I've been toying with the idea myself, I even considered writing a Valgrind plugin to keep a record of how values change in register and memory until I got scar

I've been toying with the idea myself, I even considered writing a Valgrind plugin to keep a record of how values change in register and memory until I got scared enough of the complexity and enormity of the code. I wonder how much memory will it take to run a "usefully large" program with state recording on? In theory you could compress some memory mutations pretty easily (like bzero()ing something) while other mutations would need more memory (writing single ints to various locations). I bet you could generate some nifty 10GB logs with this thing!