Put parallel sweep impl behind --enable-parallel-sweep configure option

Author: luke-gruber

configure.ac

@@ -4293,6 +4293,16 @@ AC_ARG_ENABLE(debug-env,
        AS_HELP_STRING([--enable-debug-env], [enable RUBY_DEBUG environment variable]),
        [AC_SUBST(ENABLE_DEBUG_ENV, yes)])
 
+AC_ARG_ENABLE(parallel-sweep,
+       AS_HELP_STRING([--enable-parallel-sweep],
+                      [enable background sweep thread in the default GC [[disabled by default]]]),
+       [parallel_sweep=$enableval], [parallel_sweep=no])
+AS_IF([test "x$parallel_sweep" = xyes], [
+    RUBY_APPEND_OPTIONS(CPPFLAGS, -DUSE_PARALLEL_SWEEP=1)
+], [
+    RUBY_APPEND_OPTIONS(CPPFLAGS, -DUSE_PARALLEL_SWEEP=0)
+])
+
 AS_IF([test "$gnumake" = yes], [ NULLCMD=: ], [
     AC_MSG_CHECKING([for safe null command for ${MAKE-make}])
     mkdir conftest.dir

gc.c

@@ -980,6 +980,7 @@ rb_objspace_free(void *objspace)
 
 void rb_gc_impl_parallel_sweep_start(void *objspace_ptr);
 
+#if USE_PARALLEL_SWEEP
 void
 rb_gc_parallel_sweep_start(void)
 {
@@ -989,6 +990,7 @@ rb_gc_parallel_sweep_start(void)
 #endif
     rb_gc_impl_parallel_sweep_start(rb_gc_get_objspace());
 }
+#endif
 
 size_t
 rb_gc_obj_slot_size(VALUE obj)
@@ -2484,7 +2486,7 @@ rb_gc_obj_free_concurrency_safe_vm_weak_references(VALUE obj)
     ASSUME(!RB_SPECIAL_CONST_P(obj));
     bool result = obj_free_object_id(obj, false);
 
-    if (RB_UNLIKELY(rb_obj_gen_fields_p(obj))) {
+    if (rb_obj_gen_fields_p(obj)) {
         bool freed_generic = rb_free_generic_ivar(obj);
         if (!freed_generic) result = false;
     }
@@ -2503,14 +2505,14 @@ rb_gc_obj_free_concurrency_safe_vm_weak_references(VALUE obj)
     return result;
 }
 
-bool
+void
 rb_gc_obj_free_vm_weak_references(VALUE obj)
 {
     ASSUME(!RB_SPECIAL_CONST_P(obj));
 
     obj_free_object_id(obj, true);
 
-    if (RB_UNLIKELY(rb_obj_gen_fields_p(obj))) {
+    if (rb_obj_gen_fields_p(obj)) {
         rb_free_generic_ivar(obj);
     }
 
@@ -2538,7 +2540,6 @@ rb_gc_obj_free_vm_weak_references(VALUE obj)
       default:
         break;
     }
-    return true;
 }
 
 /*
@@ -4402,7 +4403,7 @@ vm_weak_table_frozen_strings_foreach(VALUE *str, void *data)
     }
 
     if (retval == ST_DELETE) {
-        FL_UNSET(*str, RSTRING_FSTR);
+        FL_UNSET_RAW(*str, RSTRING_FSTR);
     }
 
     return retval;