Re: CRC in Ada?

[Jim Balter <jqb@netcom.com>]

Robert Dewar wrote:
> 
> Fergus said
> 
> <<It is somewhat surprising that the difference between the user CPU
> times is so large, but I'm more surprised by the speed of the second
> version than by the slowness of the first.>>
> 
> Well it does not surprise me, and I don't really understand why it
> surprises you. I would expect a large difference here, and indeed
> it is what we see!
> 
> Just shows that measurements can be a useful substitute for guesswork,
> though measurements have the weakness of being system specific, so
> nothing really substitutes for a lot of experience on a lot of difrerent
> machiens!

These measurements are totally misleading, and their use and
interpretation show a total *lack* of experience.  If the tests are
coded properly so that the buffer accesses aren't optimized out of
existence, you will see about a 3:1 difference in user cpu time between
the getchar version and the read version, solely as a consequence of the
getchar macro requiring more instructions.  Exactly the same number of
system calls are made, as anyone who attempts to *understand* the
issue instead of indulging in silly sophistry will know.  If you then
do, say, 20 instructions worth of processing per character, the ratio
is 23:21.  *Big* difference, like you'd expect.  Feh.

--
<J Q B>