src/share/vm/utilities/bitMap.inline.hpp

rev 2896 : 7121547: G1: High number mispredicted branches while iterating over the marking bitmap
Summary: There is a high number of mispredicted branches associated with calling BitMap::iteratate() from within CMBitMapRO::iterate(). Implement a version of CMBitMapRO::iterate() directly using inline-able routines.
Reviewed-by:

*** 176,187 ****
    if (res != (uintptr_t)NoBits) {
      // find the position of the 1-bit
      for (; !(res & 1); res_offset++) {
        res = res >> 1;
      }
!     assert(res_offset >= l_offset &&
!            res_offset < r_offset, "just checking");
      return MIN2(res_offset, r_offset);
    }
    // skip over all word length 0-bit runs
    for (index++; index < r_index; index++) {
      res = map(index);
--- 176,195 ----
    if (res != (uintptr_t)NoBits) {
      // find the position of the 1-bit
      for (; !(res & 1); res_offset++) {
        res = res >> 1;
      }
! 
!     // In the following assert, checking that res_offset is strictly
!     // less than r_offset is too strong. Consider the case where
!     // l_offset is bit 15 and r_offset is bit 17 of the same map word,
!     // and where bits [18:17:16:15] == [01:00:00:00]. The calculation
!     // above would yield the offset of bit 18. All we can assert is that
!     // res_offset is strictly less than size() since we know that there
!     // are set bits at offsets above, but in the same map word as, r_offset
!     // The bits in the range (r_offset:l_offset] are all 0.
!     assert(res_offset >= l_offset && res_offset < size(), "just checking");
      return MIN2(res_offset, r_offset);
    }
    // skip over all word length 0-bit runs
    for (index++; index < r_index; index++) {
      res = map(index);