Fix the IR gen for C++ method calls and refactor around CGFunctionInfo

In GenCalls, fix the IR gen for C++ method calls as under MSVC as the
calling conventions for free functions and C++ methods can be
different. This also fixes the missing inreg (on sret arguments)
issues on Windows ARM64.

In GenCalls, refactor to use CGFunctionInfo returnInfo isSretAfterThis
to detect when to reorder the sret and the this arguments under MSVC
in GenCalls.

In ClagImporter, don't drop the return type for the compound
assignment operators such as operator+= when the return value is a
reference so that the CGFunctionInfo will be correctly indicate an
indirect return for the compound assignment operators.

Author: hjyamauchi

lib/ClangImporter/ImportType.cpp

@@ -2203,10 +2203,11 @@ ImportedType ClangImporter::Implementation::importFunctionReturnType(
   // C++ operators +=, -=, *=, /= may return a reference to self. This is not
   // idiomatic in Swift, let's drop these return values.
   clang::OverloadedOperatorKind op = clangDecl->getOverloadedOperator();
-  if (op == clang::OverloadedOperatorKind::OO_PlusEqual ||
-      op == clang::OverloadedOperatorKind::OO_MinusEqual ||
-      op == clang::OverloadedOperatorKind::OO_StarEqual ||
-      op == clang::OverloadedOperatorKind::OO_SlashEqual)
+  if ((op == clang::OverloadedOperatorKind::OO_PlusEqual ||
+       op == clang::OverloadedOperatorKind::OO_MinusEqual ||
+       op == clang::OverloadedOperatorKind::OO_StarEqual ||
+       op == clang::OverloadedOperatorKind::OO_SlashEqual) &&
+      clangDecl->getReturnType()->isReferenceType())
     return {SwiftContext.getVoidType(), false};
 
   // Fix up optionality.

lib/IRGen/GenCall.cpp

@@ -1471,9 +1471,15 @@ void SignatureExpansion::expandExternalSignatureTypes() {
   // Generate function info for this signature.
   auto extInfo = clang::FunctionType::ExtInfo();
 
-  auto &FI = clang::CodeGen::arrangeFreeFunctionCall(IGM.ClangCodeGen->CGM(),
-                                             clangResultTy, paramTys, extInfo,
-                                             clang::CodeGen::RequiredArgs::All);
+  bool isCXXMethod =
+      FnType->getRepresentation() == SILFunctionTypeRepresentation::CXXMethod;
+  auto &FI = isCXXMethod ?
+      clang::CodeGen::arrangeCXXMethodCall(IGM.ClangCodeGen->CGM(),
+          clangResultTy, paramTys, extInfo, {},
+          clang::CodeGen::RequiredArgs::All) :
+      clang::CodeGen::arrangeFreeFunctionCall(IGM.ClangCodeGen->CGM(),
+          clangResultTy, paramTys, extInfo, {},
+          clang::CodeGen::RequiredArgs::All);
   ForeignInfo.ClangInfo = &FI;
 
   assert(FI.arg_size() == paramTys.size() &&

test/Interop/Cxx/class/method/Inputs/inreg-sret.h

@@ -0,0 +1,11 @@
+#ifndef TEST_INTEROP_CXX_CLASS_METHOD_INREG_SRET_H
+#define TEST_INTEROP_CXX_CLASS_METHOD_INREG_SRET_H
+
+struct OptionalBridgedBasicBlock {
+};
+
+struct BridgedFunction {
+  OptionalBridgedBasicBlock getFirstBlock() const { return {}; }
+};
+
+#endif // TEST_INTEROP_CXX_CLASS_METHOD_INREG_SRET_H

test/Interop/Cxx/class/method/Inputs/module.modulemap

@@ -19,3 +19,8 @@ module SRetWinARM64 {
   header "sret-win-arm64.h"
   requires cplusplus
 }
+
+module InRegSRet {
+  header "inreg-sret.h"
+  requires cplusplus
+}

test/Interop/Cxx/class/method/inreg-sret.swift

@@ -0,0 +1,28 @@
+// RUN: %target-swift-emit-irgen -I %S/Inputs -cxx-interoperability-mode=default %s | %FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-%target-cpu
+
+// REQUIRES: OS=windows-msvc
+
+import InRegSRet
+
+final public class BasicBlock {
+}
+
+extension OptionalBridgedBasicBlock {
+  public var block: BasicBlock? { nil }
+}
+
+final public class Function {
+  public var bridged: BridgedFunction {
+    BridgedFunction()
+  }
+
+  public var firstBlock : BasicBlock? { bridged.getFirstBlock().block }
+}
+
+// Check that inreg on the sret isn't missing
+
+// CHECK-x86_64: call void @"?getFirstBlock@BridgedFunction@@QEBA?AUOptionalBridgedBasicBlock@@XZ"(ptr {{.*}}, ptr noalias nocapture sret(%TSo25OptionalBridgedBasicBlockV) {{.*}})
+// CHECK-aarch64: call void @"?getFirstBlock@BridgedFunction@@QEBA?AUOptionalBridgedBasicBlock@@XZ"(ptr {{.*}}, ptr inreg noalias nocapture sret(%TSo25OptionalBridgedBasicBlockV) {{.*}})
+
+// CHECK-x86_64: define {{.*}} void @"?getFirstBlock@BridgedFunction@@QEBA?AUOptionalBridgedBasicBlock@@XZ"(ptr {{.*}} %{{.*}}, ptr noalias sret(%struct.OptionalBridgedBasicBlock) {{.*}} %{{.*}})
+// CHECK-aarch64: define {{.*}} void @"?getFirstBlock@BridgedFunction@@QEBA?AUOptionalBridgedBasicBlock@@XZ"(ptr {{.*}} %{{.*}}, ptr inreg noalias sret(%struct.OptionalBridgedBasicBlock) {{.*}} %{{.*}})