[Bug tree-optimization/99956] loop interchange fails when altering bwaves inner loop

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99956

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Sth as simple (and brute-force) as the following fixes this.  Somehow SCEV
must already know the "point of failure" though and eventually always
instantiating from loop to loop_nest in steps might be more efficient than
trying all possibilities on failure (or maybe we should do that on failure).

diff --git a/gcc/gimple-loop-interchange.cc b/gcc/gimple-loop-interchange.cc
index f45b9364644..215587a8406 100644
--- a/gcc/gimple-loop-interchange.cc
+++ b/gcc/gimple-loop-interchange.cc
@@ -1320,7 +1320,14 @@ compute_access_stride (class loop *loop_nest, class loop
*loop,
     }
   tree scev_base = build_fold_addr_expr (ref);
   tree scev = analyze_scalar_evolution (loop, scev_base);
-  scev = instantiate_scev (loop_preheader_edge (loop_nest), loop, scev);
+  tree orig_scev = scev;
+  scev = instantiate_scev (loop_preheader_edge (loop_nest), loop, orig_scev);
+  while (chrec_contains_undetermined (scev) && loop_nest != loop)
+    {
+      loop_nest = loop_nest->inner;
+      scev = instantiate_scev (loop_preheader_edge (loop_nest),
+                              loop, orig_scev);
+    }
   if (! chrec_contains_undetermined (scev))
     {
       tree sl = scev;