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From: kilgallen@eisner.decus.org (Larry Kilgallen)
Subject: Re: Fortran's Equivalence
Date: 1997/03/26
Message-ID: <1997Mar26.113448.1@eisner>#1/1
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Date: 1997-03-26T00:00:00+00:00
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In article <5hbcdn$i1h@top.mitre.org>, mfb@mbunix.mitre.org (Michael F Brenner) writes:

> manner. Had it done so, then about half of all bit field applications
> would have been portable using address overlays. As it is, true
> portability (without a changing global flag telling where bit one is), is
> difficult to accomplish without using specific operations (like SHIFT),
> which have effects on performance (both speed and logically overspecified
> algorithms). 

But each compiler knows which end is up and has enough information
to optimize away shifts done only for the sake of addressing this
issue.  For instance, many machines have a more efficient set of
instructions for testing bit 3 (from whichever end) without actually
shifting it to the end of a register.

Larry Kilgallen