[clang][CodeGen] Fix MSVC ABI for classes with a deleted copy assignment operator (llvm#90547)

For global functions and static methods the MSVC ABI returns
structs/classes with a deleted copy assignment operator indirectly.
From local testing this ABI holds true for all currently supported
architectures including ARM64EC.

Author: MaxEW707

clang/docs/ReleaseNotes.rst

@@ -76,6 +76,9 @@ ABI Changes in This Version
   returning a class in a register. This affects some uses of std::pair.
   (#GH86384).
 
+- Fixed Microsoft calling convention when returning classes that have a deleted
+  copy assignment operator. Such a class should be returned indirectly.
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 

clang/lib/CodeGen/MicrosoftCXXABI.cpp

@@ -1122,7 +1122,22 @@ static bool isTrivialForMSVC(const CXXRecordDecl *RD, QualType Ty,
   //   No base classes
   //   No virtual functions
   // Additionally, we need to ensure that there is a trivial copy assignment
-  // operator, a trivial destructor and no user-provided constructors.
+  // operator, a trivial destructor, no user-provided constructors and no
+  // deleted copy assignment operator.
+
+  // We need to cover two cases when checking for a deleted copy assignment
+  // operator.
+  //
+  // struct S { int& r; };
+  // The above will have an implicit copy assignment operator that is deleted
+  // and there will not be a `CXXMethodDecl` for the copy assignment operator.
+  // This is handled by the `needsImplicitCopyAssignment()` check below.
+  //
+  // struct S { S& operator=(const S&) = delete; int i; };
+  // The above will not have an implicit copy assignment operator that is
+  // deleted but there is a deleted `CXXMethodDecl` for the declared copy
+  // assignment operator. This is handled by the `isDeleted()` check below.
+
   if (RD->hasProtectedFields() || RD->hasPrivateFields())
     return false;
   if (RD->getNumBases() > 0)
@@ -1131,13 +1146,18 @@ static bool isTrivialForMSVC(const CXXRecordDecl *RD, QualType Ty,
     return false;
   if (RD->hasNonTrivialCopyAssignment())
     return false;
+  if (RD->needsImplicitCopyAssignment() && !RD->hasSimpleCopyAssignment())
+    return false;
   for (const Decl *D : RD->decls()) {
     if (auto *Ctor = dyn_cast<CXXConstructorDecl>(D)) {
       if (Ctor->isUserProvided())
         return false;
     } else if (auto *Template = dyn_cast<FunctionTemplateDecl>(D)) {
       if (isa<CXXConstructorDecl>(Template->getTemplatedDecl()))
         return false;
+    } else if (auto *MethodDecl = dyn_cast<CXXMethodDecl>(D)) {
+      if (MethodDecl->isCopyAssignmentOperator() && MethodDecl->isDeleted())
+        return false;
     }
   }
   if (RD->hasNonTrivialDestructor())

clang/test/CodeGen/x64-microsoft-arguments.cpp

@@ -0,0 +1,92 @@
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -ffreestanding -emit-llvm -O0 \
+// RUN: -x c++ -o - %s | FileCheck %s
+
+int global_i = 0;
+
+// Pass and return object with a reference type (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f1@@YA?AUS1@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S1) align 8 {{.*}})
+// CHECK: call void @"?func1@@YA?AUS1@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S1) align 8 {{.*}}, i64 {{.*}})
+struct S1 {
+  int& r;
+};
+
+S1 func1(S1 x);
+S1 f1() {
+  S1 x{ global_i };
+  return func1(x);
+}
+
+// Pass and return object with a reference type within an inner struct (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f2@@YA?AUS2@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S2) align 8 {{.*}})
+// CHECK: call void @"?func2@@YA?AUS2@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S2) align 8 {{.*}}, i64 {{.*}})
+struct Inner {
+  int& r;
+};
+
+struct S2 {
+  Inner i;
+};
+
+S2 func2(S2 x);
+S2 f2() {
+  S2 x{ { global_i } };
+  return func2(x);
+}
+
+// Pass and return object with a reference type (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f3@@YA?AUS3@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S3) align 8 {{.*}})
+// CHECK: call void @"?func3@@YA?AUS3@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S3) align 8 {{.*}}, i64 {{.*}})
+struct S3 {
+  const int& r;
+};
+
+S3 func3(S3 x);
+S3 f3() {
+  S3 x{ global_i };
+  return func3(x);
+}
+
+// Pass and return object with a reference type within an inner struct (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f4@@YA?AUS4@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S4) align 8 {{.*}})
+// CHECK: call void @"?func4@@YA?AUS4@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S4) align 8 {{.*}}, i64 {{.*}})
+struct InnerConst {
+  const int& r;
+};
+
+struct S4 {
+  InnerConst i;
+};
+
+S4 func4(S4 x);
+S4 f4() {
+  S4 x{ { global_i } };
+  return func4(x);
+}
+
+// Pass and return an object with an explicitly deleted copy assignment operator (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f5@@YA?AUS5@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S5) align 4 {{.*}})
+// CHECK: call void @"?func5@@YA?AUS5@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S5) align 4 {{.*}}, i32 {{.*}})
+struct S5 {
+  S5& operator=(const S5&) = delete;
+  int i;
+};
+
+S5 func5(S5 x);
+S5 f5() {
+  S5 x{ 1 };
+  return func5(x);
+}
+
+// Pass and return an object with an explicitly defaulted copy assignment operator that is implicitly deleted (pass directly, return indirectly).
+// CHECK: define dso_local void @"?f6@@YA?AUS6@@XZ"(ptr dead_on_unwind noalias writable sret(%struct.S6) align 8 {{.*}})
+// CHECK: call void @"?func6@@YA?AUS6@@U1@@Z"(ptr dead_on_unwind writable sret(%struct.S6) align 8 {{.*}}, i64 {{.*}})
+struct S6 {
+  S6& operator=(const S6&) = default;
+  int& i;
+};
+
+S6 func6(S6 x);
+S6 f6() {
+  S6 x{ global_i };
+  return func6(x);
+}