[RFC] COMDAT Safe Module Level Multi versioning

[Cary Coutant]

> +@item -fno-weak-comdat-functions
> +@opindex fno-weak-comdat-functions
> +Do not use weak symbol support for comdat non-virtual functions, even if it
> +is provided by the linker.  By default, G++ uses weak symbols if they are
> +available.  This option is useful when comdat functions generated in certain
> +compilation units need to be kept local to the respective units and not exposed
> +globally.  This does not apply to virtual comdat functions as their pointers
> +may be taken via virtual tables.  This can cause unintended behavior if
> +the addresses of comdat functions are used.
>
> It's not really the "weak" that is causing the problem -- it's the
> "comdat". What the option really is doing is making the functions
> static rather than comdat. (It's all gated under flag_weak because
> weak symbols are the fall-back to link-once and comdat symbols.) I'd
> suggest phrasing this more in terms of static vs. comdat.

Oh, also, I'd suggest clarifying what you mean by "if the addresses of
comdat functions are used". I think what you really mean here is "if
pointers to the [now non-comdat] functions are compared for equality."

-cary