PR36992: do not store beyond the dsize of a class object unless we know

the tail padding is not reused.

We track on the AggValueSlot (and through a couple of other
initialization actions) whether we're dealing with an object that might
share its tail padding with some other object, so that we can avoid
emitting stores into the tail padding if that's the case. We still
widen stores into tail padding when we can do so.

Differential Revision: https://reviews.llvm.org/D45306

llvm-svn: 329342

1 files changed, 6 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index b01926e1cae..6797a5453ee 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -3018,7 +3018,8 @@ static AggValueSlot createPlaceholderSlot(CodeGenFunction &CGF,
                                Ty.getQualifiers(),
                                AggValueSlot::IsNotDestructed,
                                AggValueSlot::DoesNotNeedGCBarriers,
-                               AggValueSlot::IsNotAliased);
+                               AggValueSlot::IsNotAliased,
+                               AggValueSlot::DoesNotOverlap);
 }
 
 void CodeGenFunction::EmitDelegateCallArg(CallArgList &args,
@@ -3486,7 +3487,8 @@ RValue CallArg::getRValue(CodeGenFunction &CGF) const {
   if (!HasLV)
     return RV;
   LValue Copy = CGF.MakeAddrLValue(CGF.CreateMemTemp(Ty), Ty);
-  CGF.EmitAggregateCopy(Copy, LV, Ty, LV.isVolatile());
+  CGF.EmitAggregateCopy(Copy, LV, Ty, AggValueSlot::DoesNotOverlap,
+                        LV.isVolatile());
   IsUsed = true;
   return RValue::getAggregate(Copy.getAddress());
 }
@@ -3500,7 +3502,8 @@ void CallArg::copyInto(CodeGenFunction &CGF, Address Addr) const {
   else {
     auto Addr = HasLV ? LV.getAddress() : RV.getAggregateAddress();
     LValue SrcLV = CGF.MakeAddrLValue(Addr, Ty);
-    CGF.EmitAggregateCopy(Dst, SrcLV, Ty,
+    // We assume that call args are never copied into subobjects.
+    CGF.EmitAggregateCopy(Dst, SrcLV, Ty, AggValueSlot::DoesNotOverlap,
                           HasLV ? LV.isVolatileQualified()
                                 : RV.isVolatileQualified());
   }