[PATCH] CodeGen: Fix linkage of reference temporaries

[David Majnemer]

================
Comment at: lib/CodeGen/CGDecl.cpp:133-134
@@ -132,4 @@
-    // can be uniqued.
-    if (D.isExternallyVisible()) {
-      Linkage = llvm::GlobalValue::LinkOnceODRLinkage;
-
----------------
Richard Smith wrote:
> Do you have tests for this change? It seems to imply that dllimport, dllexport, and selectany now work on static local variables and didn't previously.
It is a semantic error for any of those to appear on static local variables.  We get this wrong but I believe that's a different bug from this one.

================
Comment at: lib/CodeGen/CodeGenModule.cpp:2873-2874
@@ -2862,5 +2872,4 @@
   // Create a global variable for this lifetime-extended temporary.
-  llvm::GlobalVariable *GV =
-    new llvm::GlobalVariable(getModule(), Type, Constant,
-                             llvm::GlobalValue::PrivateLinkage,
-                             InitialValue, Name.c_str());
+  llvm::GlobalValue::LinkageTypes Linkage =
+      GetLLVMLinkageVarDefinition(VD, Constant);
+  unsigned AddrSpace = GetGlobalVarAddressSpace(
----------------
Richard Smith wrote:
> What should the linkage of the temporary be if the linkage of the variable is strong? Seems we have two choices here:
> 
>   1) Always the same as that of the variable. This creates more symbols than are strictly necessary, but allows us to emit the variable as available_externally in some circumstances.
>   2) If the variable is strong, the temporary is private.
> 
> The difference only seems to matter if it's possible to provide an available_externally definition (that is, for a static data member with an initializer specified within the class). Since our mangling doesn't collide with anyone else's, this will only matter if/when we start emitting the available_externally definitions.
I've updated this differential to implement option #2.  This //should// cause us to use `available_externally` for the reference temporary if the variable was `available_externally` while using `private` if the variable was `external`.

http://reviews.llvm.org/D3515