[clang] [clang] Fix loss of `dllexport` for exported template specialization (PR #94664)

[Andrew Ng via cfe-commits]

https://github.com/nga888 created https://github.com/llvm/llvm-project/pull/94664

When dropping DLL attributes, ensure that the most recent declaration is being checked.

>From 05175ee44ce0b796b641beb34b18d7974c8355d2 Mon Sep 17 00:00:00 2001
From: Andrew Ng <andrew.ng at sony.com>
Date: Fri, 31 May 2024 17:17:18 +0100
Subject: [PATCH] [clang] Fix loss of `dllexport` for exported template
 specialization

When dropping DLL attributes, ensure that the most recent declaration is
being checked.
---
 clang/lib/CodeGen/CodeGenModule.cpp            | 17 +++++++++++++----
 ...tiate-dllexport-template-specialization.cpp | 18 ++++++++++++++++++
 2 files changed, 31 insertions(+), 4 deletions(-)
 create mode 100644 clang/test/CodeGenCXX/windows-instantiate-dllexport-template-specialization.cpp

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index be7bf0b72dc0c..dcd80bc19e25c 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -4561,10 +4561,19 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
     }
 
     // Handle dropped DLL attributes.
-    if (D && !D->hasAttr<DLLImportAttr>() && !D->hasAttr<DLLExportAttr>() &&
-        !shouldMapVisibilityToDLLExport(cast_or_null<NamedDecl>(D))) {
-      Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
-      setDSOLocal(Entry);
+    if (D) {
+      auto SC = Entry->getDLLStorageClass();
+      if (SC != llvm::GlobalValue::DefaultStorageClass) {
+        const Decl *MRD = D->getMostRecentDecl();
+        if (((SC == llvm::GlobalValue::DLLImportStorageClass &&
+              !MRD->hasAttr<DLLImportAttr>()) ||
+             (SC == llvm::GlobalValue::DLLExportStorageClass &&
+              !MRD->hasAttr<DLLExportAttr>())) &&
+            !shouldMapVisibilityToDLLExport(cast<NamedDecl>(MRD))) {
+          Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
+          setDSOLocal(Entry);
+        }
+      }
     }
 
     // If there are two attempts to define the same mangled name, issue an
diff --git a/clang/test/CodeGenCXX/windows-instantiate-dllexport-template-specialization.cpp b/clang/test/CodeGenCXX/windows-instantiate-dllexport-template-specialization.cpp
new file mode 100644
index 0000000000000..97f341ba1f909
--- /dev/null
+++ b/clang/test/CodeGenCXX/windows-instantiate-dllexport-template-specialization.cpp
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -triple i686-windows         -fdeclspec -emit-llvm %s -o - | FileCheck %s -check-prefix CHECK-MS
+// RUN: %clang_cc1 -triple i686-windows-itanium -fdeclspec -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-scei-ps4      -fdeclspec -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-sie-ps5       -fdeclspec -emit-llvm %s -o - | FileCheck %s
+
+struct s {
+  template <bool b = true> static bool f();
+};
+
+template <typename T> bool template_using_f(T) { return s::f(); }
+
+bool use_template_using_f() { return template_using_f(0); }
+
+template<>
+bool __declspec(dllexport) s::f<true>() { return true; }
+
+// CHECK-MS: dllexport {{.*}} @"??$f@$00 at s@@SA_NXZ"
+// CHECK: dllexport {{.*}} @_ZN1s1fILb1EEEbv