[PATCH] D34972: [CodeGen] Propagate dllexport to thunks

[Shoaib Meenai via cfe-commits]

The thunks are (as far as I know, at least) only referenced from vtables, so
there's no opportunity to perform indirect calls via the IAT for them. The
vtable would just end up referencing the linker-generated import thunk
instead.

I can play around with marking the thunks as dllimport if the function being
thunked is dllimport, but I was curious if you had any specific scenarios in
mind where you thought it would make a difference.

From: cfe-commits <cfe-commits-bounces at lists.llvm.org> on behalf of David Majnemer via cfe-commits <cfe-commits at lists.llvm.org>
Reply-To: David Majnemer <david.majnemer at gmail.com>
Date: Tuesday, July 4, 2017 at 8:42 AM
To: Shoaib Meenai via Phabricator <reviews at reviews.llvm.org>, "compnerd at compnerd.org" <compnerd at compnerd.org>, "reviews+D34972+public+8a22767368a5b974 at reviews.llvm.org" <reviews+D34972+public+8a22767368a5b974 at reviews.llvm.org>, "rnk at google.com" <rnk at google.com>
Cc: "zturner at google.com" <zturner at google.com>, "cfe-commits at lists.llvm.org" <cfe-commits at lists.llvm.org>
Subject: Re: [PATCH] D34972: [CodeGen] Propagate dllexport to thunks

What about the import side?

On Mon, Jul 3, 2017 at 10:37 PM Shoaib Meenai via Phabricator via cfe-commits <cfe-commits at lists.llvm.org> wrote:
smeenai created this revision.

Under Windows Itanium, we need to export virtual and non-virtual thunks
if the functions being thunked are exported. These thunks would
previously inherit their dllexport attribute from the declaration, but
r298330 changed declarations to not have dllexport attributes. We
therefore need to add the dllexport attribute to the definition
ourselves now. This is consistent with MinGW GCC's behavior.

This redoes r306770 but limits the logic to Itanium. MicrosoftCXXABI's
setThunkLinkage ensures that thunks aren't exported under that ABI, so
I'm handling this in ItaniumCXXABI's setThunkLinkage for symmetry.


https://reviews.llvm.org/D34972

Files:
  lib/CodeGen/ItaniumCXXABI.cpp
  test/CodeGenCXX/dllexport-vtable-thunks.cpp


Index: test/CodeGenCXX/dllexport-vtable-thunks.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/dllexport-vtable-thunks.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -fdeclspec -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-itanium -fdeclspec -emit-llvm -o - %s | FileCheck %s
+
+struct __declspec(dllexport) A {
+  virtual void m();
+};
+struct __declspec(dllexport) B {
+  virtual void m();
+};
+struct __declspec(dllexport) C : A, B {
+  virtual void m();
+};
+void C::m() {}
+// CHECK: define dllexport void @_ZThn8_N1C1mEv
+
+struct Base {
+  virtual void m();
+};
+struct __declspec(dllexport) Derived : virtual Base {
+  virtual void m();
+};
+void Derived::m() {}
+// CHECK: define dllexport void @_ZTv0_n24_N7Derived1mEv
Index: lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- lib/CodeGen/ItaniumCXXABI.cpp
+++ lib/CodeGen/ItaniumCXXABI.cpp
@@ -284,6 +284,11 @@
     // linkage together with vtables when needed.
     if (ForVTable && !Thunk->hasLocalLinkage())
       Thunk->setLinkage(llvm::GlobalValue::AvailableExternallyLinkage);
+
+    // Propagate dllexport storage.
+    const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
+    if (MD->hasAttr<DLLExportAttr>())
+      Thunk->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
   }

   llvm::Value *performThisAdjustment(CodeGenFunction &CGF, Address This,


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