Re: [PATCH] Introduce a boolean "single_bit_set" function.

Robert P. J. Day
2009-06-30T10:20:23Z

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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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