[PATCH] Emit initializers for selectany globals in comdat-associative sections (PR20889)

[Reid Kleckner]

lgtm

================
Comment at: lib/CodeGen/CGDeclCXX.cpp:335
@@ -334,1 +334,3 @@
     DelayedCXXInitPosition.erase(D);
+  } else if (D->hasAttr<SelectAnyAttr>()) {
+    // SelectAny globals will be comdat-folded. Put the initializer into a COMDAT
----------------
hans wrote:
> rnk wrote:
> > Should we do the same for WeakAttr? What does GCC do?
> > 
> > I wonder if there is a more general GVA linkage check we should do. Can you add a test for static data members of implicitly instantiated dllexport class templates?
> Yes, doing the same for WeakAttr sounds reasonable.
> 
> I couldn't observe gcc doing anything special for dynamic initializers of weak objects :/
> 
> We do get this right for static data members of implicitly instantiated class templates; it's handled by your code above, and dllexport doesn't change it. I can add a test.
> 
> If we want to base this on GVA linkage, it seems to me that the associative comdat method should be applied to all symbols that can be merged, so GVA_StrongODR and GVA_DiscardableODR? But I don't think SelectAny or WeakAttr are reflected in the gva linkage.
I *think* selectany and templates are the only ways to get COMDAT data with dynamic initialization, so we should be OK with what we have then.

http://reviews.llvm.org/D5281