MappedScop::detectReductions: add missing permutability check

If the band with a detect reduction has one or more coincident members,
then it gets split and MappedScop::mapToThreads gets called on both parts.
The call on the outer part would fail to map the coincident members
to thread identifiers because it has a permutability check,
while the handling of reductions assumes that the entire originally
detected reduction band gets mapped to thread identifiers.
Add the same check to MappedScop::detectReductions to prevent this
from happening.
Note that the new check is more strict, because it will also punt
on a single-dimensional non-permutable band mapped to a single
thread identifier.
Also note that this is fairly theoretical because in practice
all bands are permutable.

Author: ftynse

tc/core/polyhedral/cuda/mapped_scop.cc

@@ -197,6 +197,11 @@ bool MappedScop::detectReductions(detail::ScheduleTree* tree) {
     return found;
   }
 
+  // Only reductions that appear in permutable bands are mapped to threads.
+  if (!band->permutable_) {
+    return false;
+  }
+
   // For now, only support reductions with a sufficient number
   // of coincident outer band members for the remaining thread identifiers.
   auto nCoincident = band->nOuterCoincident();