On 8 June 2017 at 14:31, Andy Shevchenko <[email protected]> wrote: > On Thu, Jun 8, 2017 at 5:55 AM, Matthew Wilcox <[email protected]> wrote: >> 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)
They are not. Read Matthew's example again. Assuming that argle is something non-constant (maybe an argument to the function), the value of nbits at the time of the bitmap_equal call is _not_ a compile-time-constant. However, if the compiler is smart (which at least some versions of gcc are), the compiler may deduce that nbits is either 8 or 16; there really are no other options. Hence it _is_ statically known that nbits is divisible by 8, so the expression nbits&7 _is_ compile-time constant (0), so gcc can change the bitmap_equal call to a memcmp call. (It may then either pass a run-time value of nbits>>3 and emit a single memcmp call, or it may decide to unroll the two options, creating two memcmp calls with 1 and 2 as compile-time arguments; these may or may not then in turn be "inlined" to code doing roughly *(u8*)p1 == *(u8*)p2 and similarly for u16 casts).
