[flang][runtime] Support READ after WRITE w/o positioning

[klausler]

Most Fortran implementations support a READ statement after a WRITE without repositioning on a sequential unit; it implies on ENDFILE and then hits an EOF condition.

Fixes llvm-test-suite/Fortran/gfortran/regression/backspace_2.f.

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: Peter Klausler (klausler)

Changes

Most Fortran implementations support a READ statement after a WRITE without repositioning on a sequential unit; it implies on ENDFILE and then hits an EOF condition.

Fixes llvm-test-suite/Fortran/gfortran/regression/backspace_2.f.

---

Full diff: https://github.com/llvm/llvm-project/pull/74650.diff

2 Files Affected:

• (modified) flang/runtime/unit.cpp (+9)
• (modified) flang/runtime/unit.h (+1)

diff --git a/flang/runtime/unit.cpp b/flang/runtime/unit.cpp
index 995656b9480c4..b069e93caf354 100644
--- a/flang/runtime/unit.cpp
+++ b/flang/runtime/unit.cpp
@@ -351,6 +351,7 @@ bool ExternalFileUnit::Emit(const char *data, std::size_t bytes,
   }
   positionInRecord += bytes;
   furthestPositionInRecord = furthestAfter;
+  anyWriteSinceLastPositioning_ = true;
   return true;
 }
 
@@ -447,6 +448,11 @@ bool ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
         HitEndOnRead(handler);
       }
     } else {
+      if (anyWriteSinceLastPositioning_) {
+        // Most Fortran implementations allow a READ after a WRITE;
+        // it just hits an EOF.
+        DoEndfile(handler);
+      }
       recordLength.reset();
       if (IsAtEOF()) {
         handler.SignalEnd();
@@ -611,6 +617,7 @@ void ExternalFileUnit::BackspaceRecord(IoErrorHandler &handler) {
       }
     }
     BeginRecord();
+    anyWriteSinceLastPositioning_ = false;
   }
 }
 
@@ -663,6 +670,7 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
     SetPosition(0, handler);
     currentRecordNumber = 1;
     leftTabLimit.reset();
+    anyWriteSinceLastPositioning_ = false;
   }
 }
 
@@ -924,6 +932,7 @@ void ExternalFileUnit::DoEndfile(IoErrorHandler &handler) {
   TruncateFrame(frameOffsetInFile_, handler);
   BeginRecord();
   impliedEndfile_ = false;
+  anyWriteSinceLastPositioning_ = false;
 }
 
 void ExternalFileUnit::CommitWrites() {
diff --git a/flang/runtime/unit.h b/flang/runtime/unit.h
index 1ec3013ba82b3..140fda3c4d2a8 100644
--- a/flang/runtime/unit.h
+++ b/flang/runtime/unit.h
@@ -140,6 +140,7 @@ class ExternalFileUnit : public ConnectionState,
   Direction direction_{Direction::Output};
   bool impliedEndfile_{false}; // sequential/stream output has taken place
   bool beganReadingRecord_{false};
+  bool anyWriteSinceLastPositioning_{false};
   bool directAccessRecWasSet_{false}; // REC= appeared
   // Subtle: The beginning of the frame can't be allowed to advance
   // during a single list-directed READ due to the possibility of a