On Sun, Aug 25, 2019 at 02:39:47PM +0300, Denis Efremov wrote: > 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); People aren't always terribly consistent with INT_MAX vs UINT_MAX. Also, bitmap_weight() should arguably return an unisnged int (it can't legitimately return a negative value). > >> 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? You're not making a great case for changing the implementation of memweight() here ... > 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. The problem with using XMM registers is that they have to be saved/restored. Not to mention the thermal issues caused by heavy usage of AVX instructions. > 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 Ah! Glad to see GCC knows about this problem and has worked around 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. That might be expecting rather more from our compiler than is reasonable ... > > > > 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. No it won't. The CPU will access more bytes than the `bytes' argument would seem to imply -- but it's going to have fetched that entire cacheline anyway. It might confuse a very strict bounds checking library, but usually those just check you're not accessing outside your object, which is going to be a multiple of 'sizeof(long)' anyway. If we do something like this, we'll need to use an 'inverse' of that mask on big-endian machines. ie something more like: if (bytes) { unsigned long mask; if (_BIG_ENDIAN) mask = ~0UL >> (bytes * 8); else mask = ~0UL << (bytes * 8); ret += hweight_long(*bitmap_long & ~mask); } Also we need a memweight() test to be sure we didn't get that wrong.
