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, 15 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 8fb110e4551..5bf95f878d4 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3626,6 +3626,21 @@ void ASTDeclReader::UpdateDecl(Decl *D, ModuleFile &ModuleFile,
           Reader.ReadSourceLocation(ModuleFile, Record, Idx));
       break;
 
+    case UPD_CXX_INSTANTIATED_DEFAULT_ARGUMENT: {
+      auto Param = cast<ParmVarDecl>(D);
+
+      // We have to read the default argument regardless of whether we use it
+      // so that hypothetical further update records aren't messed up.
+      // TODO: Add a function to skip over the next expr record.
+      auto DefaultArg = Reader.ReadExpr(F);
+
+      // Only apply the update if the parameter still has an uninstantiated
+      // default argument.
+      if (Param->hasUninstantiatedDefaultArg())
+        Param->setDefaultArg(DefaultArg);
+      break;
+    }
+
     case UPD_CXX_ADDED_FUNCTION_DEFINITION: {
       FunctionDecl *FD = cast<FunctionDecl>(D);
       if (Reader.PendingBodies[FD]) {