Re: label question

[craig at jcb-sc dot com]

>Given code like this:
>
>	foo()
>	{
>	  bar(&&label);
>	  return;
>
>	 label:
>	  printf("Hello world\n");
>	}
>
>is it legal for the compiler to conclude that the label is unreachable and 
>delete the instructions following it, even though its address has been taken?

I believe &&label is nothing more than a GCC extension -- that is,
I'm not aware of any other specifications GCC tries to implement
that specify &&label (though maybe C9X or something like that does;
I haven't kept track).

If it's specified to serve as a valid means to do non-local GOTO, then
the only situation where the compiler can conclude the label is
unreachable is one in which the compiler knows `bar' won't ever jump
to it.

If it's not specified to so serve, then I would think the compiler
can indeed delete the instructions following the label.  Perhaps it
could even delete the label itself, making the pointer it passes to
`bar' undefined.

My reading of the documentation of this GCC feature supports the latter
interpretation: it is not intended to provide non-local GOTO.
(I looked it up in `extend.texi' in the EGCS sources.)

But, if some other specification says &&label is for non-local GOTO,
then, to implement that specification, GCC will need to be changed
-- not just its docs, but its handling of &&label as well.

        tq vm, (burley)