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From: kdq@demott.com (Kevin D. Quitt)
Newsgroups: comp.sys.m68k
Subject: Re: inst. cache/optimization
Message-ID: <1991Jan10.200633.25442@demott.com>
Date: 10 Jan 91 20:06:33 GMT
References: <1991Jan10.104609.1939@vax5.cit.cornell.edu>
Distribution: comp
Organization: DeMott Electronics Co., Van Nuys CA
Lines: 22


    To answer your second question first, unrolling loops speeds up code
because the loop tests and branches are eliminated.  For a loop wholly
inside the cache, branches are very inexpensive, as they generally fall
into the "best" catagory - 0 or 1 cycles for most common branches.  So,
if you can make it fit in the cache, make it fit. 

    If your program doesn't fit into the cache, it's probably still a
win to keep the cache active, since the cache is filled 32 bits at a
time, and some of your instructions are only going to be 16 bits (so the
second instruction will be a hit).  It's really a tough call, since you
could look at the timing charts and build some super-efficient code
for the portion you lock in, (or if your memory is slow you'd get the
same effect).



-- 
 _
Kevin D. Quitt         demott!kdq   kdq@demott.com
DeMott Electronics Co. 14707 Keswick St.   Van Nuys, CA 91405-1266
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