No longer let the GC replace dead keys with the LJ_TDEADKEY tag.

Important: this changes the semantics of the write barrier!
Carefully read the big comment block in lj_obj.h
This helps HREFK key slot specialization and allows safely hoisting
HREF/HREFK across GC steps, too (fix for a barely reproducible bug).
Dead keys are only removed during a table resize (as before).

src/lib_base.c

@@ -54,7 +54,7 @@ LJLIB_PUSH("upval")
 LJLIB_PUSH("thread")
 LJLIB_PUSH("proto")
 LJLIB_PUSH("function")
-LJLIB_PUSH("deadkey")
+LJLIB_PUSH("")  /* Unused. */
 LJLIB_PUSH("table")
 LJLIB_PUSH(top-8)  /* userdata */
 LJLIB_PUSH("number")

src/lj_api.c

@@ -196,7 +196,8 @@ LUA_API int lua_type(lua_State *L, int idx)
     return LUA_TNONE;
   } else {  /* Magic internal/external tag conversion. ORDER LJ_T */
     int t = ~itype(o);
-    return (int)(((t < 8 ? 0x98a42110 : 0x75b6) >> 4*(t&7)) & 15u);
+    lua_assert(itype(o) != LJ_TUPVAL);
+    return (int)(((t < 8 ? 0x98042110 : 0x7506) >> 4*(t&7)) & 15u);
   }
 }
 
@@ -631,7 +632,7 @@ LUALIB_API int luaL_newmetatable(lua_State *L, const char *tname)
     GCtab *mt = lj_tab_new(L, 0, 1);
     settabV(L, tv, mt);
     settabV(L, L->top++, mt);
-    lj_gc_objbarriert(L, regt, mt);
+    lj_gc_anybarriert(L, regt);
     return 1;
   } else {
     copyTV(L, L->top++, tv);
@@ -899,7 +900,7 @@ LUA_API void lua_rawset(lua_State *L, int idx)
   key = L->top-2;
   dst = lj_tab_set(L, t, key);
   copyTV(L, dst, key+1);
-  lj_gc_barriert(L, t, dst);
+  lj_gc_anybarriert(L, t);
   L->top = key;
 }
 

src/lj_gc.c

@@ -186,13 +186,10 @@ static int gc_traverse_tab(global_State *g, GCtab *t)
     MSize i, hmask = t->hmask;
     for (i = 0; i <= hmask; i++) {
       Node *n = &node[i];
-      lua_assert(itype(&n->key) != LJ_TDEADKEY || tvisnil(&n->val));
       if (!tvisnil(&n->val)) {  /* Mark non-empty slot. */
 	lua_assert(!tvisnil(&n->key));
 	if (!(weak & LJ_GC_WEAKKEY)) gc_marktv(g, &n->key);
 	if (!(weak & LJ_GC_WEAKVAL)) gc_marktv(g, &n->val);
-      } else if (tvisgcv(&n->key)) {  /* Leave GC key in, but mark as dead. */
-	setitype(&n->key, LJ_TDEADKEY);
       }
     }
   }
@@ -424,11 +421,8 @@ static void gc_clearweak(GCobj *o)
 	Node *n = &node[i];
 	/* Clear hash slot when key or value is about to be collected. */
 	if (!tvisnil(&n->val) && (gc_mayclear(&n->key, 0) ||
-				  gc_mayclear(&n->val, 1))) {
+				  gc_mayclear(&n->val, 1)))
 	  setnilV(&n->val);
-	  if (tvisgcv(&n->key))  /* Leave GC key in, but mark as dead. */
-	    setitype(&n->key, LJ_TDEADKEY);
-	}
       }
     }
     o = gcref(t->gclist);

src/lj_gc.h

@@ -69,6 +69,8 @@ LJ_FUNC void lj_gc_barriertrace(global_State *g, void *T);
 #endif
 
 /* Barrier for stores to table objects. TValue and GCobj variant. */
+#define lj_gc_anybarriert(L, t)  \
+  { if (isblack(obj2gco(t))) lj_gc_barrierback(G(L), (t)); }
 #define lj_gc_barriert(L, t, tv) \
   { if (tviswhite(tv) && isblack(obj2gco(t))) \
       lj_gc_barrierback(G(L), (t)); }

src/lj_ir.h

@@ -317,7 +317,7 @@ typedef enum {
   IRT_THREAD,
   IRT_PROTO,
   IRT_FUNC,
-  IRT_9,		/* LJ_TDEADKEY is never used in the IR. */
+  IRT_9,		/* Never used in the IR. */
   IRT_TAB,
   IRT_UDATA,
   /* ... until here. */

src/lj_lib.c

@@ -53,7 +53,7 @@ void lj_lib_register(lua_State *L, const char *libname,
   ptrdiff_t tpos = L->top - L->base;
 
