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From: rbutterworth@orchid.UUCP
Newsgroups: comp.lang.c
Subject: Generating Boolean values on BSD 4.3
Message-ID: <10249@orchid.waterloo.edu>
Date: Fri, 14-Aug-87 11:20:35 EDT
Article-I.D.: orchid.10249
Posted: Fri Aug 14 11:20:35 1987
Date-Received: Sat, 15-Aug-87 16:42:21 EDT
Distribution: comp
Organization: U of Waterloo, Ontario
Lines: 35


The problem with trying to fake a Boolean type in C is that
a Boolean variable may end up with a value that isn't restricted
to 0 and 1.

One way to help prevent such mistakes is to have Boolean as an
enumerated type, with a macro that turns an expression into
exactly one of the two values.
e.g.

    typedef enum {FALSE=0, TRUE=1} Boolean;

    #define TRUTH(expression) ( (expression) ? TRUE : FALSE )

But of course this can generate fairly inefficient code in many
common cases, such as when the expression is already a 0 or 1.

    #define TRUTH(expression) ((Boolean) ((expression)!=0))
or
    #define TRUTH(expression) ((Boolean) (!!(expression)))

should generate better code.

On BSD 4.3 though, the third example generates much better code
than the second.  e.g. try the following:

    extern int a, b, c;
    a = ((b==c)!=0);
    a = !!(b==c);

The compiler does well with the double not, but fails to optimize
the logically equivalent double comparison verison.

Perhaps this is an obvious optimization that BSD might want to
add to its compiler?