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Re^4: 5x6-bit values into/out of a 32-bit word

by Thelonius (Curate)
on Mar 18, 2007 at 15:01 UTC ( #605360=note: print w/ replies, xml ) Need Help??


in reply to Re^3: 5x6-bit values into/out of a 32-bit word
in thread 5x6-bit values into/out of a 32-bit word

This is a very interesting point, one very much worth considering. In Perl, of course, the difference would not be worth mentioning, but in C, my unrolled-loop version is much faster than a version with a loop. I was guessing that it would be twice as fast, but when I actually measured it, it was actually five times as fast.

Now, of course, it was microseconds v. microseconds, but why are we packing 5 numbers into one 32-bit word? Presumably we care about the space usage, which would only matter if we are using a lot of them, probably millions, so all those microseconds can add up.

The maintenance concerns are, in this specific case, probably not valid. You can't pack 6x6-bit values in a 32-bit word, nor 5x7-bit values. We would have to change the algorithm if anything changed. And, if you actually write out the loop version, you've probably only saved one line of code.

I'm not disagreeing with your principles, but I think that in this case I would probably go with my version.

There's a very good essay, The Fallacy of Premature Optimization. One snippet:

Note, however, that Hoare did not say, "Forget about small efficiencies all of the time." Instead, he said "about 97% of the time." This means that about 3% of the time we really should worry about small efficiencies. That may not sound like much, but consider that this is 1 line of source code out of every 33. How many programmers worry about the small efficiencies even this often? Premature optimization is always bad, but the truth is that some concern about small efficiencies during program development is not premature.


Comment on Re^4: 5x6-bit values into/out of a 32-bit word
Re^5: 5x6-bit values into/out of a 32-bit word
by grinder (Bishop) on Mar 18, 2007 at 17:37 UTC
    In Perl, of course, the difference would not be worth mentioning, but in C, my unrolled-loop version is much faster than a version with a loop.

    Yes, but this is Perl we are talking about, a language where a cosine is really no more expensive than an addition, not C. Perl is about flexibility, C is about speed.

    if we are using a lot of them, probably millions

    If you have millions of them, then Perl is the wrong language to use.

    All I know is that seeing those multiples of 6 raises a red flag, at least as far as Perl is concerned. That code strikes me as fragile, in that it does not adapt to changing requirements readily (which is one of the reasons I code in Perl). Remember the cardinal virtue of Laziness.

    Also, while I don't know why BrowserUK wants to do this, I maintain my reasoning is as valid as yours. There are dozens of ways of packing small bitmaps into a 32-bit quantity, and far more can be packed into a 64-bit quantity, as 64-bit CPUs become more prevalent.

    • another intruder with the mooring in the heart of the Perl

Re^5: 5x6-bit values into/out of a 32-bit word
by blazar (Canon) on Mar 19, 2007 at 11:11 UTC
    Note, however, that Hoare did not say, "Forget about small efficiencies all of the time." Instead, he said "about 97% of the time." This means that about 3% of the time we really should worry about small efficiencies.

    As far as I knew, Hoare (implictly) did say "all of the time." The actual quotation being simply and forever:

    Premature optimization is the root of all evil.

    it was Knuth who paraphrased Hoare like thus:

    We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%.

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