The main goal here is enable stack traces in GHCi. After this change,
if you start GHCi like this:
ghci -fexternal-interpreter -prof
(which requires packages to be built for profiling, but not GHC
itself) then the interpreter manages cost-centre stacks during
execution and can produce a stack trace on request. Call locations
are available for all interpreted code, and any compiled code that was
built with the -fprof-auto familiy of flags.
There are a couple of ways to get a stack trace:
- error/undefined automatically get one attached
- Debug.Trace.traceStack can be used anywhere, and prints the current stack
Because the interpreter is running in a separate process, only the
interpreted code is running in profiled mode and the compiler itself
isn't slowed down by profiling.
The GHCi debugger still doesn't work with -fexternal-interpreter,
although this patch gets it a step closer. Most of the functionality
of breakpoints is implemented, but the runtime value introspection is
still not supported.
Along the way I also did some refactoring and added type arguments to
the various remote pointer types in GHCi.RemotePtr, so there's
better type safety and documentation in the bridge code between GHC