[PATCH] D71572: [ItaniumCXXABI] Use linkonce_odr instead of weak_odr for tls wrappers on Windows

Tue Dec 17 04:16:18 PST 2019

mstorsjo marked an inline comment as done.
mstorsjo added inline comments.

================
Comment at: clang/lib/CodeGen/ItaniumCXXABI.cpp:2519-2520
       return VarLinkage;
+  // On Windows, WeakODR is a no-op, boiling down to the same as normal external
+  // linkage.
+  if (CGM.getTriple().isOSWindows())
----------------
mstorsjo wrote:
> rnk wrote:
> > mstorsjo wrote:
> > > rnk wrote:
> > > > I would say that this is inaccurate. It greatly affects what the optimizer is allowed to do.
> > > > 
> > > > It looks like we forgot to put a comdat on these things, is that not the correct fix?
> > > Oh, ok.
> > > 
> > > The full case I was trying to fix (but forgot to recheck after changing this bit) is that when used with `-ffunction-sections`, the tls wrapper function ends up as comdat `one_only`, which then gives multiple definition errors. So perhaps the issue is in the handling of `-ffunction-sections` wrt weak_odr?
> > Ah, that is an interesting wrinkle. I'm surprised that things worked without -ffunction-sections, though. I would've expected the two plain external definitions to produce multiple definition errors.
> Without `-ffunction-sections`, there's no separate thread wrapper produced at all, so everything else is generated with working linkage. I just haven't happened to build code that forces generation of a tls wrapper without `-ffunction-sections` yet.
Actually, sorry, I misremembered. Yes, I think the same issue would be present even without `-ffunction-sections`.

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  https://reviews.llvm.org/D71572/new/

https://reviews.llvm.org/D71572