Re: debug.getlocal from within return hook

["书呆彭, Peng Yi" <nerditation@...>]

在 2015/5/19 20:03, Roberto Ierusalimschy 写道:
> > I checked the attached script and examined the source code of Lua.
> > [...]
> > when the function returns NOTHING (either an explicit `return' statement or implicitly),
> > Lua generate this opcode:
> >
> >    RETURN 0 1
> >
> > when the function returns some value (e.g. nil), Lua generate this opcode:
> >
> >    LOADNIL 8 0
> >    RETURN 8 2
> >
> > in the latter case, the VM still adjust the stack before calling the return hook, but since
> > the returnning values are loaded at the top of the stack, the active stack frame holding the
> > local variables is not destroyed, so your hook can now get the expected results.
> Many thanks for that detailed analysis. (BTW, as Philipp already found
> out, the problem also happens when we return a local variable, because
> Lua optimizes that case and returns the variable directly from where it
> lives, not from the top of the stack.)
>
> -- Roberto

similar problem exists with tail call hook (I have Lua 5.2, didn't check 5.3).

I think this optimization for the opcode genrator is just fine.
it is the VM that causes the 'incorrect' behavior. the VM invalidates
the slots when they are by definition still alive.

actually I found it totally unnecessary to adjust the stack top pointer.
anyway `luaD_poscall` is doing the work of moving the return values
to the caller's frame, whether or not a hook is called.

--
the nerdy Peng / 书呆彭 / Sent from Thunderbird