From Lua 5.2: __len for tables. Needs -DLUAJIT_ENABLE_LUA52COMPAT.

src/buildvm_arm.dasc

@@ -767,9 +767,16 @@ static void build_subroutines(BuildCtx *ctx)
   |  mov CARG1, L
   |   str PC, SAVE_PC
   |  bl extern lj_meta_len		// (lua_State *L, TValue *o)
-  |  // Returns TValue * (metamethod base).
+  |  // Returns NULL (retry) or TValue * (metamethod base).
   |  IOS ldr BASE, L->base
+#ifdef LUAJIT_ENABLE_LUA52COMPAT
+  |  cmp CRET1, #0
+  |  bne ->vmeta_binop			// Binop call for compatibility.
+  |  ldr TAB:CARG1, [BASE, RC]
+  |  b ->BC_LEN_Z
+#else
   |  b ->vmeta_binop			// Binop call for compatibility.
+#endif
   |
   |//-- Call metamethod ----------------------------------------------------
   |
@@ -2546,11 +2553,25 @@ static void build_ins(BuildCtx *ctx, BCOp op, int defop)
     |   ins_next3
     |2:
     |  checktab CARG2, ->vmeta_len
+#ifdef LUAJIT_ENABLE_LUA52COMPAT
+    |  ldr TAB:CARG3, TAB:CARG1->metatable
+    |  cmp TAB:CARG3, #0
+    |  bne >9
+    |3:
+#endif
+    |->BC_LEN_Z:
     |  IOS mov RC, BASE
     |  bl extern lj_tab_len		// (GCtab *t)
     |  // Returns uint32_t (but less than 2^31).
     |  IOS mov BASE, RC
     |  b <1
+#ifdef LUAJIT_ENABLE_LUA52COMPAT
+    |9:
+    |  ldrb CARG4, TAB:CARG3->nomm
+    |  tst CARG4, #1<<MM_len
+    |  bne <3				// 'no __len' flag set: done.
+    |  b ->vmeta_len
+#endif
     break;
 
   /* -- Binary ops -------------------------------------------------------- */