Enable stack traces with ghci -fexternal-interpreter -prof

Summary:
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
and ghc-iserv.

Test Plan: validate

Reviewers: bgamari, ezyang, austin, hvr, goldfire, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1747

GHC Trac Issues: #11047, #11100

1 files changed, 6 insertions, 22 deletions

diff --git a/libraries/ghci/GHCi/ResolvedBCO.hs b/libraries/ghci/GHCi/ResolvedBCO.hs
index 9234210418..a349dedaba 100644
--- a/libraries/ghci/GHCi/ResolvedBCO.hs
+++ b/libraries/ghci/GHCi/ResolvedBCO.hs
@@ -6,6 +6,7 @@ module GHCi.ResolvedBCO
 
 import SizedSeq
 import GHCi.RemoteTypes
+import GHCi.BreakArray
 
 import Data.Array.Unboxed
 import Data.Binary
@@ -32,31 +33,14 @@ instance Binary ResolvedBCO
 data ResolvedBCOPtr
   = ResolvedBCORef Int
       -- ^ reference to the Nth BCO in the current set
-  | ResolvedBCOPtr HValueRef
+  | ResolvedBCOPtr (RemoteRef HValue)
       -- ^ reference to a previously created BCO
-  | ResolvedBCOStaticPtr RemotePtr
+  | ResolvedBCOStaticPtr (RemotePtr ())
       -- ^ reference to a static ptr
   | ResolvedBCOPtrBCO ResolvedBCO
       -- ^ a nested BCO
-  | ResolvedBCOPtrLocal HValue
-      -- ^ something local, cannot be serialized
+  | ResolvedBCOPtrBreakArray (RemoteRef BreakArray)
+      -- ^ Resolves to the MutableArray# inside the BreakArray
   deriving (Generic, Show)
 
--- Manual Binary instance is needed because we cannot serialize
--- ResolvedBCOPtrLocal.  This will go away once we have support for
--- remote breakpoints.
-instance Binary ResolvedBCOPtr where
-  put (ResolvedBCORef a) = putWord8 0 >> put a
-  put (ResolvedBCOPtr a) = putWord8 1 >> put a
-  put (ResolvedBCOStaticPtr a) = putWord8 2 >> put a
-  put (ResolvedBCOPtrBCO a) = putWord8 3 >> put a
-  put (ResolvedBCOPtrLocal _) =
-    error "Cannot serialize a local pointer.  Use -fno-external-interpreter?"
-
-  get = do
-    w <- getWord8
-    case w of
-      0 -> ResolvedBCORef <$> get
-      1 -> ResolvedBCOPtr <$> get
-      2 -> ResolvedBCOStaticPtr <$> get
-      _ -> ResolvedBCOPtrBCO <$> get
+instance Binary ResolvedBCOPtr