On Thu, Jun 8, 2017 at 5:55 AM, Matthew Wilcox <[email protected]> wrote: > On Thu, Jun 08, 2017 at 04:48:04AM +0300, Andy Shevchenko wrote: >> > Commit 7dd968163f ("bitmap: bitmap_equal memcmp optimization") was >> > rather more restrictive than necessary; we can use memcmp() to implement >> > bitmap_equal() as long as the number of bits can be proved to be a >> > multiple of 8. And architectures other than s390 may be able to make >> > good use of this optimisation. >> >> > - if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0) >> > + if (__builtin_constant_p(nbits & 7) && IS_ALIGNED(nbits, 8)) >> > return !memcmp(src1, src2, nbits / 8); >> >> I'm not sure this is a fully correct change. >> What exactly ' & 7' part does? >> For me looks like you may just drop it. > > We only need to know if the bottom 3 bits are 0 to apply this optimisation. > For example, if we have a user which does this: > > nbits = 8; > if (argle) > nbits += 8; > if (bitmap_equal(ptr1, ptr2, nbits)) > blah(); > > then we can use memcmp() because gcc can deduce that the bottom 3 bits > are never set (try it! it works!). We don't need nbits as a whole to > be const.
What I'm talking about is that by my opinion the both below are equivalent. __builtin_constant_p(nbits) __builtin_constant_p(nbits & 7) Thus, again, what & 7 does there? -- With Best Regards, Andy Shevchenko
