Messages in this thread | | | Subject | Re: [PATCH] Introduce a boolean "single_bit_set" function. | From | Petr Tesarik <> | Date | Tue, 30 Jun 2009 08:12:59 +0200 |
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Robert P. J. Day píše v Po 29. 06. 2009 v 14:50 -0400: > On Mon, 29 Jun 2009, Petr Tesarik wrote: > > > Andrew Morton píše v Pá 24. 04. 2009 v 10:46 -0700: > > > On Fri, 24 Apr 2009 06:40:39 -0400 (EDT) "Robert P. J. Day" <rpjday@crashcourse.ca> wrote: > > > > > > > so it would be a simple matter to define the bit set boolean in > > > > terms of hweight_long(), yes? so what about, in bitops.h: > > > > > > > > static inline bool > > > > exactly_one_bit_set(unsigned long w) > > > > { > > > > return hweight_long(w) == 1; > > > > } > > > > > > > > static inline bool > > > > more_than_one_bit_set(unsigned long w) > > > > { > > > > return hweight_long(w) > 1; > > > > } > > > > > > > > Andrew, you must be kidding! Are you seriously suggesting to replace > > a simple and instruction with a call to an extern library function > > with 17 instructions (not including the call and ret)? > > > > I'd better check the use of hweight in the kernel to eradicate as > > many calls to it as possible... > > 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. ;-)
Petr Tesarik
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