Only instantiate a default argument once.

By storing the instantiated expression back in the ParmVarDecl,
we remove the last need for separately storing the sub-expression
of a CXXDefaultArgExpr.  This makes PCH/Modules merging quite
simple: CXXDefaultArgExpr records are serialized as references
to the ParmVarDecl, and we ignore redundant attempts to overwrite
the instantiated expression.

This has some extremely marginal impact on user-facing semantics.
However, the major effect is that it avoids IRGen errors about
conflicting definitions due to lambdas in the argument being
instantiated multiple times while sharing the same mangling.
It should also slightly improve memory usage and module file size.

rdar://23810407

llvm-svn: 256983

1 files changed, 14 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 0f50d7a42ea..e36e9185170 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4611,6 +4611,11 @@ void ASTWriter::WriteDeclUpdatesBlocks(RecordDataImpl &OffsetsRecord) {
         AddSourceLocation(Update.getLoc(), Record);
         break;
 
+      case UPD_CXX_INSTANTIATED_DEFAULT_ARGUMENT:
+        AddStmt(const_cast<Expr*>(
+                  cast<ParmVarDecl>(Update.getDecl())->getDefaultArg()));
+        break;
+
       case UPD_CXX_INSTANTIATED_CLASS_DEFINITION: {
         auto *RD = cast<CXXRecordDecl>(D);
         UpdatedDeclContexts.insert(RD->getPrimaryContext());
@@ -5779,6 +5784,15 @@ void ASTWriter::StaticDataMemberInstantiated(const VarDecl *D) {
        D->getMemberSpecializationInfo()->getPointOfInstantiation()));
 }
 
+void ASTWriter::DefaultArgumentInstantiated(const ParmVarDecl *D) {
+  assert(!WritingAST && "Already writing the AST!");
+  if (!D->isFromASTFile())
+    return;
+
+  DeclUpdates[D].push_back(
+      DeclUpdate(UPD_CXX_INSTANTIATED_DEFAULT_ARGUMENT, D));
+}
+
 void ASTWriter::AddedObjCCategoryToInterface(const ObjCCategoryDecl *CatD,
                                              const ObjCInterfaceDecl *IFD) {
   assert(!WritingAST && "Already writing the AST!");