Add the "thunk" attribute to MS ABI virtual member pointers

This attribute implies indicates that the function musttail calls
another function and returns whatever it returns. The return type of the
thunk is meaningless, as the thunk can dynamically call different
functions with different return types. So long as the callers bitcast
the thunk with the correct type, behavior is well defined.

This attribute was necessary to fix PR20944, where the indirect call
combiner noticed that the thunk returned void and replaced the results
of the indirect call instruction with undef.

Over-the-shoulder reviewed by David Majnemer.

llvm-svn: 226707

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 0741e5d7385..40f88f32a99 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1540,6 +1540,12 @@ llvm::Function *MicrosoftCXXABI::EmitVirtualMemPtrThunk(
   CGM.SetLLVMFunctionAttributes(MD, FnInfo, ThunkFn);
   CGM.SetLLVMFunctionAttributesForDefinition(MD, ThunkFn);
 
+  // Add the "thunk" attribute so that LLVM knows that the return type is
+  // meaningless. These thunks can be used to call functions with differing
+  // return types, and the caller is required to cast the prototype
+  // appropriately to extract the correct value.
+  ThunkFn->addFnAttr("thunk");
+
   // These thunks can be compared, so they are not unnamed.
   ThunkFn->setUnnamedAddr(false);