Removed internal cache for closures

The mechanism of "caching the last closure created for a prototype to
try to reuse it the next time a closure for that prototype is created"
was removed. There are several reasons:

- It is hard to find a natural example where this cache has a measurable
impact on performance.

- Programmers already perceive closure creation as something slow,
so they tend to avoid it inside hot paths. (Any case where the cache
could reuse a closure can be rewritten predefining the closure in some
variable and using that variable.)

- The implementation was somewhat complex, due to a bad interaction
with the generational collector. (Typically, new closures are new,
while prototypes are old. So, the cache breaks the invariant that
old objects should not point to new ones.)

9 files changed, 10 insertions, 123 deletions

diff --git a/lfunc.c b/lfunc.c
index 15874f88..aa6ce58f 100644
--- a/lfunc.c
+++ b/lfunc.c
@@ -219,8 +219,6 @@ Proto *luaF_newproto (lua_State *L) {
   f->p = NULL;
   f->sizep = 0;
   f->code = NULL;
-  f->cache = NULL;
-  f->cachemiss = 0;
   f->sizecode = 0;
   f->lineinfo = NULL;
   f->sizelineinfo = 0;
diff --git a/lgc.c b/lgc.c
index 9d196a18..95a8ad5b 100644
--- a/lgc.c
+++ b/lgc.c
@@ -221,27 +221,6 @@ void luaC_barrierback_ (lua_State *L, GCObject *o) {
 }
 
 
-/*
-** 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);
-  lua_assert(g->gckind != KGC_GEN || isold(p));
-  if (getage(p) == G_OLD1)  /* still need to be visited? */
-    linkgclist(p, g->grayagain);  /* link it in 'grayagain' */
-  else
-    linkgclist(p, g->protogray);  /* link it in 'protogray' */
-  black2gray(p);  /* make prototype gray */
-}
-
-
 void luaC_fix (lua_State *L, GCObject *o) {
   global_State *g = G(L);
   lua_assert(g->allgc == o);  /* object must be 1st in 'allgc' list! */
@@ -379,7 +358,7 @@ static int remarkupvals (global_State *g) {
 */
 static void restartcollection (global_State *g) {
   g->gray = g->grayagain = NULL;
-  g->weak = g->allweak = g->ephemeron = g->protogray = NULL;
+  g->weak = g->allweak = g->ephemeron = NULL;
   markobject(g, g->mainthread);
   markvalue(g, &g->l_registry);
   markmt(g);
@@ -535,39 +514,12 @@ static int traverseudata (global_State *g, Udata *u) {
 
 
 /*
-** Check the cache of a prototype, to keep invariants. If the
-** 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
-** is ok too. (It will naturally be visited again.) If the
-** 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;
-** then, it can become gray without problems for its other fields.)
-*/
-static void checkprotocache (global_State *g, Proto *p) {
-  if (p->cache) {
-    if (iswhite(p->cache))
-      p->cache = NULL;  /* allow cache to be collected */
-    else if (g->gckind == KGC_GEN && !isold(p->cache) && getage(p) >= G_OLD1) {
-      linkgclist(p, g->protogray);  /* link it in 'protogray' */
-      black2gray(p);  /* make prototype gray */
-    }
-  }
-  p->cachemiss = 0;  /* restart counting */
-}
-
-
-/*
 ** Traverse a prototype. (While a prototype is being build, its
 ** arrays can be larger than needed; the extra slots are filled with
 ** NULL, so the use of 'markobjectN')
 */
 static int traverseproto (global_State *g, Proto *f) {
   int i;
-  checkprotocache(g, f);
   markobjectN(g, f->source);
   for (i = 0; i < f->sizek; i++)  /* mark literals */
     markvalue(g, &f->k[i]);
@@ -696,19 +648,6 @@ static void convergeephemerons (global_State *g) {
 ** =======================================================
 */
 
-static void clearprotolist (global_State *g) {
-  GCObject *p = g->protogray;
-  g->protogray = NULL;
-  while (p != NULL) {
-    Proto *pp = gco2p(p);
-    GCObject *next = pp->gclist;
-    lua_assert(isgray(pp) && (pp->cache != NULL || pp->cachemiss >= MAXMISS));
-    gray2black(pp);
-    checkprotocache(g, pp);
-    p = next;
-  }
-}
-
 
