Index: flang/runtime/descriptor-io.h
===================================================================
--- flang/runtime/descriptor-io.h
+++ flang/runtime/descriptor-io.h
@@ -241,8 +241,9 @@
 template <Direction DIR>
 static bool DescriptorIO(IoStatementState &, const Descriptor &);
 
+// For default (not user-defined) derived type I/O, formatted & unformatted
 template <Direction DIR>
-static bool DefaultFormattedComponentIO(IoStatementState &io,
+static bool DefaultComponentIO(IoStatementState &io,
     const typeInfo::Component &component, const Descriptor &origDescriptor,
     const SubscriptValue origSubscripts[], Terminator &terminator) {
   if (component.genre() == typeInfo::Component::Genre::Data) {
@@ -263,29 +264,11 @@
   }
 }
 
-std::optional<bool> DefinedFormattedIo(
-    IoStatementState &, const Descriptor &, const typeInfo::SpecialBinding &);
-
 template <Direction DIR>
-static bool FormattedDerivedTypeIO(
-    IoStatementState &io, const Descriptor &descriptor) {
+static bool DefaultComponentwiseIO(IoStatementState &io,
+    const Descriptor &descriptor, const typeInfo::DerivedType &type) {
   IoErrorHandler &handler{io.GetIoErrorHandler()};
-  // Derived type information must be present for formatted I/O.
-  const DescriptorAddendum *addendum{descriptor.Addendum()};
-  RUNTIME_CHECK(handler, addendum != nullptr);
-  const typeInfo::DerivedType *type{addendum->derivedType()};
-  RUNTIME_CHECK(handler, type != nullptr);
-  if (const typeInfo::SpecialBinding *
-      special{type->FindSpecialBinding(DIR == Direction::Input
-              ? typeInfo::SpecialBinding::Which::ReadFormatted
-              : typeInfo::SpecialBinding::Which::WriteFormatted)}) {
-    if (std::optional<bool> wasDefined{
-            DefinedFormattedIo(io, descriptor, *special)}) {
-      return *wasDefined; // user-defined I/O was applied
-    }
-  }
-  // Default componentwise derived type formatting
-  const Descriptor &compArray{type->component()};
+  const Descriptor &compArray{type.component()};
   RUNTIME_CHECK(handler, compArray.rank() == 1);
   std::size_t numComponents{compArray.Elements()};
   std::size_t numElements{descriptor.Elements()};
@@ -299,7 +282,7 @@
          ++k, compArray.IncrementSubscripts(at)) {
       const typeInfo::Component &component{
           *compArray.Element<typeInfo::Component>(at)};
-      if (!DefaultFormattedComponentIO<DIR>(
+      if (!DefaultComponentIO<DIR>(
               io, component, descriptor, subscripts, handler)) {
         return false;
       }
@@ -308,6 +291,30 @@
   return true;
 }
 
+std::optional<bool> DefinedFormattedIo(
+    IoStatementState &, const Descriptor &, const typeInfo::SpecialBinding &);
+
+template <Direction DIR>
+static bool FormattedDerivedTypeIO(
+    IoStatementState &io, const Descriptor &descriptor) {
+  IoErrorHandler &handler{io.GetIoErrorHandler()};
+  // Derived type information must be present for formatted I/O.
+  const DescriptorAddendum *addendum{descriptor.Addendum()};
+  RUNTIME_CHECK(handler, addendum != nullptr);
+  const typeInfo::DerivedType *type{addendum->derivedType()};
+  RUNTIME_CHECK(handler, type != nullptr);
+  if (const typeInfo::SpecialBinding *
+      special{type->FindSpecialBinding(DIR == Direction::Input
+              ? typeInfo::SpecialBinding::Which::ReadFormatted
+              : typeInfo::SpecialBinding::Which::WriteFormatted)}) {
+    if (std::optional<bool> wasDefined{
+            DefinedFormattedIo(io, descriptor, *special)}) {
+      return *wasDefined; // user-defined I/O was applied
+    }
+  }
+  return DefaultComponentwiseIO<DIR>(io, descriptor, *type);
+}
+
 bool DefinedUnformattedIo(
     IoStatementState &, const Descriptor &, const typeInfo::SpecialBinding &);
 