   /* Avoid barriers further down. */
-  if (isblack(obj2gco(tab))) lj_gc_barrierback(G(L), tab);
+  lj_gc_anybarriert(L, tab);
   tab->nomm = 0;
 
   for (;;) {

src/lj_meta.c

@@ -134,7 +134,7 @@ TValue *lj_meta_tset(lua_State *L, cTValue *o, cTValue *k)
       TValue *tv = lj_tab_set(L, t, k);
       if (!tvisnil(tv) ||
 	  !(mo = lj_meta_fast(L, tabref(t->metatable), MM_newindex))) {
-	if (isblack(obj2gco(t))) lj_gc_barrierback(G(L), t);
+	lj_gc_anybarriert(L, t);
 	return tv;
       }
     } else if (tvisnil(mo = lj_meta_lookup(L, o, MM_newindex))) {

src/lj_obj.c

@@ -11,12 +11,12 @@
 /* Object type names. */
 LJ_DATADEF const char *const lj_obj_typename[] = {  /* ORDER LUA_T */
   "no value", "nil", "boolean", "userdata", "number", "string",
-  "table", "function", "userdata", "thread", "proto", "upval"
+  "table", "function", "userdata", "thread", "proto"
 };
 
 LJ_DATADEF const char *const lj_obj_itypename[] = {  /* ORDER LJ_T */
   "nil", "boolean", "boolean", "userdata", "string", "upval", "thread",
-  "proto", "function", "deadkey", "table", "userdata", "number"
+  "proto", "function", "" /* Unused */, "table", "userdata", "number"
 };
 
 /* Compare two objects without calling metamethods. */
@@ -25,14 +25,8 @@ int lj_obj_equal(cTValue *o1, cTValue *o2)
   if (itype(o1) == itype(o2)) {
     if (tvispri(o1))
       return 1;
-    if (!tvisnum(o1)) {
+    if (!tvisnum(o1))
-#if LJ_64
+      return gcrefeq(o1->gcr, o2->gcr);
-      if (tvislightud(o1))
-	return o1->u64 == o2->u64;
-      else
-#endif
-	return gcrefeq(o1->gcr, o2->gcr);
-    }
   } else if (!tvisnum(o1) || !tvisnum(o2)) {
     return 0;
   }

src/lj_obj.h

@@ -75,9 +75,37 @@ typedef struct GCRef {
 ** a barrier has been omitted are annotated with a NOBARRIER comment.
 **
 ** The same logic applies for stores to table slots (array part or hash
-** part). ALL uses of lj_tab_set* require a barrier for the stored *value*
+** part). ALL uses of lj_tab_set* require a barrier for the stored value
-** (if it's a GC object). The barrier for the *key* is already handled
+** *and* the stored key, based on the above rules. In practice this means
-** internally by lj_tab_newkey.
+** a barrier is needed if *either* of the key or value are a GC object.
+**
+** It's ok to LEAVE OUT the write barrier in the following special cases:
+** - The stored value is nil. The key doesn't matter because it's either
+**   not resurrected or lj_tab_newkey() will take care of the key barrier.
+** - The key doesn't matter if the *previously* stored value is guaranteed
+**   to be non-nil (because the key is kept alive in the table).
+** - The key doesn't matter if it's guaranteed not to be part of the table,
+**   since lj_tab_newkey() takes care of the key barrier. This applies
+**   trivially to new tables, but watch out for resurrected keys. Storing
+**   a nil value leaves the key in the table!
+**
+** In case of doubt use lj_gc_anybarriert() as it's rather cheap. It's used
+** by the interpreter for all table stores.
+**
+** Note: In contrast to Lua's GC, LuaJIT's GC does *not* specially mark
+** dead keys in tables. The reference is left in, but it's guaranteed to
+** be never dereferenced as long as the value is nil. It's ok if the key is
+** freed or if any object subsequently gets the same address.
+**
+** Not destroying dead keys helps to keep key hash slots stable. This avoids
+** specialization back-off for HREFK when a value flips between nil and
+** non-nil and the GC gets in the way. It also allows safely hoisting
+** HREF/HREFK across GC steps. Dead keys are only removed if a table is
+** resized (i.e. by NEWREF) and xREF must not be CSEd across a resize.
+**
+** The trade-off is that a write barrier for tables must take the key into
+** account, too. Implicitly resurrecting the key by storing a non-nil value
+** may invalidate the incremental GC invariant.
 */
 
