On Sat, Dec 26, 2020 at 7:42 AM Syed Nayyar Waris <syednwaris@xxxxxxxxx> wrote: > > This macro iterates for each group of bits (clump) with set bits, > within a bitmap memory region. For each iteration, "start" is set to > the bit offset of the found clump, while the respective clump value is > stored to the location pointed by "clump". Additionally, the > bitmap_get_value() and bitmap_set_value() functions are introduced to > respectively get and set a value of n-bits in a bitmap memory region. > The n-bits can have any size from 1 to BITS_PER_LONG. size less > than 1 or more than BITS_PER_LONG causes undefined behaviour. > Moreover, during setting value of n-bit in bitmap, if a situation arise > that the width of next n-bit is exceeding the word boundary, then it > will divide itself such that some portion of it is stored in that word, > while the remaining portion is stored in the next higher word. Similar > situation occurs while retrieving the value from bitmap. > > GCC gives warning in bitmap_set_value(): https://godbolt.org/z/rjx34r > Add explicit check to see if the value being written into the bitmap > does not fall outside the bitmap. > The situation that it is falling outside would never be possible in the > code because the boundaries are required to be correct before the > function is called. The responsibility is on the caller for ensuring the > boundaries are correct. > The code change is simply to silence the GCC warning messages > because GCC is not aware that the boundaries have already been checked. > As such, we're better off using __builtin_unreachable() here because we > can avoid the latency of the conditional check entirely. Didn't the __builtin_unreachable() end up leading to an objtool warning about incorrect stack frames for the code path that leads into the undefined behavior? I thought I saw a message from the 0day build bot about that and didn't expect to see it again after that. Can you actually measure any performance difference compared to BUG_ON() that avoids the undefined behavior? Practically all CPUs from the past 20 years have branch predictors that should completely hide measurable overhead from this. Arnd
