[clang][MSVC] Correct mangling of thread-safe static initialization variables.

[tahonermann]

Static local variables with dynamic initializers depend on implicitly defined guard variables to synchronize thread-safe initialization. These guard variables may have external linkage and therefore require a stable name for linkage purposes. The Microsoft ABI assigns these variables a local name of $TSS followed by a discriminator and mangles them as a static local variable of type int. Previously, the $TSS<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 signature of the enclosing function. This change adds the previously missing back reference registration. This matches the mangling performed by MSVC and resolves incompatibilities when inline functions with static local variables are inlined across DLL boundaries.

This is an ABI change and has the potential to cause backward compatibility issues with previous Clang releases.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Tom Honermann (tahonermann)

Changes

Static local variables with dynamic initializers depend on implicitly defined guard variables to synchronize thread-safe initialization. These guard variables may have external linkage and therefore require a stable name for linkage purposes. The Microsoft ABI assigns these variables a local name of $TSS followed by a discriminator and mangles them as a static local variable of type int. Previously, the $TSS<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 signature of the enclosing function. This change adds the previously missing back reference registration. This matches the mangling performed by MSVC and resolves incompatibilities when inline functions with static local variables are inlined across DLL boundaries.

This is an ABI change and has the potential to cause backward compatibility issues with previous Clang releases.

---

Full diff: https://github.com/llvm/llvm-project/pull/85300.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+6)
• (modified) clang/lib/AST/MicrosoftMangle.cpp (+3-2)
• (modified) clang/test/CodeGenCXX/mangle-ms-back-references.cpp (+17)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8935a610722a31..1d7902671a18fd 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -47,6 +47,12 @@ C++ Specific Potentially Breaking Changes
 
 ABI Changes in This Version
 ---------------------------
+- Fixed Microsoft name mangling of implicitly defined variables used for thread
+  safe static initialization of static local variables. This change resolves
+  incompatibilities with code compiled by MSVC but might introduce
+  incompatibilities with code compiled by earlier versions of Clang when an
+  inline member function that contains a static local variable with a dynamic
+  initializer is declared with ``__declspec(dllimport)``. (#GH83616).
 
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index b272a546573a31..aa26bb7ed46f48 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -390,6 +390,7 @@ class MicrosoftCXXNameMangler {
                           const FunctionDecl *D = nullptr,
                           bool ForceThisQuals = false,
                           bool MangleExceptionSpec = true);
+  void mangleSourceName(StringRef Name);
   void mangleNestedName(GlobalDecl GD);
 
 private:
@@ -408,7 +409,6 @@ class MicrosoftCXXNameMangler {
     mangleUnqualifiedName(GD, cast<NamedDecl>(GD.getDecl())->getDeclName());
   }
   void mangleUnqualifiedName(GlobalDecl GD, DeclarationName Name);
-  void mangleSourceName(StringRef Name);
   void mangleOperatorName(OverloadedOperatorKind OO, SourceLocation Loc);
   void mangleCXXDtorType(CXXDtorType T);
   void mangleQualifiers(Qualifiers Quals, bool IsMember);
@@ -3920,7 +3920,8 @@ void MicrosoftMangleContextImpl::mangleThreadSafeStaticGuardVariable(
   msvc_hashing_ostream MHO(Out);
   MicrosoftCXXNameMangler Mangler(*this, MHO);
 
-  Mangler.getStream() << "?$TSS" << GuardNum << '@';
+  Mangler.getStream() << "?";
+  Mangler.mangleSourceName("$TSS" + llvm::utostr(GuardNum));
   Mangler.mangleNestedName(VD);
   Mangler.getStream() << "@4HA";
 }
diff --git a/clang/test/CodeGenCXX/mangle-ms-back-references.cpp b/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
index cb95c100b3d22e..b27a9c5acacb77 100644
--- a/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
+++ b/clang/test/CodeGenCXX/mangle-ms-back-references.cpp
@@ -83,3 +83,20 @@ class H;
 
 void ManyParams(T01 &, T02 &, T03 &, T04 &, T05 &, T06 &, T07 &, T08 &, T09 &, T10 &, H<T11> &, H<T11> &) {}
 // CHECK: "?ManyParams@@YAXAAVT01@@AAVT02@@AAVT03@@AAVT04@@AAVT05@@AAVT06@@AAVT07@@AAVT08@@AAVT09@@AAVT10@@AAV?$H@VT11@@@@AAV?$H@VT11@@@@@Z"
+
+namespace NS {
+// The name "TSS0" for the name of the class below has been specifically
+// chosen to ensure that back reference lookup does not match against the
+// implicitly generated "$TSS0" name of the thread safe static initialization
+// variable.
+struct __declspec(dllexport) TSS0 {
+  static TSS0& get();
+  __forceinline static TSS0& singleton() {
+    static TSS0& lsv = get();
+    return lsv;
+  }
+};
+}
+// CHECK: "?singleton@TSS0@NS@@SAAAU12@XZ"
+// CHECK: "?lsv@?1??singleton@TSS0@NS@@SAAAU23@XZ@4AAU23@A"
+// CHECK: "?$TSS0@?1??singleton@TSS0@NS@@SAAAU23@XZ@4HA"

[zmodem]

lgtm, thanks!

Please include a "Fixes #83616" reference in the commit message.