Of the 7 implementations listed in the gdb page: * gdb's "Native" implementation is unusably slow. 1000x slowdown during recording. * Simics is expensive and AF

Of the 7 implementations listed in the gdb page: * gdb's "Native" implementation is unusably slow. 1000x slowdown during recording. * Simics is expensive and AFAIK a full-system simulator not very useful for debugging applications. * Moxie-ELF is for an obscure custom CPU. SID looks similar. * VMWare have discontinued support for reverse execution. * Chronicle-gdbserver is unusably slow, 300x recording overhead. * UndoDB is the only one that's really usable. We actually have another gdb remote backend supporting reverse execution that's in active use but not publicly available yet --- Pernosco.