Messages in this thread | | | From | "Vaittinen, Matti" <> | Subject | Re: [PATCH 1/4] bitops: Add single_bit_set() | Date | Tue, 23 Nov 2021 05:26:56 +0000 |
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Morning <please apply local timezone> Yuru, & all,
On 11/22/21 19:54, Yury Norov wrote: > On Mon, Nov 22, 2021 at 02:57:27PM +0200, Andy Shevchenko wrote: >> On Mon, Nov 22, 2021 at 12:42:21PM +0000, Vaittinen, Matti wrote: >>> On 11/22/21 13:28, Andy Shevchenko wrote: >>>> On Mon, Nov 22, 2021 at 01:03:25PM +0200, Matti Vaittinen wrote: >>>>> There are cases when it is useful to check a bit-mask has only one bit >>>>> set. Add a generic helper for it instead of baking own one for each >>>>> user. >> >>>> So, you decided to reinvent hamming weight... >>>> Please, drop this patch and use corresponding hweight() call. >> >>> Thanks Andy. >>> >>> There are few differences to hamming weight here. We scan only given >>> amount of bits - and we will end scanning immediately when we hit second >>> set bit. Oh, and obviously we only return information whether there is >>> exactly one bit set. So no, this is not hamming weight(). >> >> What do you mean by this? >> >> hweight() will return you the number of the non-zero elements in the set. >> In application to boolean based arrays it means the number of bits that >> are set. Obviously, the condition `hweight() == 1` is what you are looking >> for. > > Hi Andy, > > I think, Matti means earlier return when part of bitmap counts set > bits to a greater nubmer, and we can skip the rest. Right, Matti?
Yes.
> But in general, it might be useful for long bitmaps. > > The more complete way of doing this would be adding a new set of > functions: bitmap_weight_{eq,neq,gt,le} > > I'm looking at how bitmap_weight is used in the kernel and see > quite a lot of places where this optimization may take place. For > example otx2_remove_flow() in drivers/net/ethernet/marvell/octeontx2/nic/otx2_flows.c: > > if (bitmap_weight(&flow_cfg->dmacflt_bmap, > flow_cfg->dmacflt_max_flows) == 1) > otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL); > > may be replaced with: > > if (bitmap_weight_eq(&flow_cfg->dmacflt_bmap, flow_cfg->dmacflt_max_flows, 1) > otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL); > > Most of that places are in drivers however, and the length of bitmaps > there is typically small, so that there's no chance to get any > measurable performance improvement. > > There is always a chance that we have opencoded bitmap_weight_eq() > et all. If we add these API, it might help people wright better code. > > What do you think?
My uneducated opinion (for what it matters :]) is thet the cost of adding such functions is negligible so I am all for adding them if there are even few users who can benefit from those.
Best Regards --Matti Vaittinen
-- The Linux Kernel guy at ROHM Semiconductors
Matti Vaittinen, Linux device drivers ROHM Semiconductors, Finland SWDC Kiviharjunlenkki 1E 90220 OULU FINLAND
~~ this year is the year of a signature writers block ~~
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