[flang] Don't duplicate hermetic module file dependencies

flang/lib/Semantics/mod-file.cpp

@@ -143,18 +143,22 @@ void ModFileWriter::Write(const Symbol &symbol) {
   std::string path{context_.moduleDirectory() + '/' +
       ModFileName(symbol.name(), ancestorName, context_.moduleFileSuffix())};
 
-  UnorderedSymbolSet hermeticModules;
-  hermeticModules.insert(symbol);
+  std::set<std::string> hermeticModuleNames;
+  hermeticModuleNames.insert(symbol.name().ToString());
   UnorderedSymbolSet additionalModules;
   PutSymbols(DEREF(symbol.scope()),
       hermeticModuleFileOutput_ ? &additionalModules : nullptr);
   auto asStr{GetAsString(symbol)};
   while (!additionalModules.empty()) {
-    for (auto ref : UnorderedSymbolSet{std::move(additionalModules)}) {
-      if (hermeticModules.insert(*ref).second &&
-          !ref->owner().IsIntrinsicModules()) {
-        PutSymbols(DEREF(ref->scope()), &additionalModules);
-        asStr += GetAsString(*ref);
+    UnorderedSymbolSet nextPass{std::move(additionalModules)};
+    additionalModules.clear();
+    for (const Symbol &modSym : nextPass) {
+      if (!modSym.owner().IsIntrinsicModules() &&
+          hermeticModuleNames.find(modSym.name().ToString()) ==
+              hermeticModuleNames.end()) {
+        hermeticModuleNames.insert(modSym.name().ToString());
+        PutSymbols(DEREF(modSym.scope()), &additionalModules);
+        asStr += GetAsString(modSym);
       }
     }
   }

flang/test/Semantics/modfile77.F90

@@ -0,0 +1,37 @@
+!RUN: %flang -c -fhermetic-module-files -DWHICH=1 %s && %flang -c -fhermetic-module-files -DWHICH=2 %s && %flang -c -fhermetic-module-files %s && cat modfile77c.mod | FileCheck %s
+
+#if WHICH == 1
+module modfile77a
+  interface gen
+    procedure proc
+  end interface
+ contains
+  subroutine proc
+    print *, 'ok'
+  end
+end
+#elif WHICH == 2
+module modfile77b
+  use modfile77a
+end
+#else
+module modfile77c
+  use modfile77a
+  use modfile77b
+end
+#endif
+
+!CHECK: module modfile77c
+!CHECK: use modfile77a,only:proc
+!CHECK: use modfile77a,only:gen
+!CHECK: interface gen
+!CHECK: end interface
+!CHECK: end
+!CHECK: module modfile77a
+!CHECK: interface gen
+!CHECK: procedure::proc
+!CHECK: end interface
+!CHECK: contains
+!CHECK: subroutine proc()
+!CHECK: end
+!CHECK: end

flang/test/Semantics/modfile78.F90

@@ -0,0 +1,33 @@
+!RUN: %flang -c -fhermetic-module-files -DWHICH=1 %s && %flang -c -fhermetic-module-files -DWHICH=2 %s && %flang -c -fhermetic-module-files %s && cat modfile78c.mod | FileCheck %s
+
+#if WHICH == 1
+module modfile78a
+  integer :: global_variable = 0
+end
+#elif WHICH == 2
+module modfile78b
+  use modfile78a
+ contains
+  subroutine test
+  end
+end
+#else
+module modfile78c
+  use modfile78a
+  use modfile78b
+end
+#endif
+
+!CHECK: module modfile78c
+!CHECK: use modfile78a,only:global_variable
+!CHECK: use modfile78b,only:test
+!CHECK: end
+!CHECK: module modfile78a
+!CHECK: integer(4)::global_variable
+!CHECK: end
+!CHECK: module modfile78b
+!CHECK: use modfile78a,only:global_variable
+!CHECK: contains
+!CHECK: subroutine test()
+!CHECK: end
+!CHECK: end