Debug Info: Unique forward declarations generated for scopes and types.

This is mostly a cleanup and results slightly more efficient debug info.

rdar://problem/25965038

Author: adrian-prantl

lib/IRGen/IRGenDebugInfo.cpp

@@ -545,18 +545,21 @@ llvm::DIScope *IRGenDebugInfo::getOrCreateContext(DeclContext *DC) {
     return getOrCreateContext(DC->getParent());
   case DeclContextKind::GenericTypeDecl: {
     auto *TyDecl = cast<GenericTypeDecl>(DC);
-    if (auto *DITy = getTypeOrNull(TyDecl->getDeclaredType().getPointer()))
+    auto *Ty = TyDecl->getDeclaredType().getPointer();
+    if (auto *DITy = getTypeOrNull(Ty))
       return DITy;
 
     // Create a Forward-declared type.
     auto Loc = getDebugLoc(SM, TyDecl);
     auto File = getOrCreateFile(Loc.Filename);
     auto Line = Loc.Line;
-    auto FwdDecl = DBuilder.createForwardDecl(
+    auto FwdDecl = DBuilder.createReplaceableCompositeType(
         llvm::dwarf::DW_TAG_structure_type, TyDecl->getName().str(),
         getOrCreateContext(DC->getParent()), File, Line,
         llvm::dwarf::DW_LANG_Swift, 0, 0);
-
+    ReplaceMap.emplace_back(
+        std::piecewise_construct, std::make_tuple(Ty),
+        std::make_tuple(static_cast<llvm::Metadata *>(FwdDecl)));
     return FwdDecl;
   }
   }
@@ -1792,6 +1795,17 @@ llvm::DIType *IRGenDebugInfo::getOrCreateType(DebugTypeInfo DbgTy) {
 void IRGenDebugInfo::finalize() {
   assert(LocationStack.empty() && "Mismatch of pushLoc() and popLoc().");
 
+  // Finalize all replaceable forward declarations.
+  for (auto &Ty : ReplaceMap) {
+    llvm::TempMDNode FwdDecl(cast<llvm::MDNode>(Ty.second));
+    llvm::Metadata *Replacement;
+    if (auto *FullType = getTypeOrNull(Ty.first))
+      Replacement = FullType;
+    else
+      Replacement = Ty.second;
+    DBuilder.replaceTemporary(std::move(FwdDecl),
+                              cast<llvm::MDNode>(Replacement));
+  }
   // Finalize the DIBuilder.
   DBuilder.finalize();
 }

lib/IRGen/IRGenDebugInfo.h

@@ -70,6 +70,9 @@ class IRGenDebugInfo {
   std::vector<std::pair<const SILDebugScope *, llvm::TrackingMDNodeRef>>
       LastInlineChain;
 
+  /// A list of replaceable fwddecls that need to be RAUWed at the end.
+  std::vector<std::pair<TypeBase *, llvm::TrackingMDRef>> ReplaceMap;
+
   llvm::BumpPtrAllocator DebugInfoNames;
   StringRef CWDName;                    /// The current working directory.
   llvm::DICompileUnit *TheCU = nullptr; /// The current compilation unit.

test/DebugInfo/parent-scope.swift

@@ -0,0 +1,31 @@
+// RUN: %target-swift-frontend -g -emit-ir %s | FileCheck %s
+
+public protocol P {
+  associatedtype AT;
+}
+
+// A generic declcontext cannot be shared, so we expect a
+// forward-declared type.
+// CHECK-GEN: ![[FWD:.*]] = !DICompositeType({{.*}}, name: "Generic",
+// CHECK-GEN-SAME:                           flags: DIFlagFwdDecl
+// CHECK-GEN: linkageName: "_TFV4main7Generic3setfwx2ATT_", scope: ![[FWD]],
+public struct Generic<T : P> {
+  public func get() -> T.AT? {
+    return nil
+  }
+  public func set(_ t: T.AT) {}
+}
+
+// But only one concrete type is expected.
+// CHECK: !DISubprogram({{.*}}linkageName: "_TFV4main8Concrete3getfT_GSqSi_",
+// CHECK-SAME:          scope: ![[CONC:[0-9]+]],
+// CHECK: ![[CONC]] = !DICompositeType({{.*}}, name: "Concrete",
+// CHECK-SAME-NOT: DIFlagFwdDecl
+public struct Concrete {
+  public func get() -> Int? {
+    return nil
+  }
+  public func set(_ t: Int) {}
+}
+
+public func getConcrete() -> Concrete? { return nil; }