Re: What's wrong here?

["Kristofer Karlsson" <kristofer.karlsson@...>]

2007/3/21, Thomas Lauer <thomas.lauer@virgin.net>:

Either I am doing something so stupid that I can't see it or something
fishy is going on. Consider this:

        function x(l)
                z=0
                l=nil
                return l
        end

        print(x("test"))

... which prints "nil". Fine.

Now simply comment out the first line of x() (ie "z=0") and try again...
this time it prints "test".

It seems that the first line of x() can be any valid Lua statement other
than "l=nil" and all works fine. However, if this assignment is the
first line it just doesn't work. I am puzzled.

(In case you wonder why anyone would set a function parameter to nil:
the above is stripped down to the smallest code snippet that shows this
behaviour. The original stuff comes from a more complex wrapper around a
bunch of functions in the Lua runtime library.)

That's Lua 5.1.1 on a Win2k SP4 rollup box. I checked with both the
latest precompiled binaries and a locally compiled version.

--
cheers  thomasl

web : http://thomaslauer.com/start

Strange indeed! Compiling "function(x) x = nil return x end" on lua 5.1.1 (from ubuntu lua package) gives a chunk that directly returns x.
So, it's a compiler bug and not a virtual machine bug.
My best guess (but it's nothing more than a guess) is that the compiler sees "assign nil to register 0" but doesn't see any previous assigment to register 0, thus falsely assuming that the operation can be optimized away.