rr is the debugger (on Linux) that's almost too good to be true.
May I ask which platform you are working on? I cannot get rr to build on Ubuntu 14.04, because of issues installing the dependencies.
I installed 4.0 from the .deb fine, and then later I built it from source without issues (apt-geting the debs just worked), both on Ubuntu 15.04.
PSA: Don't run the rr test suite unless you've first checked that your CPU is supported. It will do bad things if it isn't.
Edit: Now fixed.