IA32WatchpointFunctions.setWatchpoint handles 1, 2 or 4 bytes ranges only. [type double on IA32 = 8 bytes]
Unfortunately one watchpoint = max of 4 bytes on IA32. I guess the thing to do here in the client would be if length > 4 set two+ plus watchpoints. In the case of double, one for the top half, one for the bottom half. Likely this would trigger both watchpoints one write, but the high level would have to deal with this interpretation by looking at its own length data.
*** This bug has been marked as a duplicate of 6551 ***