Fix unwinding of C -> Haskell FFI calls with -threaded

See the new note.

Test Plan:
manual testing with patched gdb

Reviewers: bgamari, simonmar, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4666

1 files changed, 3 insertions, 15 deletions

diff --git a/rts/StgStartup.cmm b/rts/StgStartup.cmm
index f67373031b..571e0637fc 100644
--- a/rts/StgStartup.cmm
+++ b/rts/StgStartup.cmm
@@ -62,24 +62,12 @@ INFO_TABLE_RET(stg_stop_thread, STOP_FRAME,
        be an info table on top of the stack).
     */
 
-    /*
-       Here we setup the stack unwinding annotation necessary to allow
-       debuggers to find their way back to the C stack.
-
-       This is a bit fiddly as we assume the layout of the stack prepared
-       for us by StgRun. Note that in most cases StgRun is written in assembler
-       and therefore has no associated unwind information. For this reason we
-       need to identify the platform stack pointer and return address values for
-       the StgRun's caller.
-     */
+    // See Note [Unwinding foreign exports on x86-64].
 #if defined(x86_64_HOST_ARCH)
-    // offset of 8 in MachSp value due to return address
-    unwind MachSp = MachSp + RESERVED_C_STACK_BYTES + STG_RUN_STACK_FRAME_SIZE + 8,
-           UnwindReturnReg = W_[MachSp + RESERVED_C_STACK_BYTES + STG_RUN_STACK_FRAME_SIZE];
+    unwind UnwindReturnReg = STG_RUN_JMP;
 #else
     // FIXME: Fill in for other platforms
-    unwind MachSp = return,
-           UnwindReturnReg = return;
+    unwind UnwindReturnReg = return;
 #endif
 
     Sp = Sp + SIZEOF_StgStopFrame - WDS(2);