obtainOuterBand: extract out ScheduleTree::makeEmptyBand

This function will be reused in the next commit.

Author: Sven Verdoolaege

tc/core/polyhedral/schedule_tree.cc

@@ -208,6 +208,14 @@ std::unique_ptr<ScheduleTree> ScheduleTree::makeBand(
   return res;
 }
 
+ScheduleTreeUPtr ScheduleTree::makeEmptyBand(const ScheduleTree* root) {
+  auto domain = root->elemAs<ScheduleTreeElemDomain>();
+  CHECK(domain);
+  auto space = domain->domain_.get_space().set_from_params();
+  auto zero = isl::multi_union_pw_aff::zero(space);
+  return ScheduleTree::makeBand(zero);
+}
+
 std::unique_ptr<ScheduleTree> ScheduleTree::makeDomain(
     isl::union_set domain,
     std::vector<ScheduleTreeUPtr>&& children) {

tc/core/polyhedral/schedule_tree.h

@@ -285,6 +285,9 @@ struct ScheduleTree {
       isl::multi_union_pw_aff mupa,
       std::vector<ScheduleTreeUPtr>&& children = {});
 
+  // Return a zero-dimensional band for use in a tree with the given root.
+  static ScheduleTreeUPtr makeEmptyBand(const ScheduleTree* root);
+
   static ScheduleTreeUPtr makeDomain(
       isl::union_set domain,
       std::vector<ScheduleTreeUPtr>&& children = {});

tc/core/polyhedral/scop.cc

@@ -488,11 +488,7 @@ detail::ScheduleTree* obtainOuterBand(detail::ScheduleTree* root) {
       continue;
     }
 
-    auto domain = root->elemAs<ScheduleTreeElemDomain>();
-    CHECK(domain);
-    auto space = domain->domain_.get_space().set_from_params();
-    auto zero = isl::multi_union_pw_aff::zero(space);
-    auto band = ScheduleTree::makeBand(zero);
+    auto band = ScheduleTree::makeEmptyBand(root);
     if (n == 0) {
       return setPermutable(insertNodeBelow(tree, std::move(band)));
     } else {