WIP: don't allow Ipoll before Istackoffset

dra27 · dra27 · commit 3b56c5b50213 · 2020-06-03T10:37:10.000+01:00
Failing test: tests/weak-ephe-final/weaklifetime (consistently failing
on Hephaestus - looks repeatable)

The end of the OCaml portion of Weak.create inserts an Ipoll before the
Istackoffset following the Iextcall to caml_weak_create. It's not at
present clear whether this is not a safe point or (more likely) the
frame_descriptor is not correctly indicating that %rax is live.

This repeatably results in the ephemeron not being marked and eventually
the page is freed which manifested (consistently) as the ephemeron
changing length!
diff --git a/asmcomp/polling.ml b/asmcomp/polling.ml
@@ -71,7 +71,7 @@ let rec insert_poll_aux delta instr =
         (* | Iop (Itailcall_ind _) *)
         (* | Iop (Itailcall_imm _) *)
         (* | Iop (Iextcall _) *)
-        | Iop (Istackoffset _)
+        (*| Iop (Istackoffset _)*)
         (* | Iop (Iload _) *)
         (* | Iop (Iintop _) (* signal_alloc.ml *) *)
         | Iop (Inegf)
