We (PyPy developers) tried this and it's unusably slow. I don't know how they got that slow, but it takes a bit forever to execute 10mln instructions. We ended up using UndoDB gdb wrapper, which runs slower than they advertise (+- the same speed as valgrind so say 30x give or take), but is a lot more usable. They had stability problems in the past, but with the recent versions, we're able to run stuff like complex python programs on top of pypy on top of UndoDB, which is very impressive. Very useful for hard GC bugs.