ScheduleTreeElemMapping: take isl::ctx in constructor

The constructor of ScheduleTreeElemMapping accepts as argument an
unordered_map between isl objects, so the context may be extracted from
any object unless the map is empty.  While the constructor is throwing
if the map is empty, the isl::ctx instance is passed to the superclass
constructor, that is before it has an opportunity to throw.  Request the
context as a separate constructor argument instead of hoping on a
non-empty map and potentially triggering an undefined behavior by
dereferencing the one-past-end iterator.

Author: ftynse

tc/core/polyhedral/schedule_tree.cc

@@ -286,7 +286,7 @@ std::unique_ptr<ScheduleTree> ScheduleTree::makeMappingUnsafe(
       << "some id is used more than once in the mapping";
   auto ctx = mappedIds[0].get_ctx();
   auto res =
-      std::unique_ptr<ScheduleTree>(new ScheduleTreeElemMapping(mapping));
+      std::unique_ptr<ScheduleTree>(new ScheduleTreeElemMapping(ctx, mapping));
   res->appendChildren(std::move(children));
   return res;
 }

tc/core/polyhedral/schedule_tree_elem.h

@@ -120,8 +120,8 @@ struct ScheduleTreeElemMapping : public ScheduleTree {
   ScheduleTreeElemMapping() = delete;
   ScheduleTreeElemMapping(const ScheduleTreeElemMapping& eb)
       : ScheduleTree(eb), mapping(eb.mapping), filter_(eb.filter_) {}
-  ScheduleTreeElemMapping(const Mapping& mapping)
-      : ScheduleTree(mapping.cbegin()->second.get_ctx(), {}, NodeType),
+  ScheduleTreeElemMapping(isl::ctx ctx, const Mapping& mapping)
+      : ScheduleTree(ctx, {}, NodeType),
         mapping(mapping),
         filter_(isl::union_set()) {
     TC_CHECK_GT(mapping.size(), 0u) << "empty mapping filter";