[Bug tree-optimization/99101] optimization bug with -ffinite-loops

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99101

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we can massage post-dom compute to make control dependence which is defined
in terms of pdom do what we want.  We have

void
dom_info::calc_dfs_tree ()
{
...
      /* In the post-dom case we may have nodes without a path to EXIT_BLOCK.
         They are reverse-unreachable.  In the dom-case we disallow such
         nodes, but in post-dom we have to deal with them.

         There are two situations in which this occurs.  First, noreturn
         functions.  Second, infinite loops.  In the first case we need to
         pretend that there is an edge to the exit block.  In the second
         case, we wind up with a forest.  We need to process all noreturn
         blocks before we know if we've got any infinite loops.  */

which then first visits blocks w/o successors, breaking our expectations.

So with that in mind indeed connecting infinite loops to exit would
circumvent the issue, but connect_infinite_loops_to_exit does not
reliably avoid making the edge from the noreturn block (if that came first),
dfs_find_deadend also likes noreturn blocks very much.

Heuristically one could use loop info, but that fails to work for irreducible
regions.  One could also use marked backedges from a forward DFS walk
(blocks with just backedges outgoing).