[flang] Fix extent computation in finalization

The code that computed the extent of a dimension of a
non-allocatable/non-automatic component array during
finalization had a reversed subtraction; fix, and
use variables to make the code a little more readable.

Differential Revision: https://reviews.llvm.org/D121163

1 files changed, 4 insertions, 2 deletions

diff --git a/flang/runtime/derived.cpp b/flang/runtime/derived.cpp
index f9e34b21c800..bb47f6b7a329 100644
--- a/flang/runtime/derived.cpp
+++ b/flang/runtime/derived.cpp
@@ -188,8 +188,10 @@ void Finalize(
       SubscriptValue extent[maxRank];
       const typeInfo::Value *bounds{comp.bounds()};
       for (int dim{0}; dim < comp.rank(); ++dim) {
-        extent[dim] = bounds[2 * dim].GetValue(&descriptor).value_or(0) -
-            bounds[2 * dim + 1].GetValue(&descriptor).value_or(0) + 1;
+        SubscriptValue lb{bounds[2 * dim].GetValue(&descriptor).value_or(0)};
+        SubscriptValue ub{
+            bounds[2 * dim + 1].GetValue(&descriptor).value_or(0)};
+        extent[dim] = ub >= lb ? ub - lb + 1 : 0;
       }
       StaticDescriptor<maxRank, true, 0> staticDescriptor;
       Descriptor &compDesc{staticDescriptor.descriptor()};