"barrier" for link prototype->cache changed to be consistent with

GC behavior (link is cleared to preserve invariant)

lgc.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.c,v 2.145 2013/08/13 17:36:44 roberto Exp roberto $
+** $Id: lgc.c,v 2.146 2013/08/16 18:55:49 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -171,28 +171,6 @@ void luaC_barrierback_ (lua_State *L, GCObject *o) {
 }
 
 
-/*
-** barrier for prototypes. When creating first closure (cache is
-** NULL), use a forward barrier; this may be the only closure of the
-** prototype (if it is a "regular" function, with a single instance)
-** and the prototype may be big, so it is better to avoid traversing
-** it again. Otherwise, use a backward barrier, to avoid marking all
-** possible instances.
-*/
-LUAI_FUNC void luaC_barrierproto_ (lua_State *L, Proto *p, Closure *c) {
-  global_State *g = G(L);
-  lua_assert(isblack(obj2gco(p)));
-  if (p->cache == NULL) {  /* first time? */
-    luaC_objbarrier(L, p, c);
-  }
-  else {  /* use a backward barrier */
-    black2gray(obj2gco(p));  /* make prototype gray (again) */
-    p->gclist = g->grayagain;
-    g->grayagain = obj2gco(p);
-  }
-}
-
-
 /*
 ** check color (and invariants) for an upvalue that is being closed,
 ** i.e., moved into the 'allgc' list

lgc.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.h,v 2.60 2013/08/13 17:36:44 roberto Exp roberto $
+** $Id: lgc.h,v 2.61 2013/08/16 18:55:49 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -128,9 +128,6 @@
    { if (nolocal(obj2gco(o)), isblack(obj2gco(p)) && iswhite(obj2gco(o))) \
 	luaC_barrierback_(L,p); }
 
-#define luaC_barrierproto(L,p,c) \
-   { if (nolocal(obj2gco(c)), isblack(obj2gco(p))) luaC_barrierproto_(L,p,c); }
-
 LUAI_FUNC void luaC_freeallobjects (lua_State *L);
 LUAI_FUNC void luaC_step (lua_State *L);
 LUAI_FUNC void luaC_forcestep (lua_State *L);

lvm.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lvm.c,v 2.176 2013/07/10 17:15:12 roberto Exp roberto $
+** $Id: lvm.c,v 2.177 2013/08/16 18:55:49 roberto Exp roberto $
 ** Lua virtual machine
 ** See Copyright Notice in lua.h
 */
@@ -399,9 +399,9 @@ static Closure *getcached (Proto *p, UpVal **encup, StkId base) {
 
 /*
 ** create a new Lua closure, push it in the stack, and initialize
-** its upvalues. Note that the call to 'luaC_barrierproto' must come
-** before the assignment to 'p->cache', as the function needs the
-** original value of that field.
+** its upvalues. Note that the closure is not cached if prototype is
+** already black (which means that 'cache' was already cleared by the
+** GC).
 */
 static void pushclosure (lua_State *L, Proto *p, UpVal **encup, StkId base,
                          StkId ra) {
@@ -418,8 +418,8 @@ static void pushclosure (lua_State *L, Proto *p, UpVal **encup, StkId base,
       ncl->l.upvals[i] = encup[uv[i].idx];
     /* new closure is white and local, so we do not need a barrier here */
   }
-  luaC_barrierproto(L, p, ncl);
-  p->cache = ncl;  /* save it on cache for reuse */
+  if (!isblack(obj2gco(p)))  /* cache will not break GC invariant? */
+    p->cache = ncl;  /* save it on cache for reuse */
 }