[flang] Handle optional TARGET associate in ASSOCIATED runtime

[jeanPerier]

Summary

The TARGET argument of ASSOCIATED may be dynamically optional, in which
case ASSOCIATED(POINTER, TARGET) is equal to ASSOCIATED(TARGET).

Make the runtime argument a pointer so that it can detect and handle
arguments that are dynamically optional.

Also fix the runtime to check if TARGET base address is not null and if
its element size is not null to match the requirement of ASSOCIATED
regarding TARGET:

• if TARGET is an object: true iff [..] TARGET is not a zerosized storage sequence
• if TARGET is a POINTER: true iff [..] POINTER and TARGET are associated

Not that ASSOCIATED will also returns false if TARGET is an unallocated allocatable.
This is not described in the standard, but is a unanimous behaviour of
existing compilers.

flang/include/flang/Runtime/pointer.h

 
 // Association inquiries for ASSOCIATED()
 
 // True when the pointer is not disassociated.
 bool RTNAME(PointerIsAssociated)(const Descriptor &);
 
 // True when the pointer is associated with a specific target.
 bool RTNAME(PointerIsAssociatedWith)(
-    const Descriptor &, const Descriptor &target);
+    const Descriptor &, const Descriptor *target);
 
 } // extern "C"
 } // namespace Fortran::runtime
 #endif // FORTRAN_RUNTIME_POINTER_H_

flang/runtime/pointer.cpp

int RTNAME(PointerDeallocate)(Descriptor &pointer, bool hasStat,
   return ReturnError(terminator, pointer.Destroy(true), errMsg, hasStat);
 }
 
 bool RTNAME(PointerIsAssociated)(const Descriptor &pointer) {
   return pointer.raw().base_addr != nullptr;
 }
 
 bool RTNAME(PointerIsAssociatedWith)(
-    const Descriptor &pointer, const Descriptor &target) {
+    const Descriptor &pointer, const Descriptor *target) {
+  if (!target) {
+    return pointer.raw().base_addr != nullptr;
+  }
+  if (!target->raw().base_addr || target->ElementBytes() == 0) {
+    return false;
+  }
   int rank{pointer.rank()};
-  if (pointer.raw().base_addr != target.raw().base_addr ||
-      pointer.ElementBytes() != target.ElementBytes() ||
-      rank != target.rank()) {
+  if (pointer.raw().base_addr != target->raw().base_addr ||
+      pointer.ElementBytes() != target->ElementBytes() ||
+      rank != target->rank()) {
     return false;
   }
   for (int j{0}; j < rank; ++j) {
     const Dimension &pDim{pointer.GetDimension(j)};
-    const Dimension &tDim{target.GetDimension(j)};
+    const Dimension &tDim{target->GetDimension(j)};
     if (pDim.Extent() != tDim.Extent() ||
         pDim.ByteStride() != tDim.ByteStride()) {
       return false;
     }
   }
   return true;
 }
 
 // TODO: PointerCheckLengthParameter, PointerAllocateSource
 
 } // extern "C"
 } // namespace Fortran::runtime