@@ -317,18 +324,24 @@
     IoStatementState &io, const Descriptor &descriptor) {
   IoErrorHandler &handler{io.GetIoErrorHandler()};
   const DescriptorAddendum *addendum{descriptor.Addendum()};
-  const typeInfo::DerivedType *type{
-      addendum ? addendum->derivedType() : nullptr};
-  if (const typeInfo::SpecialBinding *
-      special{type
-              ? type->FindSpecialBinding(DIR == Direction::Input
-                        ? typeInfo::SpecialBinding::Which::ReadUnformatted
-                        : typeInfo::SpecialBinding::Which::WriteUnformatted)
-              : nullptr}) {
-    // User-defined derived type unformatted I/O
-    return DefinedUnformattedIo(io, descriptor, *special);
+  if (const typeInfo::DerivedType *
+      type{addendum ? addendum->derivedType() : nullptr}) {
+    // derived type unformatted I/O
+    if (const typeInfo::SpecialBinding *
+        special{type->FindSpecialBinding(DIR == Direction::Input
+                ? typeInfo::SpecialBinding::Which::ReadUnformatted
+                : typeInfo::SpecialBinding::Which::WriteUnformatted)}) {
+      // User-defined derived type unformatted I/O
+      return DefinedUnformattedIo(io, descriptor, *special);
+    } else {
+      // Default derived type unformatted I/O
+      // TODO: If no component at any level has user defined READ or WRITE
+      // (as appropriate), the elements are contiguous, and no byte swapping
+      // is active, do a block transfer via the code below.
+      return DefaultComponentwiseIO<DIR>(io, descriptor, *type);
+    }
   } else {
-    // Regular derived type unformatted I/O, not user-defined
+    // intrinsic type unformatted I/O
     auto *externalUnf{io.get_if<ExternalUnformattedIoStatementState<DIR>>()};
     auto *childUnf{io.get_if<ChildUnformattedIoStatementState<DIR>>()};
     auto *inq{
@@ -336,28 +349,41 @@
     RUNTIME_CHECK(handler, externalUnf || childUnf || inq);
     std::size_t elementBytes{descriptor.ElementBytes()};
     std::size_t numElements{descriptor.Elements()};
+    std::size_t swappingBytes{elementBytes};
+    if (auto maybeCatAndKind{descriptor.type().GetCategoryAndKind()}) {
+      // Byte swapping units can be smaller than elements, namely
+      // for COMPLEX and CHARACTER.
+      if (maybeCatAndKind->first == TypeCategory::Character) {
+        // swap each character position independently
+        swappingBytes = maybeCatAndKind->second; // kind
+      } else if (maybeCatAndKind->first == TypeCategory::Complex) {
+        // swap real and imaginary components independently
+        swappingBytes /= 2;
+      }
+    }
     SubscriptValue subscripts[maxRank];
     descriptor.GetLowerBounds(subscripts);
     using CharType =
         std::conditional_t<DIR == Direction::Output, const char, char>;
-    auto Transfer{[=](CharType &x, std::size_t totalBytes,
-                      std::size_t elementBytes) -> bool {
+    auto Transfer{[=](CharType &x, std::size_t totalBytes) -> bool {
       if constexpr (DIR == Direction::Output) {
-        return externalUnf ? externalUnf->Emit(&x, totalBytes, elementBytes)
-            : childUnf     ? childUnf->Emit(&x, totalBytes, elementBytes)
-                           : inq->Emit(&x, totalBytes, elementBytes);
+        return externalUnf ? externalUnf->Emit(&x, totalBytes, swappingBytes)
+            : childUnf     ? childUnf->Emit(&x, totalBytes, swappingBytes)
+                           : inq->Emit(&x, totalBytes, swappingBytes);
       } else {
-        return externalUnf ? externalUnf->Receive(&x, totalBytes, elementBytes)
-                           : childUnf->Receive(&x, totalBytes, elementBytes);
+        return externalUnf ? externalUnf->Receive(&x, totalBytes, swappingBytes)
+                           : childUnf->Receive(&x, totalBytes, swappingBytes);
       }
     }};
-    if (descriptor.IsContiguous()) { // contiguous unformatted I/O
+    bool swapEndianness{externalUnf && externalUnf->unit().swapEndianness()};
+    if (!swapEndianness &&
+        descriptor.IsContiguous()) { // contiguous unformatted I/O
       char &x{ExtractElement<char>(io, descriptor, subscripts)};
-      return Transfer(x, numElements * elementBytes, elementBytes);
-    } else { // non-contiguous unformatted I/O
+      return Transfer(x, numElements * elementBytes);
+    } else { // non-contiguous or byte-swapped intrinsic type unformatted I/O
       for (std::size_t j{0}; j < numElements; ++j) {
         char &x{ExtractElement<char>(io, descriptor, subscripts)};
-        if (!Transfer(x, elementBytes, elementBytes)) {
+        if (!Transfer(x, elementBytes)) {
           return false;
         }
         if (!descriptor.IncrementSubscripts(subscripts) &&