[MinGW] Don't mark external variables as DSO local

Since MinGW supports automatically importing external variables from
DLLs even without the DLLImport attribute, we shouldn't mark them
as DSO local unless we actually know them to be local for sure.

Keep marking thread local variables as DSO local.

Differential Revision: https://reviews.llvm.org/D51382

llvm-svn: 340941

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 29692eedb53..c5cac6a3982 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -730,6 +730,14 @@ static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
     return false;
 
   const llvm::Triple &TT = CGM.getTriple();
+  if (TT.isWindowsGNUEnvironment()) {
+    // In MinGW, variables without DLLImport can still be automatically
+    // imported from a DLL by the linker; don't mark variables that
+    // potentially could come from another DLL as DSO local.
+    if (GV->isDeclarationForLinker() && isa<llvm::GlobalVariable>(GV) &&
+        !GV->isThreadLocal())
+      return false;
+  }
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmware builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations