Messages in this thread | | | Date | Tue, 30 Jun 2009 06:18:01 -0400 (EDT) | From | "Robert P. J. Day" <> | Subject | Re: [PATCH] Introduce a boolean "single_bit_set" function. |
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On Tue, 30 Jun 2009, Petr Tesarik wrote:
> Robert P. J. Day píše v Po 29. 06. 2009 v 14:50 -0400:
> > since i originally muttered about this, the rationale behind it > > was not for performance (obviously), but for semantic > > clarification, so that when you saw the expression "n & (n-1)", it > > was more obvious which test you were doing semantically: > > > > 1) is n a power of 2? > > 2) does n represent a single set bit? > > > > nothing ever came of that, but that was the thinking behind it. > > Yes, I can remember and I would still appreciate it. It's always > better to show _what_ the code does rather than _how_ it does it. > > IIRC Andrew rejected your patch on the grounds that it is possible > to replace the expression "n & (n-1)" with "hweight(n) == 1" if one > wants to show that it really tests for a single bit set. But I don't > like his proposal quite as much as yours, because of the big > overhead. > > In short, if you re-post your patch, I'll gladly join you in the > battle of getting it in. ;-)
i never meant for this to turn into a pitched battle of philosophies -- it just seemed like a simple way to make the code clearer, particularly since some of the patches i've submitted allowed for the removal of comments like "test that blocksize is a power of 2" given that what you're testing is now painfully obvious. :-)
if someone has a quick, simple and performance non-crippling suggestion for this, i'm all ears. but there's no point thinking about it if it's actually going to cause performance issues.
rday
p.s. a simple grep to find potential cleanup of the form n&(n-1):
grep -Ern "([^\(\)]+) ?\& ?\(\1 ?- ?1\)" * | less
--
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