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

This reverts commit cb5c2fe875965b7aedbc189012803fc62e48fb3f.

It appears to have broken OSX and Windows builds.

1 files changed, 15 insertions, 3 deletions

diff --git a/rts/StgStartup.cmm b/rts/StgStartup.cmm
index 571e0637fc..f67373031b 100644
--- a/rts/StgStartup.cmm
+++ b/rts/StgStartup.cmm
@@ -62,12 +62,24 @@ INFO_TABLE_RET(stg_stop_thread, STOP_FRAME,
        be an info table on top of the stack).
     */
 
-    // See Note [Unwinding foreign exports on x86-64].
+    /*
+       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.
+     */
 #if defined(x86_64_HOST_ARCH)
-    unwind UnwindReturnReg = STG_RUN_JMP;
+    // 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];
 #else
     // FIXME: Fill in for other platforms
-    unwind UnwindReturnReg = return;
+    unwind MachSp = return,
+           UnwindReturnReg = return;
 #endif
 
     Sp = Sp + SIZEOF_StgStopFrame - WDS(2);