[flang] Add ClassIs runtime function

[clementval]

Summary

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.

[clementval]

Add missing class

[jeanPerier]

Other than the two runtime code style nits, LGTM

flang/runtime/derived-api.cpp

46

braces around all if/else in the runtime code (also below).

48

{} init in the runtime code.

[clementval]

Fix runtime format issue

[clementval]

flang/runtime/derived-api.cpp

46

Thanks! I forgot about these style differences.

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

 #ifndef FORTRAN_RUNTIME_DERIVED_API_H_
 #define FORTRAN_RUNTIME_DERIVED_API_H_
 
 #include "flang/Runtime/entry-names.h"
 
 namespace Fortran::runtime {
 class Descriptor;
 
+namespace typeInfo {
+class DerivedType;
+}
+
 extern "C" {
 
 // Initializes and allocates an object's components, if it has a derived type
 // with any default component initialization or automatic components.
 // The descriptor must be initialized and non-null.
 void RTNAME(Initialize)(
     const Descriptor &, const char *sourceFile = nullptr, int sourceLine = 0);
 
 // Finalizes an object and its components.  Deallocates any
 // allocatable/automatic components.  Does not deallocate the descriptor's
 // storage.
 void RTNAME(Destroy)(const Descriptor &);
 
 // Intrinsic or defined assignment, with scalar expansion but not type
 // conversion.
 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

if (const DescriptorAddendum * addendum{descriptor.Addendum()}) {
     if (const auto *derived{addendum->derivedType()}) {
       if (!derived->noDestructionNeeded()) {
         Destroy(descriptor, true, *derived);
       }
     }
   }
 }
 
+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"
 } // namespace Fortran::runtime