TITLE: To divide or shift, that is the question


PROBLEM: Many programmers write shift operators instead of division.


RESPONSE: Steve Clamage (steve@taumet.com), 21 Apr 92

Several posted square-root examples contained code like this:

	s1=(s+xx/s) >> 1;
	a = a << 2;
	b = b << 1;
	a = a>>2;
	b = b>>1;

I recommend writing a multiply or divide when that is what you mean,
rather than writing a shift.  Quality compilers will convert the
multiply by a literal power of two to a shift if it is faster on the
machine, and not otherwise.  

If you write a shift, it is unlikely that the compiler will convert
it to a multiply or divide, even if it were faster.  (Right shifting
is not equivalent to division if the dividend is negative, so you
should be doing these operations with unsigned types in any case.)

If it happens that you have a dumb compiler (the code is faster when
you code a shift), then go ahead and change the code for that compiler.
Hand-optimizing with no knowledge of the characteristics of the
compiler and target machine often results in slower code than if you
let the compiler do the optimizing.