[clang] [clang] Fix loss of `dllexport` for exported template specialization (PR #93302)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Andrew Ng (nga888)

<details>
<summary>Changes</summary>

In commit 0a20f541, "Better codegen support for DLL attributes being dropped after the first declaration (PR20792)", code was added to enable "dropping" of DLL attributes. The specific issue and example given was related to `dllimport` and this was the test case that was added in that commit.

However, the code also included the "dropping" of `dllexport` but no test case for this was added. This "dropping" of `dllexport` can cause a `dllexport` template specialization to incorrectly lose its `dllexport` attribute as shown by the test case in this patch.

As there appears to be no need for the "dropping" of `dllexport`, remove this code to fix this issue.

---
Full diff: https://github.com/llvm/llvm-project/pull/93302.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenModule.cpp (+11-5) 
- (added) clang/test/CodeGenCXX/windows-instantiate-dllexport-template-specialization.cpp (+18) 


``````````diff
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index e4774a587707a..ae8104fe1c4cb 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -4554,8 +4554,11 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(
         Entry->setLinkage(llvm::Function::ExternalLinkage);
     }
 
-    // Handle dropped DLL attributes.
-    if (D && !D->hasAttr<DLLImportAttr>() && !D->hasAttr<DLLExportAttr>() &&
+    // Handle dropped dllimport.
+    if (D &&
+        (Entry->getDLLStorageClass() ==
+         llvm::GlobalVariable::DLLImportStorageClass) &&
+        !D->hasAttr<DLLImportAttr>() &&
         !shouldMapVisibilityToDLLExport(cast_or_null<NamedDecl>(D))) {
       Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
       setDSOLocal(Entry);
@@ -4849,9 +4852,12 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName, llvm::Type *Ty,
         Entry->setLinkage(llvm::Function::ExternalLinkage);
     }
 
-    // Handle dropped DLL attributes.
-    if (D && !D->hasAttr<DLLImportAttr>() && !D->hasAttr<DLLExportAttr>() &&
-        !shouldMapVisibilityToDLLExport(D))
+    // Handle dropped dllimport.
+    if (D &&
+        (Entry->getDLLStorageClass() ==
+         llvm::GlobalVariable::DLLImportStorageClass) &&
+        !D->hasAttr<DLLImportAttr>() &&
+        !shouldMapVisibilityToDLLExport(cast_or_null<NamedDecl>(D)))
       Entry->setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
 
     if (LangOpts.OpenMP && !LangOpts.OpenMPSimd && D)
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

``````````

</details>


https://github.com/llvm/llvm-project/pull/93302