Re: [PATCH v2] lib/memweight.c: open codes bitmap_weight()
From: Denis Efremov
Date: Fri Sep 13 2019 - 07:48:32 EST
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
this results in a signification simplification.
__bitmap_weight already count last bits with hweight_long as we
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?