[Module map] Introduce a private module re-export directive.

Introduce a new "export_as" directive for top-level modules, which
indicates that the current module is a "private" module whose symbols
will eventually be exported through the named "public" module. This is
in support of a common pattern in the Darwin ecosystem where a single
public framework is constructed of several private frameworks, with
(currently) header duplication and some support from the linker.

Addresses rdar://problem/34438420.

llvm-svn: 313316

2 files changed, 19 insertions, 1 deletions

diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 3dba2997905..46823030d30 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -5123,7 +5123,7 @@ ASTReader::ReadSubmoduleBlock(ModuleFile &F, unsigned ClientLoadCapabilities) {
       break;
     }
 
-    case SUBMODULE_INITIALIZERS:
+    case SUBMODULE_INITIALIZERS: {
       if (!ContextObj)
         break;
       SmallVector<uint32_t, 16> Inits;
@@ -5132,6 +5132,11 @@ ASTReader::ReadSubmoduleBlock(ModuleFile &F, unsigned ClientLoadCapabilities) {
       ContextObj->addLazyModuleInitializers(CurrentModule, Inits);
       break;
     }
+
+    case SUBMODULE_EXPORT_AS:
+      CurrentModule->ExportAsModule = Blob.str();
+      break;
+    }
   }
 }
 
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 8f4a908f540..063d6f46e34 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1130,6 +1130,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(SUBMODULE_TEXTUAL_HEADER);
   RECORD(SUBMODULE_PRIVATE_TEXTUAL_HEADER);
   RECORD(SUBMODULE_INITIALIZERS);
+  RECORD(SUBMODULE_EXPORT_AS);
 
   // Comments Block.
   BLOCK(COMMENTS_BLOCK);
@@ -2791,6 +2792,12 @@ void ASTWriter::WriteSubmodules(Module *WritingModule) {
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));    // Message
   unsigned ConflictAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
 
+  Abbrev = std::make_shared<BitCodeAbbrev>();
+  Abbrev->Add(BitCodeAbbrevOp(SUBMODULE_EXPORT_AS));
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));    // Macro name
+  unsigned ExportAsAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
+
+
   // Write the submodule metadata block.
   RecordData::value_type Record[] = {
       getNumberOfModules(WritingModule),
@@ -2925,6 +2932,12 @@ void ASTWriter::WriteSubmodules(Module *WritingModule) {
     if (!Inits.empty())
       Stream.EmitRecord(SUBMODULE_INITIALIZERS, Inits);
 
+    // Emit the name of the re-exported module, if any.
+    if (!Mod->ExportAsModule.empty()) {
+      RecordData::value_type Record[] = {SUBMODULE_EXPORT_AS};
+      Stream.EmitRecordWithBlob(ExportAsAbbrev, Record, Mod->ExportAsModule);
+    }
+    
     // Queue up the submodules of this module.
     for (auto *M : Mod->submodules())
       Q.push(M);