Statistical profiling support in the RTS

Now since GHC can produce useful debugging information (e.g. DWARF annotations and source notes) in compiled objects, it would great if we could leverage this for more efficient profiling.

While ideally we would be able to rely on external tools like perf for this task, this would require that the STG stack be walkable like a standard C stack.

A more feasible alternative is to incorporate a simple statistical profiler into the RTS.

Profiling is defined loosely here as the collection of information describing the current state of evaluation in response to certain triggers. In practice this will be the collection of the current symbol being evaluated triggered on a few events of interest,

• Memory allocation
• Periodic timers for time-based profiling
• Blackhole blocking for locating performance problems due to sharing in parallel programs