Support the tls_model attribute (PR9788)

This adds support for the tls_model attribute. This allows the user to
choose a TLS model that is better than what LLVM would select by
default. For example, a variable might be declared as:

  __thread int x __attribute__((tls_model("initial-exec")));

if it will not be used in a shared library that is dlopen'ed.

This depends on LLVM r159077.

llvm-svn: 159078

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1187,7 +1187,7 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
     new llvm::GlobalVariable(getModule(), Ty->getElementType(), false,
                              llvm::GlobalValue::ExternalLinkage,
                              0, MangledName, 0,
-                             false, AddrSpace);
+                             llvm::GlobalVariable::NotThreadLocal, AddrSpace);
 
   // Handle things which are present even on external declarations.
   if (D) {
@@ -1211,6 +1211,12 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
     }
 
     GV->setThreadLocal(D->isThreadSpecified());
+
+    // Set the TLS model if it it's explicitly specified.
+    if (D->hasAttr<TLSModelAttr>()) {
+      const TLSModelAttr *Attr = D->getAttr<TLSModelAttr>();
+      GV->setThreadLocalMode(GetLLVMTLSModel(Attr->getModel()));
+    }
   }
 
   if (AddrSpace != Ty->getAddressSpace())