Subject:
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Re: NQC 2.0 and some math questions
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Newsgroups:
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lugnet.robotics.rcx.nqc
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Date:
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Thu, 30 Sep 1999 06:01:04 GMT
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Viewed:
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2262 times
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In article <37f16547.6612368@lugnet.com>, marcob@equalis.it wrote:
> x = y % z
>
> is equivalent to (where w is an temporary integer):
>
> w = y / z;
> x = y - z * w;
That's exaclty what I was thinking of doing. Originally, the compiler had
no way of allocating temp variables which is why code for % couldn't be
emitted. The temp allocator is now pretty good, so I could emit the above
sequence, but it just didn't occur to me until a few days ago.
There is one subtle problem. The operations above won't happen
atomically, so there are some concurrency issues. To be completely safe,
I'd have to latch both y and z into temps as well. Without this, some
truly weird stuff can happen. For example, lets say that intially y is a
multiple of z, and then you decrement y in-between the computation of w
and x. The result will be that x is negative. This is a result that is
inconsistient with the decrement happening either before or after the
entire mod calculation.
My personal feeling on this, however, is that if you're running around
changing y amd z in one task while calculating with them in another,
you're just asking for trouble.
Dave Baum
--
reply to: dbaum at enteract dot com
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