Messages in this thread | | | Date | Fri, 27 Jul 2018 10:13:45 +0800 | From | Wei Wang <> | Subject | Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK |
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On 07/26/2018 08:10 PM, Yury Norov wrote: > On Thu, Jul 26, 2018 at 06:15:59PM +0800, Wei Wang wrote: >> External Email >> >> On 07/26/2018 05:37 PM, Yury Norov wrote: >>> On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote: >>>> The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if nbits is >>>> 0. This patch changes the macro to return 0 when there is no bit needs to >>>> be masked. >>> I think this is intentional behavour. Previous version did return ~0UL >>> explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h: improve >>> BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus. >> Yes, I saw that. But it seems confusing for the corner case that nbits=0 >> (no bits to mask), the macro returns with all the bits set. >> >> >>> Introducing conditional branch would affect performance. All existing >>> code checks nbits for 0 before handling last word where needed >>> explicitly. So I think we'd better change nothing here. >> I think that didn't save the conditional branch essentially, because >> it's just moved from inside this macro to the caller as you mentioned. >> If callers missed the check for some reason and passed 0 to the macro, >> they will get something unexpected. >> >> Current callers like __bitmap_weight, __bitmap_equal, and others, they have >> >> if (bits % BITS_PER_LONG) >> w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits)); >> >> we could remove the "if" check by "w += hweight_long(bitmap[k] & >> BITMAP_LAST_WORD_MASK(bits % BITS_PER_LONG));" the branch is the same. > But your patch doesn't remove external conditional, and it fact > introduces overhead, right? Also, in some cases it's not so trivial to > remove it. Consider __bitmap_intersects() for example. > > Anyway, this patch changes the very basic API. In that case you should > check every user of the macro to be safe against the change, including > possible performance downsides. > > If you find this corner case behavior of macro confusing, I think that > the better option would be introducing detailed comment to the > BITMAP_LAST_WORD_MASK(), or writing wrapper around it that handles > nbits == 0 as you expect. >
OK. Thanks Yury and Andy for the discussion. It seems the more preferred way is just to add comments as a note. Agree with that.
Best, Wei
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