Re: [PATCH] Fix PRs middle-end/15014 and middle-end/16973, removing labels with their address taken

[Jeffrey A Law <law@redhat.com>]

On Mon, 2004-10-04 at 08:46, Andrew Pinski wrote:
> The problem here is that we were getting rid of some bb as they were 
> being marked as unreachable (which is true in the code flow sense) but 
> should not
> be removed.  I fixed this by adding an edge from the first bb to the 
> label's
> bb which has its address taken.  I think this is most correct solution 
> as
> this does not happen much we should not have to worry about losing some
> optimization possibility from doing it this way.
> 
> OK? Bootstrapped and tested on powerpc-apple-darwin.
> 
> Thanks,
> Andrew Pinski
> 
> 
> Testcase:
> void
> f (void)
> {
>    static __SIZE_TYPE__ u = &&a-&&b;
>    a : b :  return;
> }
> 
> 
> ChangeLog:
> 	* tre-cfg.c (make_edges): After making the edges but before
> 	cleaning up the cfg, make an edge from the first BB to ever
> 	BB which we take the address of the label.
Didn't we decide to disallow taking the addresses of user labels
for use by anything but control flow statements?  That would
make this code ill-formed.

If we're going to allow this, then I would strongly recommend
against this approach to the problem.  Instead just move
these labels to the start of their nearest following block that
is reachable.

Inserting arbitrary edges into the CFG is generally bad.

Jeff