On Thu, Dec 18, 2014 at 5:52 PM, David Majnemer <david.majne...@gmail.com> wrote: > > On Thu, Dec 18, 2014 at 5:46 PM, Richard Smith <rich...@metafoo.co.uk> > wrote: >> >> On Thu, Dec 18, 2014 at 2:30 PM, David Majnemer <david.majne...@gmail.com >> > wrote: >>> >>> On Wednesday, December 17, 2014, Richard Smith <rich...@metafoo.co.uk> >>> wrote: >>> >>>> On Wed, Dec 17, 2014 at 3:52 PM, Rafael Espíndola < >>>> rafael.espind...@gmail.com> wrote: >>>>> >>>>> > Based on talking to David, we think the frontend should be >>>>> responsible for >>>>> > picking whether things are ODR or not, and then LLVM optimizations >>>>> can >>>>> > continue to be mostly ignorant of comdats. For example, the D5 >>>>> destructor >>>>> > comdat group functions should all be inlinable. >>>>> >>>>> +1 :-) >>>>> >>>> >>>> I think that makes a lot of sense. Does this imply that globals within >>>> a comdat would need to have some kind of weak linkage? >>>> >>> >>> The only non-weak linkage we allow in COMDATs is external. The closest >>> analog is weak_odr but it has subtly different semantics wrt duplicate >>> symbol definitions. >>> >>> I'll try to dig up a concrete example as soon as I can get to my laptop. >>> >> >> I think Rafael's right here: external symbols in a comdat are meaningful >> (and presumably they mean that you can't have that symbol defined in any >> other object file you link in, so you either get your definition or no >> definition), and I can imagine (rare) cases when they might be what you >> want. (And if not, don't do that...) >> > > I think it means something different from what you are suggesting. > STB_WEAK is orthogonal to COMDAT-ness. Clang when targeting MSVC uses > COMDATs with external linkage to mean: > - This symbol is in some COMDAT group > - It is not weak, don't replace my symbol with some non-weak definition. > - This symbol cannot be discarded. > > I don't think there is another combination of linkage and comdat-ness > which gets the same effects. >
... in that case, all our interprocedural optimizations *do* need to know about comdats, and being in a comdat needs to make an external symbol effectively weak for optimization purposes (you can't propagate information out of it, because it might be replaced by a different definition). This violates Reid's desire that optimizations can be mostly ignorant of comdats.
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