[PATCH] MS ABI: Use musttail for vtable thunks that pass arguments by value

[David Majnemer]

LGTM with comments addressed.

================
Comment at: lib/CodeGen/CGVTables.cpp:260-261
@@ -248,5 +259,4 @@
   // Add the rest of the arguments.
-  for (FunctionDecl::param_const_iterator I = MD->param_begin(),
-       E = MD->param_end(); I != E; ++I)
-    EmitDelegateCallArg(CallArgs, *I, (*I)->getLocStart());
+  for (const ParmVarDecl *PD : MD->params())
+    EmitDelegateCallArg(CallArgs, PD, PD->getLocStart());
 
----------------
Commit this separately.

================
Comment at: lib/CodeGen/CGVTables.cpp:317-319
@@ +316,5 @@
+  // the exception of 'this'.
+  SmallVector<llvm::Value *, 8> Args;
+  for (llvm::Argument &A : CurFn->args())
+    Args.push_back(&A);
+
----------------
`SmallVector<llvm::Value *, 8> Args(CurFn->arg_begin(), CurFn->arg_end());` would be more efficient, it will reserve the "right" amount up-front.

================
Comment at: lib/CodeGen/MicrosoftCXXABI.cpp:1470
@@ -1469,27 +1469,3 @@
 
-  unsigned CallingConv;
-  CodeGen::AttributeListType AttributeList;
-  CGM.ConstructAttributeList(FnInfo, MD, AttributeList, CallingConv, true);
-  llvm::AttributeSet Attrs =
-      llvm::AttributeSet::get(CGF.getLLVMContext(), AttributeList);
-
-  // Do a musttail call with perfect argument forwarding.  Any inalloca argument
-  // will be forwarded in place without any copy.
-  SmallVector<llvm::Value *, 8> Args;
-  for (llvm::Argument &A : ThunkFn->args())
-    Args.push_back(&A);
-  llvm::CallInst *Call = CGF.Builder.CreateCall(Callee, Args);
-  Call->setTailCallKind(llvm::CallInst::TCK_MustTail);
-  Call->setAttributes(Attrs);
-  Call->setCallingConv(static_cast<llvm::CallingConv::ID>(CallingConv));
-
-  if (Call->getType()->isVoidTy())
-    CGF.Builder.CreateRetVoid();
-  else
-    CGF.Builder.CreateRet(Call);
-
-  // Finish the function to maintain CodeGenFunction invariants.
-  // FIXME: Don't emit unreachable code.
-  CGF.EmitBlock(CGF.createBasicBlock());
-  CGF.FinishFunction();
+  CGF.EmitCallAndReturnForThunk(Callee, 0);
 
----------------
`CGF.EmitCallAndReturnForThunk(Callee, /*Thunk=*/nullptr);`

http://reviews.llvm.org/D4613