Re: [PATCH v2] lib/memweight.c: open codes bitmap_weight()

[efremov]

Sorry, no question, pointer alignment of course.

Denis Efremov ÐÐÑÐÐ 2019-09-13 14:48:
> Hi,
>
> Sorry for reviving this conversation, but it looks to me like
> this function could be reduced to a single bitmap_weight call:
>
> static inline size_t memweight(const void *ptr, size_t bytes)
> {
>         BUG_ON(bytes >= UINT_MAX / BITS_PER_BYTE);
>         return bitmap_weight(ptr, bytes * BITS_PER_BYTE);
> }
>
> Comparing to the current implementation
> https://elixir.bootlin.com/linux/latest/source/lib/memweight.c#L11
> this results in a signification simplification.
>
> __bitmap_weight already count last bits with hweight_long as we
> discussed earlier.
>
> int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
> {
> 	...
> 	if (bits % BITS_PER_LONG)
> 		w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
> 	...
> }
>
> and __arch_hweight* functions use popcnt instruction.
>
> I've briefly tested the equivalence of 2 implementations on x86_64 with
> fuzzing here: 
> https://gist.github.com/evdenis/95a8b9b8041e09368b31c3a9510491a5
>
> What do you think making this function static inline and moving it
> to include/linux/string.h? I could prepare a patch for it and add some 
> tests for
> memweight and bitmap_weight. Or maybe I miss something again?
>
> Best regards,
> Denis