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Re: [RFC] COMDAT Safe Module Level Multi versioning

>>>> 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.

+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"

+@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.

This looks like the right way to go, though.


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