Towards a new implementation of arrays

The array part of a table wastes too much space, due to padding.
To avoid that, we need to store values in the array as something
different from a TValue. Therefore, the API for table access
should not assume that any value in a table lives in a *TValue.
This commit is the first step to remove that assumption: functions
luaH_get*, instead of returning a *TValue where the value lives,
receive a *TValue where to put the value being accessed.
(We still have to change the luaH_set* functions.)

lapi.c

@@ -637,16 +637,16 @@ LUA_API int lua_pushthread (lua_State *L) {
 
 
 l_sinline int auxgetstr (lua_State *L, const TValue *t, const char *k) {
-  const TValue *slot;
+  int aux;
   TString *str = luaS_new(L, k);
-  if (luaV_fastget(L, t, str, slot, luaH_getstr)) {
-    setobj2s(L, L->top.p, slot);
+  luaV_fastget1(t, str, s2v(L->top.p), luaH_getstr1, aux);
+  if (aux == HOK) {
     api_incr_top(L);
   }
   else {
     setsvalue2s(L, L->top.p, str);
     api_incr_top(L);
-    luaV_finishget(L, t, s2v(L->top.p - 1), L->top.p - 1, slot);
+    luaV_finishget1(L, t, s2v(L->top.p - 1), L->top.p - 1, aux);
   }
   lua_unlock(L);
   return ttype(s2v(L->top.p - 1));
@@ -672,15 +672,13 @@ LUA_API int lua_getglobal (lua_State *L, const char *name) {
 
 
 LUA_API int lua_gettable (lua_State *L, int idx) {
-  const TValue *slot;
+  int aux;
   TValue *t;
   lua_lock(L);
   t = index2value(L, idx);
-  if (luaV_fastget(L, t, s2v(L->top.p - 1), slot, luaH_get)) {
-    setobj2s(L, L->top.p - 1, slot);
-  }
-  else
-    luaV_finishget(L, t, s2v(L->top.p - 1), L->top.p - 1, slot);
+  luaV_fastget1(t, s2v(L->top.p - 1), s2v(L->top.p - 1), luaH_get1, aux);
+  if (aux != HOK)
+    luaV_finishget1(L, t, s2v(L->top.p - 1), L->top.p - 1, aux);
   lua_unlock(L);
   return ttype(s2v(L->top.p - 1));
 }
@@ -694,16 +692,14 @@ LUA_API int lua_getfield (lua_State *L, int idx, const char *k) {
 
 LUA_API int lua_geti (lua_State *L, int idx, lua_Integer n) {
   TValue *t;
-  const TValue *slot;
+  int aux;
   lua_lock(L);
   t = index2value(L, idx);
-  if (luaV_fastgeti(L, t, n, slot)) {
-    setobj2s(L, L->top.p, slot);
-  }
-  else {
-    TValue aux;
-    setivalue(&aux, n);
-    luaV_finishget(L, t, &aux, L->top.p, slot);
+  luaV_fastgeti1(t, n, s2v(L->top.p), aux);
+  if (aux != HOK) {
+    TValue key;
+    setivalue(&key, n);
+    luaV_finishget1(L, t, &key, L->top.p, aux);
   }
   api_incr_top(L);
   lua_unlock(L);

ltable.c

@@ -752,6 +752,21 @@ const TValue *luaH_getint (Table *t, lua_Integer key) {
 }
 
 
+static int finishnodeget (const TValue *val, TValue *res) {
+  if (!ttisnil(val)) {
+    setobj(((lua_State*)NULL), res, val);
+    return HOK;  /* success */
+  }
+  else
+    return HNOTFOUND;  /* could not get value */
+}
+
+
+int luaH_getint1 (Table *t, lua_Integer key, TValue *res) {
+  return finishnodeget(luaH_getint(t, key), res);
+}
+
+
 /*
 ** search function for short strings
 */
@@ -771,6 +786,11 @@ const TValue *luaH_getshortstr (Table *t, TString *key) {
 }
 
 
+int luaH_getshortstr1 (Table *t, TString *key, TValue *res) {
+  return finishnodeget(luaH_getshortstr(t, key), res);
+}
+
+
 const TValue *luaH_getstr (Table *t, TString *key) {
   if (key->tt == LUA_VSHRSTR)
     return luaH_getshortstr(t, key);
@@ -782,6 +802,11 @@ const TValue *luaH_getstr (Table *t, TString *key) {
 }
 
 
+int luaH_getstr1 (Table *t, TString *key, TValue *res) {
+  return finishnodeget(luaH_getstr(t, key), res);
+}
+
+
 /*
 ** main search function
 */
@@ -802,6 +827,11 @@ const TValue *luaH_get (Table *t, const TValue *key) {
 }
 
