Re: Ravi bug related to setting of L->top in OP_RETURN

[Roberto Ierusalimschy <roberto@...>]

> I am investigating a particularly difficult bug related to setting of
> L->top, and would appreciate some insight on what L->top should be
> when a function returns via OP_RETURN.
> 
> The Lua code does following in OP_RETURN:
> 
>         int b = GETARG_B(i);
>         if (cl->p->sizep > 0) luaF_close(L, base);
>         b = luaD_poscall(L, ra, (b != 0 ? b - 1 : L->top - ra));
>         if (!(ci->callstatus & CIST_REENTRY))  /* 'ci' still the called one */
>           return;  /* external invocation: return */
>         else {  /* invocation via reentry: continue execution */
>           ci = L->ci;
>           if (b) L->top = ci->top;
>           lua_assert(isLua(ci));
>           lua_assert(GET_OPCODE(*((ci)->u.l.savedpc - 1)) == OP_CALL);
>           goto newframe;  /* restart luaV_execute over new Lua function */
>         }
> 
> It is not clear to me why in the external invocation case, if
> following code is not executed:
> 
>           ci = L->ci;
>           if (b) L->top = ci->top;

Both statements here prepare 'luaV_execute' to run the function
"OP_RETURN" is returning *to*. (The previous call to 'luaD_poscall'
already updated 'L->ci' to point to this previous entry.) When
the running function (the one returning *from*) was called
externally (e.g., from a C function), 'luaV_execute' returns,
so this preparation would be meaningless.

When 'b' is zero, this return must return all values on the stack
(e.g., 'return ...', and L->top must be kept for the next instruction
(after the OP_CALL to where we are returning) to know how many
results the function have (see next answer).


> A related question is what is the meaning of the return value from
> luaD_poscall() - it seems to be a signal for L-top to be reset to
> ci->top, but I am not sure I understand when this happens.

It happens when the function that called the one being returned needs
multiple returns. As I explained, in that case L->top limits the
number of values being returned, and will be corrected by the
instruction following the OP_CALL (see comments in 'lopcdes.h').

-- Roberto