[TLS on Darwin] change how we handle globals with linkonce or weak linkage.

This is about how we handle static member of a template. Before this commit,
we use internal linkage for the IR thread-local variable, which is inefficient.
With this commit, we will start to follow Itanium C++ ABI.

rdar://problem/23415206

Reviewed by John McCall.

llvm-svn: 252814

2 files changed, 16 insertions, 11 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 5d7794e4a30..d36a85df54c 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2315,12 +2315,17 @@ void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D) {
   llvm::GlobalValue::LinkageTypes Linkage =
       getLLVMLinkageVarDefinition(D, GV->isConstant());
 
-  // On Darwin, the backing variable for a C++11 thread_local variable always
-  // has internal linkage; all accesses should just be calls to the
+  // On Darwin, if the normal linkage of a C++ thread_local variable is
+  // LinkOnce or Weak, we keep the normal linkage to prevent multiple
+  // copies within a linkage unit; otherwise, the backing variable has
+  // internal linkage and all accesses should just be calls to the
   // Itanium-specified entry point, which has the normal linkage of the
-  // variable.
+  // variable. This is to preserve the ability to change the implementation
+  // behind the scenes.
   if (!D->isStaticLocal() && D->getTLSKind() == VarDecl::TLS_Dynamic &&
-      Context.getTargetInfo().getTriple().isMacOSX())
+      Context.getTargetInfo().getTriple().isMacOSX() &&
+      !llvm::GlobalVariable::isLinkOnceLinkage(Linkage) &&
+      !llvm::GlobalVariable::isWeakLinkage(Linkage))
     Linkage = llvm::GlobalValue::InternalLinkage;
 
   GV->setLinkage(Linkage);
diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index 0eea014b3d9..a6402f233c3 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -2163,12 +2163,10 @@ getThreadLocalWrapperLinkage(const VarDecl *VD, CodeGen::CodeGenModule &CGM) {
     return VarLinkage;
 
   // If the thread wrapper is replaceable, give it appropriate linkage.
-  if (isThreadWrapperReplaceable(VD, CGM)) {
-    if (llvm::GlobalVariable::isLinkOnceLinkage(VarLinkage) ||
-        llvm::GlobalVariable::isWeakODRLinkage(VarLinkage))
-      return llvm::GlobalVariable::WeakAnyLinkage;
-    return VarLinkage;
-  }
+  if (isThreadWrapperReplaceable(VD, CGM))
+    if (!llvm::GlobalVariable::isLinkOnceLinkage(VarLinkage) &&
+        !llvm::GlobalVariable::isWeakODRLinkage(VarLinkage))
+      return VarLinkage;
   return llvm::GlobalValue::WeakODRLinkage;
 }
 
@@ -2194,7 +2192,9 @@ ItaniumCXXABI::getOrCreateThreadLocalWrapper(const VarDecl *VD,
       llvm::Function::Create(FnTy, getThreadLocalWrapperLinkage(VD, CGM),
                              WrapperName.str(), &CGM.getModule());
   // Always resolve references to the wrapper at link time.
-  if (!Wrapper->hasLocalLinkage() && !isThreadWrapperReplaceable(VD, CGM))
+  if (!Wrapper->hasLocalLinkage() && !(isThreadWrapperReplaceable(VD, CGM) &&
+      !llvm::GlobalVariable::isLinkOnceLinkage(Wrapper->getLinkage()) &&
+      !llvm::GlobalVariable::isWeakODRLinkage(Wrapper->getLinkage())))
     Wrapper->setVisibility(llvm::GlobalValue::HiddenVisibility);
   return Wrapper;
 }