Debug Info: Add basic support for external types references.
This is a necessary prerequisite for bootstrapping the emission
of debug info inside modules.

- Adds a FlagExternalTypeRef to DICompositeType.
  External types must have a unique identifier.
- External type references are emitted using a forward declaration
  with a DW_AT_signature([DW_FORM_ref_sig8]) based on the UID.

http://reviews.llvm.org/D9612

llvm-svn: 242302
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 3555822..799398a 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -277,6 +277,13 @@
                dwarf::DW_FORM_ref_sig8, DIETypeSignature(Type));
 }
 
+void DwarfUnit::addDIETypeSignature(DIE &Die, dwarf::Attribute Attribute,
+                                    StringRef Identifier) {
+  uint64_t Signature = DD->makeTypeSignature(Identifier);
+  Die.addValue(DIEValueAllocator, Attribute, dwarf::DW_FORM_ref_sig8,
+               DIEInteger(Signature));
+}
+
 void DwarfUnit::addDIEEntry(DIE &Die, dwarf::Attribute Attribute,
                             DIEEntry Entry) {
   const DIE *DieCU = Die.getUnitOrNull();
@@ -700,7 +707,8 @@
 
   constructTypeDIE(TyDIE, cast<DICompositeType>(Ty));
 
-  updateAcceleratorTables(Context, Ty, TyDIE);
+  if (!Ty->isExternalTypeRef())
+    updateAcceleratorTables(Context, Ty, TyDIE);
   return &TyDIE;
 }
 
@@ -899,6 +907,13 @@
 }
 
 void DwarfUnit::constructTypeDIE(DIE &Buffer, const DICompositeType *CTy) {
+  if (CTy->isExternalTypeRef()) {
+    StringRef Identifier = CTy->getIdentifier();
+    assert(!Identifier.empty() && "external type ref without identifier");
+    addFlag(Buffer, dwarf::DW_AT_declaration);
+    return addDIETypeSignature(Buffer, dwarf::DW_AT_signature, Identifier);
+  }
+
   // Add name if not anonymous or intermediate type.
   StringRef Name = CTy->getName();