MS ABI: Pass 'sret' as the second parameter of instance methods

Summary:
MSVC always passes 'sret' after 'this', unlike GCC.  This required
changing a number of places in Clang that assumed the sret parameter was
always first in LLVM IR.

This fixes win64 MSVC ABI compatibility for methods returning structs.

Reviewers: rsmith, majnemer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3618

llvm-svn: 208458

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -622,7 +622,10 @@ void CodeGenFunction::StartFunction(GlobalDecl GD,
              !hasScalarEvaluationKind(CurFnInfo->getReturnType())) {
     // Indirect aggregate return; emit returned value directly into sret slot.
     // This reduces code size, and affects correctness in C++.
-    ReturnValue = CurFn->arg_begin();
+    auto AI = CurFn->arg_begin();
+    if (CurFnInfo->getReturnInfo().isSRetAfterThis())
+      ++AI;
+    ReturnValue = AI;
   } else if (CurFnInfo->getReturnInfo().getKind() == ABIArgInfo::InAlloca &&
              !hasScalarEvaluationKind(CurFnInfo->getReturnType())) {
     // Load the sret pointer from the argument struct and return into that.