 
+int luaH_get1 (Table *t, const TValue *key, TValue *res) {
+  return finishnodeget(luaH_get(t, key), res);
+}
+
+
 /*
 ** Finish a raw "set table" operation, where 'slot' is where the value
 ** should have been (the result of a previous "get table").

ltable.h

@@ -35,6 +35,27 @@
 #define nodefromval(v)	cast(Node *, (v))
 
 
+/* results from get/set */
+#define HOK		0
+#define HNOTFOUND	1
+#define HNOTATABLE	2
+
+
+/* fast access to components of array values */
+#define getArrTag(t,k)	(&(t)->array[k - 1].tt_)
+#define getArrVal(t,k)	(&(t)->array[k - 1].value_)
+
+#define tagisempty(tag)  (novariant(tag) == LUA_TNIL)
+
+#define arr2val(h,k,tag,res)  \
+  ((res)->tt_ = tag, (res)->value_ = *getArrVal(h,k))
+
+
+LUAI_FUNC int luaH_getshortstr1 (Table *t, TString *key, TValue *res);
+LUAI_FUNC int luaH_getstr1 (Table *t, TString *key, TValue *res);
+LUAI_FUNC int luaH_get1 (Table *t, const TValue *key, TValue *res);
+LUAI_FUNC int luaH_getint1 (Table *t, lua_Integer key, TValue *res);
+
 LUAI_FUNC const TValue *luaH_getint (Table *t, lua_Integer key);
 LUAI_FUNC void luaH_setint (lua_State *L, Table *t, lua_Integer key,
                                                     TValue *value);

lvm.c

@@ -284,12 +284,12 @@ static int floatforloop (StkId ra) {
 ** if 'slot' is NULL, 't' is not a table; otherwise, 'slot' points to
 ** t[k] entry (which must be empty).
 */
-void luaV_finishget (lua_State *L, const TValue *t, TValue *key, StkId val,
-                      const TValue *slot) {
+void luaV_finishget1 (lua_State *L, const TValue *t, TValue *key, StkId val,
+                      int aux) {
   int loop;  /* counter to avoid infinite loops */
   const TValue *tm;  /* metamethod */
   for (loop = 0; loop < MAXTAGLOOP; loop++) {
-    if (slot == NULL) {  /* 't' is not a table? */
+    if (aux == HNOTATABLE) {  /* 't' is not a table? */
       lua_assert(!ttistable(t));
       tm = luaT_gettmbyobj(L, t, TM_INDEX);
       if (l_unlikely(notm(tm)))
@@ -297,7 +297,6 @@ void luaV_finishget (lua_State *L, const TValue *t, TValue *key, StkId val,
       /* else will try the metamethod */
     }
     else {  /* 't' is a table */
-      lua_assert(isempty(slot));
       tm = fasttm(L, hvalue(t)->metatable, TM_INDEX);  /* table's metamethod */
       if (tm == NULL) {  /* no metamethod? */
         setnilvalue(s2v(val));  /* result is nil */
@@ -310,10 +309,9 @@ void luaV_finishget (lua_State *L, const TValue *t, TValue *key, StkId val,
       return;
     }
     t = tm;  /* else try to access 'tm[key]' */
-    if (luaV_fastget(L, t, key, slot, luaH_get)) {  /* fast track? */
-      setobj2s(L, val, slot);  /* done */
-      return;
-    }
+    luaV_fastget1(t, key, s2v(val), luaH_get1, aux);
+    if (aux == HOK)
+      return;  /* done */
     /* else repeat (tail call 'luaV_finishget') */
   }
   luaG_runerror(L, "'__index' chain too long; possible loop");
@@ -1250,58 +1248,51 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
       }
       vmcase(OP_GETTABUP) {
         StkId ra = RA(i);
-        const TValue *slot;
         TValue *upval = cl->upvals[GETARG_B(i)]->v.p;
         TValue *rc = KC(i);
         TString *key = tsvalue(rc);  /* key must be a string */
-        if (luaV_fastget(L, upval, key, slot, luaH_getshortstr)) {
-          setobj2s(L, ra, slot);
-        }
-        else
-          Protect(luaV_finishget(L, upval, rc, ra, slot));
+        int aux;
+        luaV_fastget1(upval, key, s2v(ra), luaH_getshortstr1, aux);
+        if (aux != HOK)
+          Protect(luaV_finishget1(L, upval, rc, ra, aux));
         vmbreak;
       }
       vmcase(OP_GETTABLE) {
         StkId ra = RA(i);
-        const TValue *slot;
         TValue *rb = vRB(i);
         TValue *rc = vRC(i);
-        lua_Unsigned n;
-        if (ttisinteger(rc)  /* fast track for integers? */
-            ? (cast_void(n = ivalue(rc)), luaV_fastgeti(L, rb, n, slot))
-            : luaV_fastget(L, rb, rc, slot, luaH_get)) {
-          setobj2s(L, ra, slot);
+        int aux;
+        if (ttisinteger(rc)) {  /* fast track for integers? */
+          luaV_fastgeti1(rb, ivalue(rc), s2v(ra), aux);
         }
         else
-          Protect(luaV_finishget(L, rb, rc, ra, slot));
+          luaV_fastget1(rb, rc, s2v(ra), luaH_get1, aux);
+        if (aux != HOK)  /* fast track for integers? */
+          Protect(luaV_finishget1(L, rb, rc, ra, aux));
         vmbreak;
       }
       vmcase(OP_GETI) {
         StkId ra = RA(i);
-        const TValue *slot;
         TValue *rb = vRB(i);
         int c = GETARG_C(i);
-        if (luaV_fastgeti(L, rb, c, slot)) {
-          setobj2s(L, ra, slot);
-        }
-        else {
+        int aux;
+        luaV_fastgeti1(rb, c, s2v(ra), aux);
+        if (aux != HOK) {
           TValue key;
           setivalue(&key, c);
-          Protect(luaV_finishget(L, rb, &key, ra, slot));
+          Protect(luaV_finishget1(L, rb, &key, ra, aux));
         }
         vmbreak;
       }
       vmcase(OP_GETFIELD) {
         StkId ra = RA(i);
-        const TValue *slot;
         TValue *rb = vRB(i);
         TValue *rc = KC(i);
         TString *key = tsvalue(rc);  /* key must be a string */
-        if (luaV_fastget(L, rb, key, slot, luaH_getshortstr)) {
-          setobj2s(L, ra, slot);
-        }
-        else
-          Protect(luaV_finishget(L, rb, rc, ra, slot));
+        int aux;
+        luaV_fastget1(rb, key, s2v(ra), luaH_getshortstr1, aux);
+        if (aux != HOK)
+          Protect(luaV_finishget1(L, rb, rc, ra, aux));
         vmbreak;
       }
       vmcase(OP_SETTABUP) {
@@ -1381,16 +1372,14 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
       }
       vmcase(OP_SELF) {
         StkId ra = RA(i);
-        const TValue *slot;
+        int aux;
         TValue *rb = vRB(i);
         TValue *rc = RKC(i);
         TString *key = tsvalue(rc);  /* key must be a string */
         setobj2s(L, ra + 1, rb);
-        if (luaV_fastget(L, rb, key, slot, luaH_getstr)) {
-          setobj2s(L, ra, slot);
-        }
-        else
-          Protect(luaV_finishget(L, rb, rc, ra, slot));
+        luaV_fastget1(rb, key, s2v(ra), luaH_getstr1, aux);
+        if (aux != HOK)
+          Protect(luaV_finishget1(L, rb, rc, ra, aux));
         vmbreak;
       }
       vmcase(OP_ADDI) {

