Messages in this thread | | | Date | Wed, 7 Jun 2017 19:55:56 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH 3/3] bitmap: Use memcmp optimisation in more situations |
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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.
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