complete thread mapping in a single band

Since mapping nested bands to threads is invalid, we may complete the
mapping by assigning unused thread dimensions to zero immediately after
mapping coincident and reduction members to threads.  Note that it is
still necessary to insert synchronizations if some band outside the one
mapped to threads has non-coincident members.  The mapping also needs to
be completed before inserting synchronizations between children of a
sequence node, some of which may not be mapped to threads at all.

Author: ftynse

tc/core/polyhedral/cuda/mapped_scop.cc

@@ -467,20 +467,24 @@ size_t MappedScop::mapInnermostBandsToThreads(detail::ScheduleTree* st) {
   if (auto band = st->elemAs<detail::ScheduleTreeElemBand>()) {
     if (n == 0) {
       // If children were not mapped to threads, the current band can be mapped.
+      // First, map the coincidence and reduction dimension to threads.
+      // Then, if some threads were mapped, fix unused thread dimensions to 0
+      // because we cannot map parent bands anyway.
       auto nMapped = mapToThreads(st);
-      markUnroll(scop_->scheduleRoot(), st, unroll);
-      return nMapped;
+      if (nMapped > 0) {
+        mapRemaining<mapping::ThreadId>(st, nMapped, numThreads.view.size());
+        markUnroll(scop_->scheduleRoot(), st, unroll);
+        return numThreads.view.size();
+      }
     } else if (anyNonCoincidentMember(band)) {
       // If children were mapped to threads, and this band has a non-coincident
       // member, insert a synchronization after its last child.
-      // This also implies the mapping must be completed first.
       // The node must have children if some of them were mapped to threads,
       // double-check.  Note that a band node has at most one child.
       CHECK_EQ(st->numChildren(), 1);
-      mapRemaining<mapping::ThreadId>(
-          st->child({0}), n, numThreads.view.size());
+      // The mapping should be always complete, double-check.
+      CHECK_EQ(n, numThreads.view.size());
       scop_->insertSyncAfter(st->child({0}));
-      return numThreads.view.size();
     }
   }