 /*
 ** clear entries with unmarked keys from all weaktables in list 'l'
@@ -1439,7 +1378,6 @@ static lu_mem atomic (lua_State *L) {
   clearbyvalues(g, g->weak, origweak);
   clearbyvalues(g, g->allweak, origall);
   luaS_clearcache(g);
-  clearprotolist(g);
   g->currentwhite = cast_byte(otherwhite(g));  /* flip current white */
   lua_assert(g->gray == NULL);
   return work;  /* estimate of slots marked by 'atomic' */
diff --git a/lgc.h b/lgc.h
index e0a8806b..6b1c2861 100644
--- a/lgc.h
+++ b/lgc.h
@@ -164,9 +164,6 @@
         (isblack(p) && iswhite(o)) ? \
         luaC_barrier_(L,obj2gco(p),obj2gco(o)) : cast_void(0))
 
-#define luaC_protobarrier(L,p,o) \
-        (isblack(p) ? luaC_protobarrier_(L,p) : cast_void(0))
-
 LUAI_FUNC void luaC_fix (lua_State *L, GCObject *o);
 LUAI_FUNC void luaC_freeallobjects (lua_State *L);
 LUAI_FUNC void luaC_step (lua_State *L);
@@ -175,7 +172,6 @@ LUAI_FUNC void luaC_fullgc (lua_State *L, int isemergency);
 LUAI_FUNC GCObject *luaC_newobj (lua_State *L, int tt, size_t sz);
 LUAI_FUNC void luaC_barrier_ (lua_State *L, GCObject *o, GCObject *v);
 LUAI_FUNC void luaC_barrierback_ (lua_State *L, GCObject *o);
-LUAI_FUNC void luaC_protobarrier_ (lua_State *L, Proto *p);
 LUAI_FUNC void luaC_checkfinalizer (lua_State *L, GCObject *o, Table *mt);
 LUAI_FUNC void luaC_changemode (lua_State *L, int newmode);
 
diff --git a/lobject.h b/lobject.h
index ea3511de..df31a1a0 100644
--- a/lobject.h
+++ b/lobject.h
@@ -505,7 +505,6 @@ typedef struct Proto {
   lu_byte numparams;  /* number of fixed (named) parameters */
   lu_byte is_vararg;
   lu_byte maxstacksize;  /* number of registers needed by this function */
-  lu_byte cachemiss;  /* count for successive misses for 'cache' field */
   int sizeupvalues;  /* size of 'upvalues' */
   int sizek;  /* size of 'k' */
   int sizecode;
@@ -516,7 +515,6 @@ typedef struct Proto {
   int linedefined;  /* debug information  */
   int lastlinedefined;  /* debug information  */
   TValue *k;  /* constants used by the function */
-  struct LClosure *cache;  /* last-created closure with this prototype */
   Instruction *code;  /* opcodes */
   struct Proto **p;  /* functions defined inside the function */
   Upvaldesc *upvalues;  /* upvalue information */
diff --git a/lstate.c b/lstate.c
index 4a2453d1..9d399959 100644
--- a/lstate.c
+++ b/lstate.c
@@ -340,7 +340,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   g->finobjsur = g->finobjold = g->finobjrold = NULL;
   g->sweepgc = NULL;
   g->gray = g->grayagain = NULL;
-  g->weak = g->ephemeron = g->allweak = g->protogray = NULL;
+  g->weak = g->ephemeron = g->allweak = NULL;
   g->twups = NULL;
   g->totalbytes = sizeof(LG);
   g->GCdebt = 0;
diff --git a/lstate.h b/lstate.h
index f08c2355..ce337707 100644
--- a/lstate.h
+++ b/lstate.h
@@ -43,8 +43,6 @@
 ** '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.
-** There is also a list 'protogray' for prototypes that need to have
-** their caches cleared.
 
 */
 
@@ -186,7 +184,6 @@ typedef struct global_State {
   GCObject *weak;  /* list of tables with weak values */
   GCObject *ephemeron;  /* list of ephemeron tables (weak keys) */
   GCObject *allweak;  /* list of all-weak tables */
-  GCObject *protogray;  /* list of prototypes with "new" caches */
   GCObject *tobefnz;  /* list of userdata to be GC */
   GCObject *fixedgc;  /* list of objects not to be collected */
   /* fields for generational collector */
diff --git a/ltests.c b/ltests.c
index 7cf08891..aba44b87 100644
--- a/ltests.c
+++ b/ltests.c
@@ -283,7 +283,6 @@ static void checkudata (global_State *g, Udata *u) {
 static void checkproto (global_State *g, Proto *f) {
   int i;
   GCObject *fgc = obj2gco(f);
-  checkobjref(g, fgc, f->cache);
   checkobjref(g, fgc, f->source);
   for (i=0; i<f->sizek; i++) {
     if (ttisstring(f->k + i))
@@ -417,8 +416,6 @@ static void checkobject (global_State *g, GCObject *o, int maybedead,
         getage(o) == G_TOUCHED1 ||
         getage(o) == G_OLD0 ||
         o->tt == LUA_TTHREAD ||
-        (o->tt == LUA_TPROTO &&
-            (gco2p(o)->cache != NULL || gco2p(o)->cachemiss >= MAXMISS)) ||
         (o->tt == LUA_TUPVAL && upisopen(gco2upv(o))));
       }
     }
@@ -452,7 +449,6 @@ static void checkgrays (global_State *g) {
   checkgraylist(g, g->grayagain);
   checkgraylist(g, g->weak);
   checkgraylist(g, g->ephemeron);
-  checkgraylist(g, g->protogray);
 }
 
