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From: uad1077@dircon.co.uk (Ian Kemmish)
Newsgroups: comp.arch
Subject: Re: Loop instructions
Message-ID: <1991Apr25.181104.5293@dircon.co.uk>
Date: 25 Apr 91 18:11:04 GMT
References: <1991Apr16.152438.3445@waikato.ac.nz> <12739@pt.cs.cmu.edu>
Organization: The Direct Connection, UK
Lines: 32

lindsay@gandalf.cs.cmu.edu (Donald Lindsay) writes:

>In article <1991Apr16.152438.3445@waikato.ac.nz> 
>	ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University) writes:
>>On the 68000 family, the DBcc loop instructions decrement the 16-bit count
>>register and stop looping when it reaches -1 (or 65535), not 0.

>Compiler writers dislike this instruction, but not because of the
>test semantics. The killer is that the count is 16 bits, on a machine
>where variables and expressions are naturally 32 bits. This means
>that the compiler can only use the instruction when it possesses
>range information. 

>A sophisticated compiler (with optimization enabled) can often deduce
>ranges. However, when that machine came out, the instruction was
>essentially only usable by assembler programmers.
>-- 
>Don		D.C.Lindsay 	Carnegie Mellon Robotics Institute
The Sun compiler seemed to have a hack that let people use that
instruction.  The sources to NeWS 1.0 included bits of the
pixrect library.  The *second* register variable in a proc.
was a short, and was fudged so that loops ran to -1, and the
grotty code that did this was always surrounded by comments to
the effect that this ``let the compiler use dbra''.....

And yes, it did seem to matter whether it was the *second* or not:->

-- 
Ian D. Kemmish                    Tel. +44 767 601 361
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