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Comments about OLD0/OLD1 ages 

Improved the comments in file 'lgc.c' explaining the roles of "ages"
OLD0 and OLD1 in the generacional collector.
This commit is contained in:
Roberto Ierusalimschy 2018-07-18 11:43:45 -03:00
parent 2e297d6ab3
commit ccae0f5aad
1 changed files with 30 additions and 24 deletions
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54
lgc.c
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@ -181,9 +181,13 @@ static int iscleared (global_State *g, const GCObject *o) {
/*
** barrier that moves collector forward, that is, mark the white object
** being pointed by a black object. (If in sweep phase, clear the black
** object to white [sweep it] to avoid other barrier calls for this
** same object.)
** 'v' being pointed by the black object 'o'. (If in sweep phase, clear
** the black object to white [sweep it] to avoid other barrier calls for
** this same object.) In the generational mode, 'v' must also become
** old, if 'o' is old; however, it cannot be changed directly to OLD,
** because it may still point to non-old objects. So, it is marked as
** OLD0. In the next cycle it will become OLD1, and in the next it
** will finally become OLD (regular old).
*/
void luaC_barrier_ (lua_State *L, GCObject *o, GCObject *v) {
global_State *g = G(L);
@ -218,13 +222,14 @@ void luaC_barrierback_ (lua_State *L, GCObject *o) {
/*
** Barrier for prototype's cache of closures. For an 'old1'
** object, making it gray stops it from being visited by 'markold',
** so it is linked in the 'grayagain' list to ensure it will be
** visited. Otherwise, it goes to 'protogray', as only its 'cache' field
** needs to be revisited. (A prototype to be in this barrier must be
** already finished, so its other fields cannot change and do not need
** to be revisited.)
** Barrier for prototype's cache of closures. It turns the prototype
** back to gray (it was black). For an 'OLD1' prototype, making it
** gray stops it from being visited by 'markold', so it is linked in
** the 'grayagain' list to ensure it will be visited. For other ages,
** it goes to the 'protogray' list, as only its 'cache' field needs to
** be revisited. (A prototype to be in this barrier must be already
** finished, so its other fields cannot change and do not need to be
** revisited.)
*/
LUAI_FUNC void luaC_protobarrier_ (lua_State *L, Proto *p) {
global_State *g = G(L);
@ -233,7 +238,7 @@ LUAI_FUNC void luaC_protobarrier_ (lua_State *L, Proto *p) {
linkgclist(p, g->grayagain); /* link it in 'grayagain' */
else
linkgclist(p, g->protogray); /* link it in 'protogray' */
black2gray(p); /* make prototype gray (to avoid other barriers) */
black2gray(p); /* make prototype gray */
}
@ -533,9 +538,9 @@ static int traverseudata (global_State *g, Udata *u) {
** cache is white, clear it. (A cache should not prevent the
** collection of its reference.) Otherwise, if in generational
** mode, check the generational invariant. If the cache is old,
** everything is ok. If the prototype is 'old0', everything
** everything is ok. If the prototype is 'OLD0', everything
** is ok too. (It will naturally be visited again.) If the
** prototype is older than 'old0', then its cache (which is new)
** prototype is older than 'OLD0', then its cache (which is new)
** must be visited again in the next collection, so the prototype
** goes to the 'protogray' list. (If the prototype has a cache,
** it is already immutable and does not need other barriers;
@ -1085,9 +1090,9 @@ static void whitelist (global_State *g, GCObject *p) {
/*
** Correct a list of gray objects. Because this correction is
** done after sweeping, young objects can be white and still
** be in the list. They are only removed.
** Correct a list of gray objects.
** Because this correction is done after sweeping, young objects might
** be turned white and still be in the list. They are only removed.
** For tables and userdata, advance 'touched1' to 'touched2'; 'touched2'
** objects become regular old and are removed from the list.
** For threads, just remove white ones from the list.
@ -1104,13 +1109,14 @@ static GCObject **correctgraylist (GCObject **p) {
changeage(curr, G_TOUCHED1, G_TOUCHED2);
p = next; /* go to next element */
}
else {
if (!iswhite(curr)) {
else { /* not touched in this cycle */
if (!iswhite(curr)) { /* not white? */
lua_assert(isold(curr));
if (getage(curr) == G_TOUCHED2)
changeage(curr, G_TOUCHED2, G_OLD);
if (getage(curr) == G_TOUCHED2) /* advance from G_TOUCHED2... */
changeage(curr, G_TOUCHED2, G_OLD); /* ... to G_OLD */
gray2black(curr); /* make it black */
}
/* else, object is white: just remove it from this list */
*p = *next; /* remove 'curr' from gray list */
}
break;
@ -1146,7 +1152,7 @@ static void correctgraylists (global_State *g) {
/*
** Mark 'old1' objects when starting a new young collection.
** Mark 'OLD1' objects when starting a new young collection.
** Gray objects are already in some gray list, and so will be visited
** in the atomic step.
*/
@ -1177,9 +1183,9 @@ static void finishgencycle (lua_State *L, global_State *g) {
/*
** Does a young collection. First, mark 'old1' objects. (Only survival
** and "recent old" lists can contain 'old1' objects. New lists cannot
** contain 'old1' objects, at most 'old0' objects that were already
** Does a young collection. First, mark 'OLD1' objects. (Only survival
** and "recent old" lists can contain 'OLD1' objects. New lists cannot
** contain 'OLD1' objects, at most 'OLD0' objects that were already
** visited when marked old.) Then does the atomic step. Then,
** sweep all lists and advance pointers. Finally, finish the collection.
*/