a style question

[Henry Spencer]

In article <DAVIS.90Oct1025438 at pacific.mps.ohio-state.edu> davis at pacific.mps.ohio-state.edu  (John E. Davis) writes:
> [ `<' vs `!=' in comparisons ]
>Which generates faster code?  It seems to me that it is easier to tell if two
>values are unequal than to tell if one is greater than the other...

Most machines do all comparisons in the same length of time.  When it
does make a difference, the nature of the difference is highly machine-
specific.

What *can* make a difference is to run the loop backwards, algorithm
permitting:

	for (x = 99; x >= 0; x--)
		...

so that the termination test is a comparison against zero.  That very
often *is* faster than comparison to an arbitrary constant.  When the
algorithm requires an up-counter, it can even be faster to run a down-
counter in parallel and use it for the termination test:

	for (i = 99, x = 0; i >= 0; i--, x++)
		/* code using x */

Obviously this depends somewhat on the availability of registers and
other machine-specific details.
-- 
Imagine life with OS/360 the standard  | Henry Spencer at U of Toronto Zoology
operating system.  Now think about X.  |  henry at zoo.toronto.edu   utzoo!henry