On Thursday, October 14, 2010 11:49:23 pm Garrett Cooper wrote: > On Thu, Oct 14, 2010 at 6:37 AM, John Baldwin <j...@freebsd.org> wrote: > > On Thursday, October 14, 2010 7:58:32 am Andriy Gapon wrote: > >> on 14/10/2010 00:30 Garrett Cooper said the following: > >> > I was talking to someone today about this macro, and he noted that > >> > the algorithm is incorrect -- it fails the base case with ((x) == 0 -- > >> > which makes sense because 2^(x) cannot equal 0 (mathematically > >> > impossible, unless you consider the limit as x goes to negative > >> > infinity as log (0) / log(2) is undefined). I tested out his claim and > >> > he was right: > >> > >> That's kind of obvious given the code. > >> I think that this might be an intentional optimization. > >> I guess that it doesn't really make sense to apply powerof2 to zero and > >> the users > >> of the macro should do the check on their own if they expect zero as input > >> (many > >> places in the do not allow that). > > But the point is that this could be micro-optimizing things > incorrectly. I'm running simple iteration tests to see what the > performance is like, but the runtime is going to take a while to > produce stable results. > > Mathematically there is a conflict with the definition of the macro, > so it might confuse folks who pay attention to the math as opposed to > the details (if you want I'll gladly add a comment around the macro in > a patch to note the caveats of using powerof2).
We aren't dealing with mathematicians, but programmers. > > I agree, the current macro is this way on purpose (and straight out of > > "Hacker's Delight"). And this book trumps you on that case. Using the powerof2() macro as it currently stands is a widely-used practice among folks who write systems-level code. If you were writing a powerof2() function for a higher level language where performance doesn't matter and bit twiddling isn't common, then a super-safe variant of powerof2() might be appropriate. However, this is C, and C programmers are expected to know how this stuff works. > > sys/net/flowtable.c: ft->ft_lock_count = > > 2*(powerof2(mp_maxid + 1) ? (mp_maxid + 1): > > > > Clearly, 'mp_maxid + 1' will not be zero (barring a bizarre overflow case > > which will not happen until we support 2^32 CPUs), so this is fine. > > But that should be caught by the mp_machdep code, correct? Yes, hence "bizarre". It is also way unrealistic and not worth excessive pessimizations scattered throughout the tree. > What about the other patches? The mfiutil and mptutil ones at least > get the two beforementioned tools in sync with sys/param.h at least, > so I see some degree of value in the patches (even if they're just > cleanup). No, powerof2() should not change. It would most likely be a POLA violation to change how it works given 1) it's historical behavior, and 2) it's underlying idiom's common (and well-understood) use among the software world. -- John Baldwin _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
