https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99101
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Another fix for the finite-loop issue would be to avoid considering loops not
post-dominated by the exit block as finite. Like with the imperfect
diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
index 3817ec423e7..388c03ee6fc 100644
--- a/gcc/tree-ssa-loop-niter.c
+++ b/gcc/tree-ssa-loop-niter.c
@@ -2865,6 +2865,16 @@ finite_loop_p (class loop *loop)
FOR_EACH_VEC_ELT (exits, i, ex)
if (!(ex->flags & (EDGE_EH | EDGE_ABNORMAL | EDGE_FAKE)))
{
+ /* But do not count noreturn regions as exit.
+ ??? This is a to simple check, we'd have to either
+ use post-dominance by the exit block (which isn't
+ computed) or a DFS walk. See PR99101 where the
+ the fact that control dependence (and post-dominance)
+ is not well-defined for not backward reachable regions
+ causes issues otherwise. */
+ if (ex->dest->index != EXIT_BLOCK
+ && EDGE_COUNT (ex->dest->succs) == 0)
+ continue;
if (dump_file)
fprintf (dump_file, "Assume loop %i to be finite: it has an exit
"
"and -ffinite-loops is on.\n", loop->num);
but that still leaves post-dominance and thus control dependence not
well-defined in not backwards reachable regions and thus possibly "bogus"
control-dependent DCE decisions in those. Note even with the above
CD-DCE will then use control-dependence to make the loop control necessary:
FOR_EACH_LOOP (loop, 0)
if (!finite_loop_p (loop))
{
if (dump_file)
fprintf (dump_file, "cannot prove finiteness of loop %i\n",
loop->num);
mark_control_dependent_edges_necessary (loop->latch, false);
}
Which means to be absolutely sure we could use non-CD DCE whenever there
are not backwards reachable blocks in the function.