 /* -- Common type definitions --------------------------------------------- */
@@ -136,10 +164,7 @@ typedef const TValue cTValue;
 
 /* More external and GCobj tags for internal objects. */
 #define LAST_TT		LUA_TTHREAD
-
 #define LUA_TPROTO	(LAST_TT+1)
-#define LUA_TUPVAL	(LAST_TT+2)
-#define LUA_TDEADKEY	(LAST_TT+3)
 
 /* Internal object tags.
 **
@@ -170,7 +195,7 @@ typedef const TValue cTValue;
 #define LJ_TTHREAD		(-7)
 #define LJ_TPROTO		(-8)
 #define LJ_TFUNC		(-9)
-#define LJ_TDEADKEY		(-10)
+/* Unused			(-10) */
 #define LJ_TTAB			(-11)
 #define LJ_TUDATA		(-12)
 /* This is just the canonical number type used in some places. */
@@ -689,7 +714,7 @@ static LJ_AINLINE int32_t lj_num2bit(lua_Number n)
 /* -- Miscellaneous object handling --------------------------------------- */
 
 /* Names and maps for internal and external object tags. */
-LJ_DATA const char *const lj_obj_typename[1+LUA_TUPVAL+1];
+LJ_DATA const char *const lj_obj_typename[1+LUA_TPROTO+1];
 LJ_DATA const char *const lj_obj_itypename[~LJ_TNUMX+1];
 
 #define typename(o)	(lj_obj_itypename[itypemap(o)])

src/lj_opt_fold.c

@@ -1351,8 +1351,9 @@ LJFOLDF(fwd_xload)
 /* Write barriers are amenable to CSE, but not across any incremental
 ** GC steps.
 **
-** The same logic applies to open upvalue references, because the stack
+** The same logic applies to open upvalue references, because a stack
-** may be resized during a GC step.
+** may be resized during a GC step (not the current stack, but maybe that
+** of a coroutine).
 */
 LJFOLD(TBAR any)
 LJFOLD(OBAR any any)

src/lj_parse.c

@@ -189,6 +189,7 @@ static BCReg const_gc(FuncState *fs, GCobj *gc, int itype)
   lua_State *L = fs->L;
   TValue o, *val;
   setgcV(L, &o, &gc->gch, itype);
+  /* NOBARRIER: the key is new or kept alive. */
   val = lj_tab_set(L, fs->kt, &o);
   if (tvisnum(val))
     return val->u32.lo;
@@ -206,6 +207,7 @@ static BCReg const_str(FuncState *fs, ExpDesc *e)
 /* Anchor string constant to avoid GC. */
 GCstr *lj_parse_keepstr(LexState *ls, const char *str, size_t len)
 {
+  /* NOBARRIER: the key is new or kept alive. */
   lua_State *L = ls->L;
   GCstr *s = lj_str_new(L, str, len);
   TValue *tv = lj_tab_setstr(L, ls->fs->kt, s);
@@ -1202,6 +1204,7 @@ static GCproto *fs_finish(LexState *ls, BCLine line)
   lua_assert(ls->fs != NULL || ls->token == TK_eof);
   /* Re-anchor last string token to avoid GC. */
   if (ls->token == TK_name || ls->token == TK_string) {
+    /* NOBARRIER: the key is new or kept alive. */
     TValue *tv = lj_tab_setstr(ls->L, ls->fs->kt, strV(&ls->tokenval));
     if (tvisnil(tv)) setboolV(tv, 1);
   }
@@ -1346,8 +1349,7 @@ static void expr_table(LexState *ls, ExpDesc *e)
       vcall = 0;
       expr_kvalue(&k, &key);
       expr_kvalue(lj_tab_set(fs->L, t, &k), &val);
-      if (val.k == VKSTR)
+      lj_gc_anybarriert(fs->L, t);
-	lj_gc_objbarriert(fs->L, t, val.u.sval);
     } else {
       if (val.k != VCALL) { expr_toanyreg(fs, &val); vcall = 0; }
       if (expr_isk(&key)) expr_index(fs, e, &key);

