Don't use special return for finalizer running

For now, we'll handle this by checking if there's anything in the
finalizer queue each time we exit a frame. It's not ideal, in that it
costs something every frame exit, but it unblocks eliminating the
special return mechanism, which will in turn unlock removing many other
checks on frame exit, so it's a reasonable trade-off.

src/core/callstack.c

@@ -732,6 +732,8 @@ MVMFrame * MVM_callstack_unwind_frame(MVMThreadContext *tc, MVMuint8 exceptional
                 MVM_panic(1, "Unknown call stack record type in unwind");
         }
     } while (tc->stack_top && !is_bytecode_frame(tc->stack_top->kind));
+    if (tc->num_finalizing && !exceptional && MVM_gc_finalize_run_handler(tc))
+        *thunked = 1;
     return tc->stack_top ? MVM_callstack_record_to_frame(tc->stack_top) : NULL;
 }
 

src/gc/finalize.c

@@ -26,39 +26,6 @@ void MVM_gc_finalize_add_to_queue(MVMThreadContext *tc, MVMObject *obj) {
     tc->num_finalize++;
 }
 
-/* Sets the passed thread context's thread up so that we'll run a finalize
- * handler on it in the near future. */
-static void finalize_handler_caller(MVMThreadContext *tc, void *sr_data) {
-    MVMCode *handler = MVM_hll_current(tc)->finalize_handler;
-    if (handler) {
-        /* Drain the finalizing queue to an array. */
-        MVMObject *drain;
-        MVMROOT(tc, handler, {
-            drain = MVM_repr_alloc_init(tc, tc->instance->boot_types.BOOTArray);
-            while (tc->num_finalizing > 0)
-                MVM_repr_push_o(tc, drain, tc->finalizing[--tc->num_finalizing]);
-        });
-
-        /* Invoke the handler. */
-        MVMCallStackArgsFromC *args_record = MVM_callstack_allocate_args_from_c(tc,
-                MVM_callsite_get_common(tc, MVM_CALLSITE_ID_OBJ));
-        args_record->args.source[0].o = drain;
-        MVM_frame_dispatch_from_c(tc, handler, args_record, NULL, MVM_RETURN_VOID);
-    }
-}
-static void setup_finalize_handler_call(MVMThreadContext *tc) {
-    MVMFrame *install_on = tc->cur_frame;
-    while (install_on) {
-        if (!install_on->extra || !install_on->extra->special_return)
-            if (install_on->static_info->body.cu->body.hll_config)
-                break;
-        install_on = install_on->caller;
-    }
-    if (install_on)
-        MVM_frame_special_return(tc, install_on, finalize_handler_caller, NULL,
-            NULL, NULL);
-}
-
 /* Walks through the per-thread finalize queues, identifying objects that
  * should be finalized, pushing them onto a finalize list, and then marking
  * that list entry. Assumes the world is stopped. */
@@ -108,11 +75,31 @@ void MVM_finalize_walk_queues(MVMThreadContext *tc, MVMuint8 gen) {
     while (cur_thread) {
         if (cur_thread->body.tc) {
             walk_thread_finalize_queue(cur_thread->body.tc, gen);
-            if (cur_thread->body.tc->num_finalizing > 0) {
+            if (cur_thread->body.tc->num_finalizing > 0)
                 MVM_gc_collect(cur_thread->body.tc, MVMGCWhatToDo_Finalizing, gen);
-                setup_finalize_handler_call(cur_thread->body.tc);
-            }
         }
         cur_thread = cur_thread->body.next;
     }
 }
+
+/* Try to run a finalization handler. Returns a true value if we do so */
+MVMint32 MVM_gc_finalize_run_handler(MVMThreadContext *tc) {
+    MVMCode *handler = MVM_hll_current(tc)->finalize_handler;
+    if (handler) {
+        /* Drain the finalizing queue to an array. */
+        MVMObject *drain;
+        MVMROOT(tc, handler, {
+            drain = MVM_repr_alloc_init(tc, tc->instance->boot_types.BOOTArray);
+            while (tc->num_finalizing > 0)
+                MVM_repr_push_o(tc, drain, tc->finalizing[--tc->num_finalizing]);
+        });
+
+        /* Invoke the handler. */
+        MVMCallStackArgsFromC *args_record = MVM_callstack_allocate_args_from_c(tc,
+                MVM_callsite_get_common(tc, MVM_CALLSITE_ID_OBJ));
+        args_record->args.source[0].o = drain;
+        MVM_frame_dispatch_from_c(tc, handler, args_record, NULL, MVM_RETURN_VOID);
+        return 1;
+    }
+    return 0;
+}

src/gc/finalize.h

@@ -1,3 +1,4 @@
 void MVM_gc_finalize_set(MVMThreadContext *tc, MVMObject *type, MVMint64 finalize);
 void MVM_gc_finalize_add_to_queue(MVMThreadContext *tc, MVMObject *obj);
 void MVM_finalize_walk_queues(MVMThreadContext *tc, MVMuint8 gen);
+MVMint32 MVM_gc_finalize_run_handler(MVMThreadContext *tc);