[ms-cxxabi] Add a new calling convention that swaps 'this' and 'sret'

MSVC always places the 'this' parameter for a method first.  The
implicit 'sret' pointer for methods always comes second.  We already
implement this for __thiscall by putting sret parameters on the stack,
but __cdecl methods require putting both parameters on the stack in
opposite order.

Using a special calling convention allows frontends to keep the sret
parameter first, which avoids breaking lots of assumptions in LLVM and
Clang.

Fixes PR15768 with the corresponding change in Clang.

Reviewers: ributzka, majnemer

Differential Revision: http://llvm-reviews.chandlerc.com/D2663

llvm-svn: 200561

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/include/llvm/IR/CallingConv.h b/llvm/include/llvm/IR/CallingConv.h
index 1eaf4f7f469..af44e8a30c3 100644
--- a/llvm/include/llvm/IR/CallingConv.h
+++ b/llvm/include/llvm/IR/CallingConv.h
@@ -137,7 +137,13 @@ namespace CallingConv {
     /// convention differs from the more common \c X86_64_SysV convention
     /// in a number of ways, most notably in that XMM registers used to pass
     /// arguments are shadowed by GPRs, and vice versa.
-    X86_64_Win64 = 79
+    X86_64_Win64 = 79,
+
+    /// \brief The calling convention used for __cdecl methods on win32.
+    /// Differs from the C calling convention only in that the order of the
+    /// first parameter and the sret parameter are swapped.
+    X86_CDeclMethod = 80
+
   };
 } // End CallingConv namespace