Optimization in 'markold'

OLD1 objects can be potentially anywhere in the 'allgc' list (up
to 'reallyold'), but frequently they are all after 'old1' (natural
evolution of survivals) or do not exist at all (when all objects die
young). So, instead of 'markold' starts looking for them always
from the start of 'allgc', the collector keeps an extra pointer,
'firstold1', that points to the first OLD1 object in the 'allgc' list,
or is NULL if there are no OLD1 objects in that list.

lstate.h

@@ -46,6 +46,15 @@
 ** lists. Moreover, barriers can age young objects in young lists as
 ** OLD0, which then become OLD1. However, a list never contains
 ** elements younger than their main ages.
+**
+** The generational collector also uses a pointer 'firstold1', which
+** points to the first OLD1 object in the list. It is used to optimize
+** 'markold'. (Potentially OLD1 objects can be anywhere between 'allgc'
+** and 'reallyold', but often the list has no OLD1 objects or they are
+** after 'old1'.) Note the difference between it and 'old1':
+** 'firstold1': no OLD1 objects before this point; there can be all
+**   ages after it.
+** 'old1': no objects younger than OLD1 after this point.
 */
 
 /*
@@ -54,7 +63,7 @@
 ** can become gray have such a field. The field is not the same
 ** in all objects, but it always has this name.)  Any gray object
 ** must belong to one of these lists, and all objects in these lists
-** must be gray:
+** must be gray (with one exception explained below):
 **
 ** 'gray': regular gray objects, still waiting to be visited.
 ** 'grayagain': objects that must be revisited at the atomic phase.
@@ -65,6 +74,12 @@
 ** 'weak': tables with weak values to be cleared;
 ** 'ephemeron': ephemeron tables with white->white entries;
 ** 'allweak': tables with weak keys and/or weak values to be cleared.
+**
+** The exception to that "gray rule" is the TOUCHED2 objects in
+** generational mode. Those objects stay in a gray list (because they
+** must be visited again at the end of the cycle), but they are marked
+** black (because assignments to them must activate barriers, to move
+** them back to TOUCHED1).
 */
 
 
@@ -266,6 +281,7 @@ typedef struct global_State {
   GCObject *survival;  /* start of objects that survived one GC cycle */
   GCObject *old1;  /* start of old1 objects */
   GCObject *reallyold;  /* objects more than one cycle old ("really old") */
+  GCObject *firstold1;  /* first OLD1 object in the list (if any) */
   GCObject *finobjsur;  /* list of survival objects with finalizers */
   GCObject *finobjold1;  /* list of old1 objects with finalizers */
   GCObject *finobjrold;  /* list of really old objects with finalizers */