CodeGen: Copy tail padding when we're not dealing with a trivial copy assign or move assign operator.

This fixes a regression from r162254, the optimizer has problems reasoning
about the smaller memcpy as it's often not safe to widen a store but making it
smaller is.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164917 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 25 insertions, 7 deletions

diff --git a/lib/CodeGen/CGExprAgg.cpp b/lib/CodeGen/CGExprAgg.cpp
index c9ffc19a8b..200b43aa6d 100644
--- a/lib/CodeGen/CGExprAgg.cpp
+++ b/lib/CodeGen/CGExprAgg.cpp
@@ -1274,7 +1274,8 @@ LValue CodeGenFunction::EmitAggExprToLValue(const Expr *E) {
 void CodeGenFunction::EmitAggregateCopy(llvm::Value *DestPtr,
                                         llvm::Value *SrcPtr, QualType Ty,
                                         bool isVolatile,
-                                        CharUnits alignment) {
+                                        CharUnits alignment,
+                                        bool isAssignment) {
   assert(!Ty->isAnyComplexType() && "Shouldn't happen for complex");
 
   if (getContext().getLangOpts().CPlusPlus) {
@@ -1303,9 +1304,13 @@ void CodeGenFunction::EmitAggregateCopy(llvm::Value *DestPtr,
   // implementation handles this case safely.  If there is a libc that does not
   // safely handle this, we can add a target hook.
 
-  // Get data size and alignment info for this aggregate.
-  std::pair<CharUnits, CharUnits> TypeInfo = 
-    getContext().getTypeInfoDataSizeInChars(Ty);
+  // Get data size and alignment info for this aggregate. If this is an
+  // assignment don't copy the tail padding. Otherwise copying it is fine.
+  std::pair<CharUnits, CharUnits> TypeInfo;
+  if (isAssignment)
+    TypeInfo = getContext().getTypeInfoDataSizeInChars(Ty);
+  else
+    TypeInfo = getContext().getTypeInfoInChars(Ty);
 
   if (alignment.isZero())
     alignment = TypeInfo.second;
diff --git a/lib/CodeGen/CGExprCXX.cpp b/lib/CodeGen/CGExprCXX.cpp
index 7276440a47..16311963b3 100644
--- a/lib/CodeGen/CGExprCXX.cpp
+++ b/lib/CodeGen/CGExprCXX.cpp
@@ -241,7 +241,7 @@ RValue CodeGenFunction::EmitCXXMemberCallExpr(const CXXMemberCallExpr *CE,
       // We don't like to generate the trivial copy/move assignment operator
       // when it isn't necessary; just produce the proper effect here.
       llvm::Value *RHS = EmitLValue(*CE->arg_begin()).getAddress();
-      EmitAggregateCopy(This, RHS, CE->getType());
+      EmitAggregateAssign(This, RHS, CE->getType());
       return RValue::get(This);
     }
     
@@ -378,7 +378,7 @@ CodeGenFunction::EmitCXXOperatorMemberCallExpr(const CXXOperatorCallExpr *E,
       MD->isTrivial()) {
     llvm::Value *Src = EmitLValue(E->getArg(1)).getAddress();
     QualType Ty = E->getType();
-    EmitAggregateCopy(This, Src, Ty);
+    EmitAggregateAssign(This, Src, Ty);
     return RValue::get(This);
   }
 
diff --git a/lib/CodeGen/CodeGenFunction.h b/lib/CodeGen/CodeGenFunction.h
index fc930ec158..4a0c35bd82 100644
--- a/lib/CodeGen/CodeGenFunction.h
+++ b/lib/CodeGen/CodeGenFunction.h
@@ -1653,13 +1653,26 @@ public:
   void EmitExprAsInit(const Expr *init, const ValueDecl *D,
                       LValue lvalue, bool capturedByInit);
 
+  /// EmitAggregateCopy - Emit an aggrate assignment.
+  ///
+  /// The difference to EmitAggregateCopy is that tail padding is not copied.
+  /// This is required for correctness when assigning non-POD structures in C++.
+  void EmitAggregateAssign(llvm::Value *DestPtr, llvm::Value *SrcPtr,
+                           QualType EltTy, bool isVolatile=false,
+                           CharUnits Alignment = CharUnits::Zero()) {
+    EmitAggregateCopy(DestPtr, SrcPtr, EltTy, isVolatile, Alignment, true);
+  }
+
   /// EmitAggregateCopy - Emit an aggrate copy.
   ///
   /// \param isVolatile - True iff either the source or the destination is
   /// volatile.
+  /// \param isAssignment - If false, allow padding to be copied.  This often
+  /// yields more efficient.
   void EmitAggregateCopy(llvm::Value *DestPtr, llvm::Value *SrcPtr,
                          QualType EltTy, bool isVolatile=false,
-                         CharUnits Alignment = CharUnits::Zero());
+                         CharUnits Alignment = CharUnits::Zero(),
+                         bool isAssignment = false);
 
   /// StartBlock - Start new block named N. If insert block is a dummy block
   /// then reuse it.