Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: a style question
Message-ID: <1990Oct1.174941.22195@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <7341@darkstar.ucsc.edu> <1990Sep30.050655.13212@zoo.toronto.edu> <1990Sep30.172917.2951@Neon.Stanford.EDU> <DAVIS.90Oct1025438@pacific.mps.ohio-state.edu>
Date: Mon, 1 Oct 90 17:49:41 GMT

In article <DAVIS.90Oct1025438@pacific.mps.ohio-state.edu> davis@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@zoo.toronto.edu   utzoo!henry