[llvm-branch-commits] [clang] release/18.x: [clang codegen] Fix MS ABI detection of user-provided constructors. (#90151) (PR #90639)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/90639

Backport 3ab4ae9e58c09dfd8203547ba8916f3458a0a481

Requested by: @efriedma-quic

>From ffbaa2afd157c678f4535b233519229f7e5b1ad3 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma at quicinc.com>
Date: Mon, 29 Apr 2024 12:00:12 -0700
Subject: [PATCH] [clang codegen] Fix MS ABI detection of user-provided
 constructors. (#90151)

In the context of determining whether a class counts as an "aggregate",
a constructor template counts as a user-provided constructor.

Fixes #86384

(cherry picked from commit 3ab4ae9e58c09dfd8203547ba8916f3458a0a481)
---
 clang/docs/ReleaseNotes.rst                      |  6 ++++++
 clang/lib/CodeGen/MicrosoftCXXABI.cpp            | 12 +++++++++---
 clang/test/CodeGen/arm64-microsoft-arguments.cpp | 15 +++++++++++++++
 3 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1e88b58725bd95..e533ecfd5aeba5 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -149,6 +149,12 @@ ABI Changes in This Version
 - Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer
   be split between a register and a stack slot.
 
+- Fixed Microsoft calling convention for returning certain classes with a
+  templated constructor. If a class has a templated constructor, it should
+  be returned indirectly even if it meets all the other requirements for
+  returning a class in a register. This affects some uses of std::pair.
+  (#GH86384).
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 - When dumping a sugared type, Clang will no longer print the desugared type if
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 172c4c937b9728..4d0f4c63f843b8 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1135,9 +1135,15 @@ static bool isTrivialForMSVC(const CXXRecordDecl *RD, QualType Ty,
     return false;
   if (RD->hasNonTrivialCopyAssignment())
     return false;
-  for (const CXXConstructorDecl *Ctor : RD->ctors())
-    if (Ctor->isUserProvided())
-      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;
+    }
+  }
   if (RD->hasNonTrivialDestructor())
     return false;
   return true;
diff --git a/clang/test/CodeGen/arm64-microsoft-arguments.cpp b/clang/test/CodeGen/arm64-microsoft-arguments.cpp
index e8309888dcfe21..85472645acb3b3 100644
--- a/clang/test/CodeGen/arm64-microsoft-arguments.cpp
+++ b/clang/test/CodeGen/arm64-microsoft-arguments.cpp
@@ -201,3 +201,18 @@ S11 f11() {
   S11 x;
   return func11(x);
 }
+
+// GH86384
+// Pass and return object with template constructor (pass directly,
+// return indirectly).
+// CHECK: define dso_local void @"?f12@@YA?AUS12@@XZ"(ptr dead_on_unwind inreg noalias writable sret(%struct.S12) align 4 {{.*}})
+// CHECK: call void @"?func12@@YA?AUS12@@U1@@Z"(ptr dead_on_unwind inreg writable sret(%struct.S12) align 4 {{.*}}, i64 {{.*}})
+struct S12 {
+  template<typename T> S12(T*) {}
+  int x;
+};
+S12 func12(S12 x);
+S12 f12() {
+  S12 x((int*)0);
+  return func12(x);
+}