SATBMarkQueue presently has a bool _active member for the active flag. The _index member is scaled to a byte index into the word-valued buffer, so the low LogBytesPerWord bits are always zero. We could also encode the (inverted) active state in _index, with a value of min_intx indicating inactive. This would allow a smaller and faster dispatch among (1) inactive, (2) active fast path, and (3) active slow path. Rather than the existing sequence of two load-test-branch and a later subtraction on the enqueue path, we could have something like (for x86 &etc, for example)

  mov rindex, index_offset(rthread)
  sub rindex, #BytesPerWord
  jo #inactive        // branch on overflow (min_intx - BytesPerWord overflows)
  jl #slow_path     // branch on negative (0 - BytesPerWord is negative)
  // fast path code
  // can use rindex directly, with no further adjustments needed.

[Updated for JDK-8258255, which moved the active flag from PtrQueue to SATBMarkQueue.]