On 25.08.2019 09:11, Matthew Wilcox wrote: > On Sat, Aug 24, 2019 at 01:01:02PM +0300, Denis Efremov wrote: >> This patch open codes the bitmap_weight() call. The direct >> invocation of hweight_long() allows to remove the BUG_ON and >> excessive "longs to bits, bits to longs" conversion. > > Honestly, that's not the problem with this function. Take a look > at https://danluu.com/assembly-intrinsics/ for a _benchmarked_ > set of problems with popcnt. > >> BUG_ON was required to check that bitmap_weight() will return >> a correct value, i.e. the computed weight will fit the int type >> of the return value. > > What? No. Look at the _arguments_ of bitmap_weight(): > > static __always_inline int bitmap_weight(const unsigned long *src, unsigned int nbits) I'm not sure why it is INT_MAX then? I would expect in case we care only about arguments something like: BUG_ON(longs >= UINT_MAX / BITS_PER_LONG); > >> With this patch memweight() controls the >> computation directly with size_t type everywhere. Thus, the BUG_ON >> becomes unnecessary. > > Why are you bothering? How are you allocating half a gigabyte of memory? > Why are you calling memweight() on half a gigabyte of memory? > No, we don't use such big arrays. However, it's possible to remove BUG_ON and make the code more "straight". Why do we need to "artificially" limit this function to arrays of a particular size if we can relatively simple omit this restriction? > > If you really must change anything, I'd rather see this turned into a > loop: > > while (longs) { > unsigned int nbits; > > if (longs >= INT_MAX / BITS_PER_LONG) > nbits = INT_MAX + 1; > else > nbits = longs * BITS_PER_LONG; > > ret += bitmap_weight((unsigned long *)bitmap, sz); > bytes -= nbits / 8; > bitmap += nbits / 8; > longs -= nbits / BITS_PER_LONG; > } > > then we only have to use Dan Luu's optimisation in bitmap_weight() > and not in memweight() as well. I don't know how the implementation of this optimization will look like in it's final shape, because of different hardware/compiler issues. It looks there are a number of different ways to do it https://arxiv.org/pdf/1611.07612.pdf, http://0x80.pl/articles/sse-popcount.html. However, if it will be based on popcnt instruction I would expect that hweight_long will also contain this intrinsics. Since version 4.9.2 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62011#c13 GCC knows of the false-dependency in popcnt and generates code to handle it (e.g. xor https://godbolt.org/z/Q7AW_d) Thus, I would expect that it's possible to use popcnt intrinsics in hweight_long that would be natively optimized in all loops like "for (...) { res += hweight_long() }" without requiring manual unrolling like in builtin_popcnt_unrolled_errata_manual example of Dan Luu's optimization. > > Also, why does the trailer do this: > > for (; bytes > 0; bytes--, bitmap++) > ret += hweight8(*bitmap); > > instead of calling hweight_long on *bitmap & mask? > Do you mean something like this? longs = bytes; bytes = do_div(longs, sizeof(long)); bitmap_long = (const unsigned long *)bitmap; if (longs) { for (; longs > 0; longs--, bitmap_long++) ret += hweight_long(*bitmap_long); } if (bytes) { ret += hweight_long(*bitmap_long & ((0x1 << bytes * BITS_PER_BYTE) - 1)); } The *bitmap_long will lead to buffer overflow here. Thanks, Denis
