memory promotion: ignore tensors with no rectangular approximation

If we cannot compute a rectangular approximation of a reference's
footprint, do not introduce groups for any other references to this
tensor and thus avoid promoting it.

Author: ftynse

src/core/polyhedral/memory_promotion.cc

@@ -321,13 +321,21 @@ void addSingletonReferenceGroups(
   // access relations have a shape :: [D -> ref] -> O
   // use currying to isolate the D part before intersecting with the domain
   // Compute initial groups with single reference per group.
+  std::unordered_set<isl::id, isl::IslIdIslHash> unapproximatable;
   for (auto a : isl::UnionAsVector<isl::union_map>(accesses)) {
     if (isl::union_map(a.curry()).intersect_domain(domain).is_empty()) {
       continue;
     }
 
     auto tensorId = a.get_tuple_id(isl::dim_type::out);
-    addSingletonReferenceGroup(tensorGroups, tensorId, schedule, a, type);
+    if (unapproximatable.count(tensorId) != 0) {
+      continue;
+    }
+    try {
+      addSingletonReferenceGroup(tensorGroups, tensorId, schedule, a, type);
+    } catch (const promotion::GroupingError& err) {
+      unapproximatable.insert(tensorId);
+    }
   }
 }
 } // namespace