Path: utzoo!utgpu!water!watmath!clyde!rutgers!labrea!decwrl!pyramid!voder!tolerant!vsi1!steve
From: steve@vsi1.UUCP (Steve Maurer)
Newsgroups: comp.lang.c
Subject: While talking about useful additions, how about this one??
Summary: An addition which resolves exponentiation & other problems elegantly.
Message-ID: <253@vsi1.UUCP>
Date: 13 Jan 88 05:14:42 GMT
Reply-To: steve@vsi1.UUCP (Steve Maurer)
Organization: Vicom Systems Inc. San Jose, Cal.
Lines: 45
References:

[l]
[line]
[lineater]
> > Why is everyone so interested in a power operator compared to, say, 
> > square root or logarithm?
>
> Basically, it's because exponentiation involving one or two integers
> can usually be done much faster than through use of the floating-point
> pow() function, and in the common case **2 can be highly optimized.

    While we are on the subject of useful additions to C, how about this
    one: pre-compiler switching operators '??' and '::', which ask for
    evaluation at compile time, instead of run time.

    For example, if a novice C user wished to create an optimized form of 
    pow(), without knowing the true implementation of it he might write:

    #define POW(X,Y) (Y) == 0.5 ? sqrt(X) : ( (Y) == 0 ? 1 : ( (Y) == 1 ? X :\ 
		     ( (Y) == 2 ? X * X : pow(X,Y) ) ) )

    intending to 'optimize' the resulting code.   Unfortunately, because
    of the way these operators work, the resulting code is horrendous.

    Consider however, the efficiency gained if the operators are evaluated
    at compile time.    Presuming of course that (Y) above is a CONSTANT, 
    various in line efficiencies could be squeezed out of a machine
    without resorting to numerous additions to the language.

    The actual definitions of '??' and '::' would be:

	* if in the precompiler, the expression evaluates to TRUE, then
	  the statement immediately following ?? is produced, else the statment
	  following the :: is produced.   If no :: is present, and the
	  expression value cannot be determined in the precompiler, then
	  no code is produced.

    So in the above example, with all "?" replaced by "??" and ":" replaced
    by "::", the expression POW(i, 20/10) would be replaced by i * i, with no
    comparisons in the resulting assembly.  { Of course you would still
    have to deal with the macro incremental assignment problem -
    e.g. POW(i++,3) - but such is life }

    Ok, boys.     Shoot it down.......

					    Steve Maurer