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I think Mike added support for them, if you make compile with -DLUAJIT_ENABLE_LUA52COMPAT=1 

src/lj_obj.h:

/* Metamethods. ORDER MM */
#ifdef LUAJIT_ENABLE_LUA52COMPAT
#define MMDEF_52(_) _(pairs) _(ipairs)
#else
#define MMDEF_52(_)
#endif

E:\p\luajit\src\buildvm_x86.dasc
  |
  |.ffunc_1 pairs
  |  mov TAB:RB, [BASE]
  |  cmp dword [BASE+4], LJ_TTAB;  jne ->fff_fallback
#ifdef LUAJIT_ENABLE_LUA52COMPAT
  |  cmp dword TAB:RB->metatable, 0; jne ->fff_fallback
#endif
  |  mov CFUNC:RB, [BASE-8]
  |  mov CFUNC:RD, CFUNC:RB->upvalue[0]
  |  mov PC, [BASE-4]
  |  mov dword [BASE-4], LJ_TFUNC
  |  mov [BASE-8], CFUNC:RD
  |  mov dword [BASE+12], LJ_TNIL
  |  mov RD, 1+3
  |  jmp ->fff_res

On 2/21/2012 10:56 AM, Axel Kittenberger wrote:

This needs Lua 5.2, with 5.1 or LuaJIT you're mostly out of luck (as
far I know). Also note this will make a copy of original at the moment
of write, not of the moment made the COW, so if you change the
original before, it changes in background. But I suppose you consider
that one immutable.

function makeCOW(o)
     return setmetatable({}, {
         __index= o,
         __pairs = function() return pairs(o) end,
         __ipairs = function() return ipairs(o) end,
         __newindex =
             function(t, k, v)
                 setmetatable(t, nil)
                 for ck, cv in pairs(o) do t[ck] = cv end
                 t[k] = v
             end,
     })
end

local org = {1,2,3,4,5,6,7,8,9}
local cpy = makeCOW(org);

for k, v in ipairs(cpy) do print(k, v) end
print('----')
cpy[2] = 5
for k, v in ipairs(cpy) do print(k, v) end