[Bug tree-optimization/100781] [12 Regression] Emitted binary code changes when -g is enabled at -O2

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100781

--- Comment #6 from Andrew Macleod <amacleod at redhat dot com> ---
So the new code base provides a more complete/consistent export list, and makes
use of imports now.  An import is the incoming value which can change an
outgoing value.
In this particular case we see:
<bb 3> :
  _2 = (long int) f$b_17;
 _3 = _2 <= e_12(D);
 h_13 = (int) _3;
 i_14 = (int) d_10(D);
 _5 = h_13 >= i_14;
 n_15 = (int) _5;
 if (h_13 < i_14)
   goto <bb 4>; [INV]
 else
   goto <bb 5>; [INV]

 <bb 4> :
 // predicted unlikely by goto predictor.
 // predicted unlikely by goto predictor.
 if (d_10(D) != 0)
   goto <bb 6>; [INV]
 else
   goto <bb 7>; [INV]

We look at more of these names than we use to, and f$b_17 is now taken into
account. bb3 has a back edge from bb9, so we do a little back edge tracking
earlier than we use to.
Long story short:  During the initial traversal, none of the other names in the
block have yet been fully fleshed out when we arrive back at bb3 from the
bottom, so the propagation engine uses global values as an initial value.
In particular, we use h_13 which has none, so defaults to VARYING. And in "if
h_13 < i_14", that gives us nothing for d_10.
however, we later process h_13 = (int)_3, and discover it is actually [0,1]
since _3 is a boolean.
We propagate this new [0,1] value, but since it isnt in the dependency chain
for d_10, we never re-evaluate d_10.

Previously, the first time we evaluate d_10 we see that the edge 3->4 requires  
[0,1] < i_14 to be true, which means i_14 = [1, +INF] which also means that
d_10 is non-zero and we fold the condition.

The next round of GORI code I'm about to check in makes it fair straightforward
to grab an initial value for h_13 even if it hasn't been fully evaluated yet..
so instead of VARYING, we'll get the [0,1] range for h_13, and everything will
return to the way it was.

There is more update/propagation/ordering stuff to come, but I suspect the
forthcoming changes will resolve most of these sorts of issues.