Add support for putting constructors and destructos in explicit comdats.

There are situations when clang knows that the C1 and C2 constructors
or the D1 and D2 destructors are identical. We already optimize some
of these cases, but cannot optimize it when the GlobalValue is
weak_odr.

The problem with weak_odr is that an old TU seeing the same code will
have a C1 and a C2 comdat with the corresponding symbols. We cannot
suddenly start putting the C2 symbol in the C1 comdat as we cannot
guarantee that the linker will not pick a .o with only C1 in it.

The solution implemented by GCC is to expand the ABI to have a comdat
whose name uses a C5/D5 suffix and always has both symbols. That is
what this patch implements.

llvm-svn: 217874

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index 72869d8c..941e998 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -1292,6 +1292,9 @@ void CodeGenFunction::EmitDestructorBody(FunctionArgList &Args) {
   // we'd introduce *two* handler blocks.  In the Microsoft ABI, we 
   // always delegate because we might not have a definition in this TU.
   switch (DtorType) {
+  case Dtor_Comdat:
+    llvm_unreachable("not expecting a COMDAT");
+
   case Dtor_Deleting: llvm_unreachable("already handled deleting case");
 
   case Dtor_Complete: