Re: [RFC] COMDAT Safe Module Level Multi versioning

[Sriraman Tallam <tmsriram at google dot com>]

On Tue, Aug 18, 2015 at 2:14 PM, Cary Coutant <ccoutant@gmail.com> wrote:
>>>>> Based on Richard's suggestion, I have a patch to localize comdat
>>>>> functions which seems like a very effective solution to this problem.
>>>>> The text size increase is limited to the extra comdat copies generated
>>>>> for the specialized modules (modules with unsafe options) which is
>>>>> usually only a few.   Since -fweak does something similar for
>>>>> functions,  I have called the new option -fweak-comdat-functions.
>>>>> This does not apply to virtual comdat functions as their addresses can
>>>>> always be leaked via the vtable. Using this flag with virtual
>>>>> functions generates a warning.
>
> +fweak-comdat-functions
> +C++ Var(flag_weak_comdat_functions) Init(1)
> +Specific to comdat functions(-fno-weak-comdat-functions : Localize
> Comdat Functions).
> +With -fno-weak-comdat-functions, virtual comdat functions are still linked as
> +weak functions.  With -fno-weak-comdat-functions, the address of the comdat
> +functions that are localized will be unique and this can cause unintended
> +behavior when addresses of comdat functions are used.
>
> Is one of those "With -fno-weak-comdat-functions" supposed to be "With
> -fweak-comdat-functions"? This description doesn't really say what the
> flag (without the "no") does, and doesn't explain what "localize"
> means.
>
> +@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.

Thanks, will make those changes.  Do you recommend a different name
for this flag like -fmake-comdat-functions-static?

Sri

>
> This looks like the right way to go, though.
>
> -cary