Il 02/02/2013 16:11, Eric Blake ha scritto: > On 02/02/2013 06:30 AM, Peter Maydell wrote: > >>> - * Undefined if no bit exists, so code should check against 0 first. >>> + * Returns -1 if no bit exists. Note that compared to the C library >>> + * routine ffsl, this one returns one less. >>> */ >> >> Do any of our callers actually use the "-1 on 0 input" semantics? >> (I guess that means "your new code you added" since the previous >> callers all were happy with the undefined-on-zero semantics). > > Yes, Paolo's code was replacing: > > ffsl(var) - 1 > > with > > bitops_ctzl(var) > > where var==0 was a definite possibility, so we DO have code that depends > on this semantic of returning -1.
Actually I'm pretty sure that var == 0 is not possible in hbitmap. But I still prefer having total functions, and also keeping the function monotonic. For example, I would use the number of bits in word for clz(0), since clz(x) is monotonic decreasing. Paolo >> It seems an odd choice, since I can see a justification for: >> (a) "return number of bits in word" [every bit in the word is >> a trailing zero in some sense] >> (b) "undefined" [matches gcc builtin_ctz semantics] > > For all non-zero values of var, ffsl(var) == bitops_ctzl(var)+1. > Extending the equivalency for var==0 makes the function usable in the > most places. >
