[flang][runtime] Handle endian-swapped record headers & footers on input

[klausler]

Summary

The runtime I/O library correctly handles endianness conversions on payload
data I/O and on the output of sequential record headers and footers, but
does not swap endianness when required when reading sequential record headers
and footers back in for READ and BACKSPACE statements. Mea culpa. Fix.

Fixes https://github.com/llvm/llvm-project/issues/57126

[jpenix-quic]

LGTM, thank you!

flang/runtime/unit.h

   void CommitWrites();
   bool CheckDirectAccess(IoErrorHandler &);
   void HitEndOnRead(IoErrorHandler &);
+  std::int32_t ReadHeaderOrFooter(std::int64_t frameOffset);
 
   Lock lock_;
   // TODO: replace with a thread ID

flang/runtime/unit.cpp

   }
 }
 
-static void SwapEndianness(
+static inline void SwapEndianness(
     char *data, std::size_t bytes, std::size_t elementBytes) {
   if (elementBytes > 1) {
     auto half{elementBytes >> 1};

               "record #%jd (file offset %jd): truncated record header";
     }
   } else {
-    std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
+    header = ReadHeaderOrFooter(recordOffsetInFrame_);
     recordLength = sizeof header + header; // does not include footer
     need = recordOffsetInFrame_ + *recordLength + sizeof footer;
     got = ReadFrame(frameOffsetInFile_, need, handler);

               "record #%jd (file offset %jd): hit EOF reading record with "
               "length %jd bytes";
     } else {
-      std::memcpy(&footer, Frame() + recordOffsetInFrame_ + *recordLength,
-          sizeof footer);
+      footer = ReadHeaderOrFooter(recordOffsetInFrame_ + *recordLength);
       if (footer != header) {
         error = "Unformatted variable-length sequential file input failed at "
                 "record #%jd (file offset %jd): record header has length %jd "

 
 void ExternalFileUnit::BackspaceVariableUnformattedRecord(
     IoErrorHandler &handler) {
-  std::int32_t header{0}, footer{0};
+  std::int32_t header{0};
   auto headerBytes{static_cast<std::int64_t>(sizeof header)};
   frameOffsetInFile_ += recordOffsetInFrame_;
   recordOffsetInFrame_ = 0;

     handler.SignalError(IostatShortRead);
     return;
   }
-  std::memcpy(&footer, Frame(), sizeof footer);
-  recordLength = footer;
+  recordLength = ReadHeaderOrFooter(0);
   if (frameOffsetInFile_ < *recordLength + 2 * headerBytes) {
     handler.SignalError(IostatBadUnformattedRecord);
     return;

     handler.SignalError(IostatShortRead);
     return;
   }
-  std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
+  header = ReadHeaderOrFooter(recordOffsetInFrame_);
   if (header != *recordLength) {
     handler.SignalError(IostatBadUnformattedRecord);
     return;

   }
 }
 
+std::int32_t ExternalFileUnit::ReadHeaderOrFooter(std::int64_t frameOffset) {
+  std::int32_t word;
+  char *wordPtr{reinterpret_cast<char *>(&word)};
+  std::memcpy(wordPtr, Frame() + frameOffset, sizeof word);
+  if (swapEndianness_) {
+    SwapEndianness(wordPtr, sizeof word, sizeof word);
+  }
+  return word;
+}
+
 void ChildIo::EndIoStatement() {
   io_.reset();
   u_.emplace<std::monostate>();