[flang] Add ClassIs runtime function

Add a `ClassIs` function that takes a descriptor and a
type desc to implement the check needed by the CLASS IS type guard
in SELECT TYPE construct.
Since the kind type parameter are directly folded in the type itself
in Flang and the type descriptor is a global, the function just check
if the type descriptor address of the descriptor is equivalent to
the type descriptor address of the global. If not, it check in the
parents of the descriptor's type descriptor.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D138279

Author: clementval

flang/include/flang/Runtime/derived-api.h

@@ -20,6 +20,10 @@
 namespace Fortran::runtime {
 class Descriptor;
 
+namespace typeInfo {
+class DerivedType;
+}
+
 extern "C" {
 
 // Initializes and allocates an object's components, if it has a derived type
@@ -38,6 +42,10 @@ void RTNAME(Destroy)(const Descriptor &);
 void RTNAME(Assign)(const Descriptor &, const Descriptor &,
     const char *sourceFile = nullptr, int sourceLine = 0);
 
+// Perform the test of the CLASS IS type guard statement of the SELECT TYPE
+// construct.
+bool RTNAME(ClassIs)(const Descriptor &, const typeInfo::DerivedType &);
+
 } // extern "C"
 } // namespace Fortran::runtime
 #endif // FORTRAN_RUNTIME_DERIVED_API_H_

flang/runtime/derived-api.cpp

@@ -39,6 +39,25 @@ void RTNAME(Destroy)(const Descriptor &descriptor) {
   }
 }
 
+bool RTNAME(ClassIs)(
+    const Descriptor &descriptor, const typeInfo::DerivedType &derivedType) {
+  if (const DescriptorAddendum * addendum{descriptor.Addendum()}) {
+    if (const auto *derived{addendum->derivedType()}) {
+      if (derived == &derivedType) {
+        return true;
+      }
+      const typeInfo::DerivedType *parent{derived->GetParentType()};
+      while (parent) {
+        if (parent == &derivedType) {
+          return true;
+        }
+        parent = parent->GetParentType();
+      }
+    }
+  }
+  return false;
+}
+
 // TODO: Assign()
 
 } // extern "C"