Fix PR64878: do not jump thread across two loop iterations

[Jakub Jelinek]

On Thu, Feb 05, 2015 at 08:21:46AM +0100, Richard Biener wrote:
> On February 4, 2015 10:20:06 PM CET, Sebastian Pop <sebpop@gmail.com> wrote:
> >The attached patch stops the recursion in the detection of FSM
> >jump-threads at
> >loop phi nodes after having visited a loop phi node.  This avoids
> >jump-threading
> >two iterations forward that were possible due to a flip-flop operation
> >that
> >exchange the value of the switch control variable as illustrated in the
> >testcase:
> >
> >do {
> >  c = read_from_memory;
> >  switch (a) {
> >  case 0:
> >    if (c == ' ')
> >     [...]
> >    else
> >      a = b;   // flip-flop
> >    break;
> >  case 1:
> >    a = 0;
> >    b = 15; // this will jump-thread to 15
> >    break;
> >
> >  case 15:
> >  [...]
> >  }
> >} while (...);
> >
> >The patch has passed bootstrap and regression testing on x86_64-linux.
> >Ok to commit?
> 
> But is sounds like a useful optimization?

Only if we could do it reliably.  If it isn't fixable in a different way for
GCC 5.0, then I think it is better to live with a possibly
missed-optimization than wrong-code.

	Jakub