Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!mintaka!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: a style question
Message-ID: <18365@haddock.ima.isc.com>
Date: 2 Oct 90 02:35:08 GMT
References: <7341@darkstar.ucsc.edu> <1990Sep30.050655.13212@zoo.toronto.edu> <1990Sep30.172917.2951@Neon.Stanford.EDU> <DAVIS.90Oct1025438@pacific.mps.ohio-state.edu>
Reply-To: karl@kelp.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 16

In article <DAVIS.90Oct1025438@pacific.mps.ohio-state.edu> davis@pacific.mps.ohio-state.edu (John E. Davis) writes:
>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.

On vaxlike machines, both forms of comparison will generate a compare
instruction (which sets condition codes) followed by a conditional branch,
the only difference being whether the "can't happen" case is included or
excluded from the conditions that cause a branch.  I wouldn't expect any
timing difference.

On a segmented machine where pointers are being compared, relational compares
are *faster*, since the compiler is allowed to assume that they are members of
the same array, and hence the same segment.  Equality compares have to check
the segment part as well.

Karl W. Z. Heuer (karl@kelp.ima.isc.com or ima!kelp!karl), The Walking Lint