 
diff --git a/lvm.c b/lvm.c
index 1535700f..d9671055 100644
--- a/lvm.c
+++ b/lvm.c
@@ -684,30 +684,8 @@ lua_Integer luaV_shiftl (lua_Integer x, lua_Integer y) {
 
 
 /*
-** check whether cached closure in prototype 'p' may be reused, that is,
-** whether there is a cached closure with the same upvalues needed by
-** new closure to be created.
-*/
-static LClosure *getcached (Proto *p, UpVal **encup, StkId base) {
-  LClosure *c = p->cache;
-  if (c != NULL) {  /* is there a cached closure? */
-    int nup = p->sizeupvalues;
-    Upvaldesc *uv = p->upvalues;
-    int i;
-    for (i = 0; i < nup; i++) {  /* check whether it has right upvalues */
-      TValue *v = uv[i].instack ? s2v(base + uv[i].idx) : encup[uv[i].idx]->v;
-      if (c->upvals[i]->v != v)
-        return NULL;  /* wrong upvalue; cannot reuse closure */
-    }
-    p->cachemiss = 0;  /* got a hit */
-  }
-  return c;  /* return cached closure (or NULL if no cached closure) */
-}
-
-
-/*
 ** create a new Lua closure, push it in the stack, and initialize
-** its upvalues. ???
+** its upvalues.
 */
 static void pushclosure (lua_State *L, Proto *p, UpVal **encup, StkId base,
                          StkId ra) {
@@ -724,13 +702,6 @@ static void pushclosure (lua_State *L, Proto *p, UpVal **encup, StkId base,
       ncl->upvals[i] = encup[uv[i].idx];
     luaC_objbarrier(L, ncl, ncl->upvals[i]);
   }
-  if (p->cachemiss >= MAXMISS)  /* too many missings? */
-    p->cache = NULL;  /* give up cache */
-  else {
-    p->cache = ncl;  /* save it on cache for reuse */
-    luaC_protobarrier(L, p, ncl);
-    p->cachemiss++;
-  }
 }
 
 
@@ -1811,13 +1782,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
       }
       vmcase(OP_CLOSURE) {
         Proto *p = cl->p->p[GETARG_Bx(i)];
-        LClosure *ncl = getcached(p, cl->upvals, base);  /* cached closure */
-        if (ncl == NULL) {  /* no match? */
-          savestate(L, ci);  /* in case of allocation errors */
-          pushclosure(L, p, cl->upvals, base, ra);  /* create a new one */
-        }
-        else
-          setclLvalue2s(L, ra, ncl);  /* push cashed closure */
+        halfProtect(pushclosure(L, p, cl->upvals, base, ra));
         checkGC(L, ra + 1);
         vmbreak;
       }
diff --git a/testes/closure.lua b/testes/closure.lua
index 5d090d91..cdeaebaa 100644
--- a/testes/closure.lua
+++ b/testes/closure.lua
@@ -44,18 +44,17 @@ assert(B.g == 19)
 
 -- testing equality
 a = {}
-collectgarbage"stop"
-for i = 1, 5 do  a[i] = function (x) return x + a + _ENV end  end
-collectgarbage"restart"
-assert(a[3] == a[4] and a[4] == a[5])
 
 for i = 1, 5 do  a[i] = function (x) return i + a + _ENV end  end
 assert(a[3] ~= a[4] and a[4] ~= a[5])
 
-local function f()
-  return function (x)  return math.sin(_ENV[x])  end
+do
+  local a = function (x)  return math.sin(_ENV[x])  end
+  local function f()
+    return a
+  end
+  assert(f() == f())
 end
-assert(f() == f())
 
 
 -- testing closures with 'for' control variable