Maintain cost-centre stacks in the interpreter

Summary:
Breakpoints become SCCs, so we have detailed call-stack info for
interpreted code.  Currently this only works when GHC is compiled with
-prof, but D1562 (Remote GHCi) removes this constraint so that in the
future call stacks will be available without building your own GHCi.

How can you get a stack trace?

* programmatically: GHC.Stack.currentCallStack
* I've added an experimental :where command that shows the stack when
  stopped at a breakpoint
* `error` attaches a call stack automatically, although since calls to
  `error` are often lifted out to the top level, this is less useful
  than it might be (ImplicitParams still works though).
* Later we might attach call stacks to all exceptions

Other related changes in this diff:

* I reduced the number of places that get ticks attached for
  breakpoints.  In particular there was a breakpoint around the whole
  declaration, which was often redundant because it bound no variables.
  This reduces clutter in the stack traces and speeds up compilation.

* I tidied up some RealSrcSpan stuff in InteractiveUI, and made a few
  other small cleanups

Test Plan: validate

Reviewers: ezyang, bgamari, austin, hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1595

GHC Trac Issues: #11047

1 files changed, 9 insertions, 0 deletions

diff --git a/rts/Profiling.c b/rts/Profiling.c
index 982b9461a0..2c2981a02f 100644
--- a/rts/Profiling.c
+++ b/rts/Profiling.c
@@ -227,6 +227,15 @@ freeProfiling (void)
     arenaFree(prof_arena);
 }
 
+CostCentre *mkCostCentre (char *label, char *module, char *srcloc)
+{
+    CostCentre *cc = stgMallocBytes (sizeof(CostCentre), "mkCostCentre");
+    cc->label = label;
+    cc->module = module;
+    cc->srcloc = srcloc;
+    return cc;
+}
+
 static void
 initProfilingLogFile(void)
 {