

[ hey, chris, here is topic #2 for your posting ]

which, as this discussion went a few months ago, is exactly the case.  the
code you've writen above, modulo any blindness on my part, should work perfect
as an exclusive-or function.  remember, exclusive-or is the not-equivalant
operator, it returns true when the two operands are not boolean equivalant.
the implied comparision against zero is performed automagically in the case
of || and &&.

as far as the optimizer is concerned, it should still be able to perform
any optimizations which would be allowed for any other boolean operators,
except you can't short circuit != or == as you can && and ||.  in particular,
i believe (a == 0)^(b == 0) most likely can't be as well optimized as
(a == 0)!=(b == 0).  i can't prove this to be true, but i very well suspect
it may be.

- john.

ps - your .signature is thought provoking.  i can't decide whether i'd
     rather live in a large communist country which oppresses blacks
     or a small capitalist country which oppresses blacks.  hard choice.
-- 
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