MS ABI: Disallow dllimported/exported variables from having TLS

Windows TLS relies on indexing through a tls_index in order to get at
the DLL's thread local variables.  However, this index is not exported
along with the variable: it is assumed that all accesses to thread local
variables are inside the same module which created the variable in the
first place.

While there are several implementation techniques we could adopt to fix
this (notably, the Itanium ABI gets this for free), it is not worth the
heroics.

Instead, let's just ban this combination.  We could revisit this in the
future if we need to.

This fixes PR21111.

llvm-svn: 219049

1 files changed, 12 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 03dd2a7093c..c3c436d8e31 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -9534,8 +9534,11 @@ Sema::FinalizeDeclaration(Decl *ThisDecl) {
     }
   }
 
+  // Grab the dllimport or dllexport attribute off of the VarDecl.
+  const InheritableAttr *DLLAttr = getDLLAttr(VD);
+
   // Imported static data members cannot be defined out-of-line.
-  if (const DLLImportAttr *IA = VD->getAttr<DLLImportAttr>()) {
+  if (const auto *IA = dyn_cast_or_null<DLLImportAttr>(DLLAttr)) {
     if (VD->isStaticDataMember() && VD->isOutOfLine() &&
         VD->isThisDeclarationADefinition()) {
       // We allow definitions of dllimport class template static data members
@@ -9556,6 +9559,14 @@ Sema::FinalizeDeclaration(Decl *ThisDecl) {
     }
   }
 
+  // dllimport/dllexport variables cannot be thread local, their TLS index
+  // isn't exported with the variable.
+  if (DLLAttr && VD->getTLSKind()) {
+    Diag(VD->getLocation(), diag::err_attribute_dll_thread_local) << VD
+                                                                  << DLLAttr;
+    VD->setInvalidDecl();
+  }
+
   if (UsedAttr *Attr = VD->getAttr<UsedAttr>()) {
     if (!Attr->isInherited() && !VD->isThisDeclarationADefinition()) {
       Diag(Attr->getLocation(), diag::warn_attribute_ignored) << Attr;