[flang] [llvm] [flang] Enhance show_descriptor intrinsic to avoid extra copies and extra descriptor creation (PR #173461)

[Slava Zakharin via llvm-commits]

================
@@ -7893,9 +7893,24 @@ void IntrinsicLibrary::genShowDescriptor(
   assert(args.size() == 1 && "expected single argument for show_descriptor");
   const mlir::Value descriptor = fir::getBase(args[0]);
 
-  assert(fir::isa_box_type(descriptor.getType()) &&
-         "argument must have been lowered to box type");
-  fir::runtime::genShowDescriptor(builder, loc, descriptor);
+  // If it's already a reference to a box, pass it directly.
+  if (fir::isa_ref_type(descriptor.getType()) &&
+      fir::isa_box_type(fir::unwrapRefType(descriptor.getType()))) {
+    fir::runtime::genShowDescriptor(builder, loc, descriptor);
+    return;
+  }
+
+  mlir::Value descrAddr = nullptr;
+  if (fir::isa_box_type(descriptor.getType())) {
+    // Spill it to the stack
+    descrAddr = builder.createTemporary(loc, descriptor.getType());
+    builder.createStoreWithConvert(loc, descriptor, descrAddr);
+  } else {
+    // If descriptor is not a box type (and not ref<box>), pass null.
----------------
vzakhari wrote:

```suggestion
    // If argument is not a box type (and not ref<box>), pass null.
```

https://github.com/llvm/llvm-project/pull/173461