Add unwind information to Cmm

Unwind information allows the debugger to discover more information
about a program state, by allowing it to "reconstruct" other states of
the program. In practice, this means that we explain to the debugger
how to unravel stack frames, which comes down mostly to explaining how
to find their Sp and Ip register values.

* We declare yet another new constructor for CmmNode - and this time
  there's actually little choice, as unwind information can and will
  change mid-block. We don't actually make use of these capabilities,
  and back-end support would be tricky (generate new labels?), but it
  feels like the right way to do it.

* Even though we only use it for Sp so far, we allow CmmUnwind to specify
  unwind information for any register. This is pretty cheap and could
  come in useful in future.

* We allow full CmmExpr expressions for specifying unwind values. The
  advantage here is that we don't have to make up new syntax, and can e.g.
  use the WDS macro directly. On the other hand, the back-end will now
  have to simplify the expression until it can sensibly be converted
  into DWARF byte code - a process which might fail, yielding NCG panics.
  On the other hand, when you're writing Cmm by hand you really ought to
  know what you're doing.

(From Phabricator D169)

1 files changed, 1 insertions, 0 deletions

diff --git a/rts/StgMiscClosures.cmm b/rts/StgMiscClosures.cmm
index f57fc04263..dd2570617d 100644
--- a/rts/StgMiscClosures.cmm
+++ b/rts/StgMiscClosures.cmm
@@ -47,6 +47,7 @@ INFO_TABLE_RET (stg_stack_underflow_frame, UNDERFLOW_FRAME,
 
 INFO_TABLE_RET (stg_restore_cccs, RET_SMALL, W_ info_ptr, W_ cccs)
 {
+    unwind Sp = Sp + WDS(2);
 #if defined(PROFILING)
     CCCS = Sp(1);
 #endif