src/lj_record.c

@@ -994,6 +994,7 @@ static TRef rec_idx(jit_State *J, RecordIndex *ix)
     return res;
   } else {  /* Indexed store. */
     GCtab *mt = tabref(tabV(&ix->tabv)->metatable);
+    int keybarrier = tref_isgcv(ix->key) && !tref_isnil(ix->val);
     if (tvisnil(oldv)) {  /* Previous value was nil? */
       /* Need to duplicate the hasmm check for the early guards. */
       int hasmm = 0;
@@ -1004,7 +1005,8 @@ static TRef rec_idx(jit_State *J, RecordIndex *ix)
       if (hasmm)
 	emitir(IRTG(loadop, IRT_NIL), xref, 0);  /* Guard for nil value. */
       else if (xrefop == IR_HREF)
-	emitir(IRTG(oldv == niltvg(J2G(J)) ? IR_EQ : IR_NE, IRT_PTR), xref, lj_ir_kptr(J, niltvg(J2G(J))));
+	emitir(IRTG(oldv == niltvg(J2G(J)) ? IR_EQ : IR_NE, IRT_PTR),
+	       xref, lj_ir_kptr(J, niltvg(J2G(J))));
       if (ix->idxchain && rec_mm_lookup(J, ix, MM_newindex)) { /* Metamethod? */
 	lua_assert(hasmm);
 	goto handlemm;
@@ -1015,6 +1017,7 @@ static TRef rec_idx(jit_State *J, RecordIndex *ix)
 	if (tref_isinteger(key))  /* NEWREF needs a TValue as a key. */
 	  key = emitir(IRTN(IR_TONUM), key, 0);
 	xref = emitir(IRT(IR_NEWREF, IRT_PTR), ix->tab, key);
+	keybarrier = 0;  /* NEWREF already takes care of the key barrier. */
       }
     } else if (!lj_opt_fwd_wasnonnil(J, loadop, tref_ref(xref))) {
       /* Cannot derive that the previous value was non-nil, must do checks. */
@@ -1030,11 +1033,13 @@ static TRef rec_idx(jit_State *J, RecordIndex *ix)
 	  emitir(IRTG(loadop, t), xref, 0);  /* Guard for non-nil value. */
 	}
       }
+    } else {
+      keybarrier = 0;  /* Previous non-nil value kept the key alive. */
     }
     if (tref_isinteger(ix->val))  /* Convert int to number before storing. */
       ix->val = emitir(IRTN(IR_TONUM), ix->val, 0);
     emitir(IRT(loadop+IRDELTA_L2S, tref_type(ix->val)), xref, ix->val);
-    if (tref_isgcv(ix->val))
+    if (keybarrier || tref_isgcv(ix->val))
       emitir(IRT(IR_TBAR, IRT_NIL), ix->tab, 0);
     /* Invalidate neg. metamethod cache for stores with certain string keys. */
     if (!nommstr(J, ix->key)) {

src/lj_tab.c

@@ -192,7 +192,7 @@ GCtab * LJ_FASTCALL lj_tab_dup(lua_State *L, const GCtab *kt)
       Node *kn = &knode[i];
       Node *n = &node[i];
       Node *next = nextnode(kn);
-      /* Don't use copyTV here, since it asserts on a copy of a DEADKEY. */
+      /* Don't use copyTV here, since it asserts on a copy of a dead key. */
       n->val = kn->val; n->key = kn->key;
       setmref(n->next, next == NULL? next : (Node *)((char *)next + d));
     }
@@ -448,7 +448,7 @@ TValue *lj_tab_newkey(lua_State *L, GCtab *t, cTValue *key)
   n->key.u64 = key->u64;
   if (LJ_UNLIKELY(tvismzero(&n->key)))
     n->key.u64 = 0;
-  lj_gc_barriert(L, t, key);
+  lj_gc_anybarriert(L, t);
   lua_assert(tvisnil(&n->val));
   return &n->val;
 }
@@ -517,9 +517,7 @@ static uint32_t keyindex(lua_State *L, GCtab *t, cTValue *key)
   if (!tvisnil(key)) {
     Node *n = hashkey(t, key);
     do {
-      if (lj_obj_equal(&n->key, key) ||
+      if (lj_obj_equal(&n->key, key))
-	  (itype(&n->key) == LJ_TDEADKEY && tvisgcv(key) &&
-	   gcV(&n->key) == gcV(key)))
 	return t->asize + (uint32_t)(n - noderef(t->node));
 	/* Hash key indexes: [t->asize..t->asize+t->nmask] */
     } while ((n = nextnode(n)));