[flang] Accept source files with no program units (flang-compiler/f18#658)

update documentation

Original-commit: flang-compiler/f18@3dfb831499
Reviewed-on: https://github.com/flang-compiler/f18/pull/859

flang/documentation/Extensions.md

@@ -109,8 +109,10 @@ Extensions, deletions, and legacy features supported by default
 * When a dummy argument is `POINTER` or `ALLOCATABLE` and is `INTENT(IN)`, we
   relax enforcement of some requirements on actual arguments that must otherwise
   hold true for definable arguments.
-* Assignment of `LOGICAL` to `INTEGER` and vice versa (but not other types).
-  The values are normalized.
+* Assignment of `LOGICAL` to `INTEGER` and vice versa (but not other types) is
+  allowed.  The values are normalized.
+* An effectively empty source file (no program unit) is accepted and
+  produces an empty relocatable output file.
 
 Extensions supported when enabled by options
 --------------------------------------------

flang/lib/common/Fortran-features.h

@@ -33,7 +33,8 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     Hollerith, ArithmeticIF, Assign, AssignedGOTO, Pause, OpenMP,
     CruftAfterAmpersand, ClassicCComments, AdditionalFormats, BigIntLiterals,
     RealDoControls, EquivalenceNumericWithCharacter, AdditionalIntrinsics,
-    AnonymousParents, OldLabelDoEndStatements, LogicalIntegerAssignment)
+    AnonymousParents, OldLabelDoEndStatements, LogicalIntegerAssignment,
+    EmptySourceFile)
 
 using LanguageFeatures = EnumSet<LanguageFeature, LanguageFeature_enumSize>;
 

flang/lib/parser/grammar.h

@@ -276,10 +276,12 @@ constexpr auto scalarIntConstantExpr{scalar(intConstantExpr)};
 // Consequently, a program unit END statement should be the last statement
 // on its line.  We parse those END statements via unterminatedStatement()
 // and then skip over the end of the line here.
-TYPE_PARSER(construct<Program>(some(StartNewSubprogram{} >>
-                Parser<ProgramUnit>{} / skipMany(";"_tok) / space /
-                    recovery(endOfLine, SkipPast<'\n'>{}))) /
-    skipStuffBeforeStatement)
+TYPE_PARSER(construct<Program>(
+    extension<LanguageFeature::EmptySourceFile>(skipStuffBeforeStatement >>
+        !nextCh >> defaulted(cut >> some(Parser<ProgramUnit>{}))) ||
+    some(StartNewSubprogram{} >> Parser<ProgramUnit>{} / skipMany(";"_tok) /
+            space / recovery(endOfLine, SkipPast<'\n'>{})) /
+        skipStuffBeforeStatement))
 
 // R502 program-unit ->
 //        main-program | external-subprogram | module | submodule | block-data

flang/lib/parser/parsing.cc

@@ -50,11 +50,6 @@ const SourceFile *Parsing::Prescan(const std::string &path, Options options) {
     return sourceFile;
   }
   CHECK(sourceFile);
-  if (sourceFile->bytes() == 0) {
-    ProvenanceRange range{allSources.AddCompilerInsertion(path)};
-    messages_.Say(range, "file is empty"_err_en_US);
-    return sourceFile;
-  }
 
   if (!options.isModuleFile) {
     // For .mod files we always want to look in the search directories.
@@ -86,6 +81,11 @@ const SourceFile *Parsing::Prescan(const std::string &path, Options options) {
   ProvenanceRange range{allSources.AddIncludedFile(
       *sourceFile, ProvenanceRange{}, options.isModuleFile)};
   prescanner.Prescan(range);
+  if (cooked_.BufferedBytes() == 0 && !options.isModuleFile) {
+    // Input is empty.  Append a newline so that any warning
+    // message about nonstandard usage will have provenance.
+    cooked_.Put('\n', range.start());
+  }
   cooked_.Marshal();
   if (options.needProvenanceRangeToCharBlockMappings) {
     cooked_.CompileProvenanceRangeToOffsetMappings();

flang/lib/parser/provenance.cc

@@ -461,6 +461,8 @@ std::optional<CharBlock> CookedSource::GetCharBlock(
   }
 }
 
+std::size_t CookedSource::BufferedBytes() const { return buffer_.bytes(); }
+
 void CookedSource::Marshal() {
   CHECK(provenanceMap_.SizeInBytes() == buffer_.bytes());
   provenanceMap_.Put(allSources_.AddCompilerInsertion("(after end of source)"));

flang/lib/parser/provenance.h

@@ -262,6 +262,7 @@ public:
     provenanceMap_.Put(pm);
   }
 
+  std::size_t BufferedBytes() const;
   void Marshal();  // marshals text into one contiguous block
   void CompileProvenanceRangeToOffsetMappings();
   std::string AcquireData() { return std::move(data_); }

flang/lib/parser/source.cc

@@ -223,17 +223,11 @@ bool SourceFile::ReadFile(std::string errorPath, std::stringstream *error) {
     --openFileDescriptors;
   }
   fileDescriptor_ = -1;
-  if (buffer.bytes() == 0) {
-    // empty file
-    address_ = content_ = nullptr;
-    size_ = bytes_ = 0;
-  } else {
-    normalized_ = buffer.MarshalNormalized();
-    address_ = normalized_.data();
-    size_ = normalized_.size();
-    IdentifyPayload();
-    RecordLineStarts();
-  }
+  normalized_ = buffer.MarshalNormalized();
+  address_ = normalized_.c_str();
+  size_ = normalized_.size();
+  IdentifyPayload();
+  RecordLineStarts();
   return true;
 }