MappedScop::map: add extra sanity check on mappings

This check is similar to one performed by tightenLaunchBounds,
which will be removed when tightenLaunchBounds gets implemented
in terms of the actual mapping instead of the filter derived
from the mapping.

Author: Sven Verdoolaege

tc/core/polyhedral/cuda/mapped_scop.cc

@@ -58,11 +58,17 @@ namespace {
  * the minimal value of the mapping) may not help much because
  * those identifiers may get used in other parts of the tree.
  * Furthermore, at that point it is too late to change the mapping.
+ *
+ * Also double check that the minimum is non-negative.
+ * This should always be the case since the affine functions in "list"
+ * are all the result of a modulo operation.
+ * A value of NaN means that the domain is empty.
  */
 static void checkMinimum(isl::union_set domain, isl::union_pw_aff_list list) {
   for (auto upa : list) {
     upa = upa.intersect_domain(domain);
     auto min = upa.min_val();
+    TC_CHECK(min.is_nonneg()) << "mapping to negative block/thread" << min;
     LOG_IF(WARNING, min.is_pos())
         << "Opportunity for shifting mapping -> min:" << min;
   }