lvm.h

@@ -89,6 +89,10 @@ typedef enum {
       !isempty(slot)))  /* result not empty? */
 
 
+#define luaV_fastget1(t,k,res,f, aux) \
+  (aux = (!ttistable(t) ? HNOTATABLE : f(hvalue(t), k, res)))
+
+
 /*
 ** Special case of 'luaV_fastget' for integers, inlining the fast case
 ** of 'luaH_getint'.
@@ -100,6 +104,15 @@ typedef enum {
               ? &hvalue(t)->array[k - 1] : luaH_getint(hvalue(t), k), \
       !isempty(slot)))  /* result not empty? */
 
+#define luaV_fastgeti1(t,k,val,aux) \
+  if (!ttistable(t)) aux = HNOTATABLE; \
+  else { Table *h = hvalue(t); lua_Unsigned u = l_castS2U(k); \
+    if ((u - 1u < h->alimit)) { \
+      int tag = *getArrTag(h,u); \
+      if (tagisempty(tag)) aux = HNOTFOUND; \
+      else { arr2val(h, u, tag, val); aux = HOK; }} \
+    else { aux = luaH_getint1(h, u, val); }}
+
 
 /*
 ** Finish a fast set operation (when fast get succeeds). In that case,
@@ -125,8 +138,8 @@ LUAI_FUNC int luaV_tointeger (const TValue *obj, lua_Integer *p, F2Imod mode);
 LUAI_FUNC int luaV_tointegerns (const TValue *obj, lua_Integer *p,
                                 F2Imod mode);
 LUAI_FUNC int luaV_flttointeger (lua_Number n, lua_Integer *p, F2Imod mode);
-LUAI_FUNC void luaV_finishget (lua_State *L, const TValue *t, TValue *key,
-                               StkId val, const TValue *slot);
+LUAI_FUNC void luaV_finishget1 (lua_State *L, const TValue *t, TValue *key,
+                                              StkId val, int aux);
 LUAI_FUNC void luaV_finishset (lua_State *L, const TValue *t, TValue *key,
                                TValue *val, const TValue *slot);
 LUAI_FUNC void luaV_finishOp (lua_State *L);