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Small optimization in 'convergeephemerons'

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When converging marks on ephemeron tables, change the direction the
tables are traversed at each iteration, to try to avoid bad-case
scenarios with linked lists of entries in a table.
This commit is contained in:
Roberto Ierusalimschy 2019-08-12 15:32:44 -03:00
parent 09b4e527a0
commit f64a1b175a
1 changed files with 20 additions and 9 deletions
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29
lgc.c
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@ -413,13 +413,13 @@ static void traverseweakvalue (global_State *g, Table *h) {
** (in the atomic phase). In generational mode, it (like all visited
** tables) must be kept in some gray list for post-processing.
*/
static int traverseephemeron (global_State *g, Table *h) {
static int traverseephemeron (global_State *g, Table *h, int inv) {
int marked = 0; /* true if an object is marked in this traversal */
int hasclears = 0; /* true if table has white keys */
int hasww = 0; /* true if table has entry "white-key -> white-value" */
Node *n, *limit = gnodelast(h);
unsigned int i;
unsigned int asize = luaH_realasize(h);
unsigned int nsize = sizenode(h);
/* traverse array part */
for (i = 0; i < asize; i++) {
if (valiswhite(&h->array[i])) {
@ -427,8 +427,10 @@ static int traverseephemeron (global_State *g, Table *h) {
reallymarkobject(g, gcvalue(&h->array[i]));
}
}
/* traverse hash part */
for (n = gnode(h, 0); n < limit; n++) {
/* traverse hash part; if 'inv', traverse descending
(see 'convergeephemerons') */
for (i = 0; i < nsize; i++) {
Node *n = inv ? gnode(h, nsize - 1 - i) : gnode(h, i);
if (isempty(gval(n))) /* entry is empty? */
clearkey(n); /* clear its key */
else if (iscleared(g, gckeyN(n))) { /* key is not marked (yet)? */
@ -490,7 +492,7 @@ static lu_mem traversetable (global_State *g, Table *h) {
if (!weakkey) /* strong keys? */
traverseweakvalue(g, h);
else if (!weakvalue) /* strong values? */
traverseephemeron(g, h);
traverseephemeron(g, h, 0);
else /* all weak */
linkgclist(h, g->allweak); /* nothing to traverse now */
}
@ -620,21 +622,30 @@ static lu_mem propagateall (global_State *g) {
}
/*
** Traverse all ephemeron tables propagating marks from keys to values.
** Repeat until it converges, that is, nothing new is marked. 'dir'
** inverts the direction of the traversals, trying to speed up
** convergence on chains in the same table.
**
*/
static void convergeephemerons (global_State *g) {
int changed;
int dir = 0;
do {
GCObject *w;
GCObject *next = g->ephemeron; /* get ephemeron list */
g->ephemeron = NULL; /* tables may return to this list when traversed */
changed = 0;
while ((w = next) != NULL) {
next = gco2t(w)->gclist;
if (traverseephemeron(g, gco2t(w))) { /* traverse marked some value? */
while ((w = next) != NULL) { /* for each ephemeron table */
next = gco2t(w)->gclist; /* list is rebuilt during loop */
if (traverseephemeron(g, gco2t(w), dir)) { /* marked some value? */
propagateall(g); /* propagate changes */
changed = 1; /* will have to revisit all ephemeron tables */
}
}
} while (changed);
dir = !dir; /* invert direction next time */
} while (changed); /* repeat until no more changes */
}
/* }====================================================== */