[clang] [clang] Correct Microsoft mangling of lifetime extended temporary objects. (PR #85529)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann created https://github.com/llvm/llvm-project/pull/85529

Lifetime extended temporary objects that are bound to references with static storage duration may have external linkage and therefore require mangled symbol names.  Clang uses an extension of the Microsoft ABI to give these symbols an implicit name of '$RT' followed by a discriminator and then mangles them similarly to the variable they are bound to.  Clang's mangling scheme differs from the one used by MSVC.

Previously, the `$RT<discriminator>` portion of the name was not registered as a back reference candidate and this resulted in incorrect back references for enclosing class and/or namespace scopes that might be referenced in the type of the object.

This is an ABI change and has the potential to cause backward compatibility issues with previous Clang releases.  Since MSVC uses a different mangling scheme, this change does not affect compatibility with MSVC.

This fixes one of the name mangling concerns reported in #85423.

>From 4c0149e934aae9d061e22400eb3eace47e4906be Mon Sep 17 00:00:00 2001
From: Tom Honermann <tom.honermann at intel.com>
Date: Sat, 16 Mar 2024 07:09:58 -0700
Subject: [PATCH] [clang] Correct Microsoft mangling of lifetime extended
 temporary objects.

Lifetime extended temporary objects that are bound to references with static
storage duration may have external linkage and therefore require mangled symbol
names.  Clang uses an extension of the Microsoft ABI to give these symbols an
implicit name of '$RT' followed by a discriminator and then mangles them
similarly to the variable they are bound to.  Clang's mangling scheme differs
from the one used by MSVC.

Previously, the '$RT<discriminator>' portion of the name was not registered as
a back reference candidate and this resulted in incorrect back references for
enclosing class and/or namespace scopes that might be referenced in the
type of the object.

This is an ABI change and has the potential to cause backward compatibility
issues with previous Clang releases.  Since MSVC uses a different mangling
scheme, this change does not affect compatibility with MSVC.
---
 clang/docs/ReleaseNotes.rst                         |  6 ++++++
 clang/lib/AST/MicrosoftMangle.cpp                   |  3 ++-
 clang/test/CodeGenCXX/mangle-ms-back-references.cpp | 13 +++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1ae35e6881d2f8..f0da6ca497b575 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -54,6 +54,12 @@ ABI Changes in This Version
   inline member function that contains a static local variable with a dynamic
   initializer is declared with ``__declspec(dllimport)``. (#GH83616).
 
+- Fixed Microsoft name mangling of lifetime extended temporary objects. This
+  change corrects missing back reference registrations that could result in
+  incorrect back reference indexes and suprising demangled name results. Since
+  MSVC uses a different mangling for these objects, compatibility is not affected.
+  (#GH85423).
+
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index aa26bb7ed46f48..cf9c2093a8f6a1 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -3911,7 +3911,8 @@ void MicrosoftMangleContextImpl::mangleReferenceTemporary(
   msvc_hashing_ostream MHO(Out);
   MicrosoftCXXNameMangler Mangler(*this, MHO);
 
-  Mangler.getStream() << "?$RT" << ManglingNumber << '@';
+  Mangler.getStream() << "?";
+  Mangler.mangleSourceName("$RT" + llvm::utostr(ManglingNumber));
   Mangler.mangle(VD, "");
 }
 
diff --git a/clang/test/CodeGenCXX/mangle-ms-back-references.cpp b/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
index b27a9c5acacb77..8707bff9534070 100644
--- a/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
+++ b/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
@@ -1,5 +1,18 @@
 // RUN: %clang_cc1 -fms-extensions -fblocks -emit-llvm %s -o - -triple=i386-pc-win32 | FileCheck %s
 
+namespace NS {
+// The name "RT1" for the name of the class below has been specifically
+// chosen to ensure that back reference lookup does not match against the
+// implicitly generated "$RT1" name of the reference temporary symbol.
+struct RT1 {
+  static const RT1& singleton;
+  int i;
+};
+const RT1& RT1::singleton = RT1{1};
+}
+// CHECK: "?$RT1 at singleton@RT1 at NS@@2ABU23 at B"
+// CHECK: "?singleton at RT1@NS@@2ABU12 at B"
+
 void f1(const char* a, const char* b) {}
 // CHECK: "?f1@@YAXPBD0 at Z"