On Tue, Jun 16, 2015 at 4:22 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Tue, May 19, 2015 at 9:11 AM, Xinliang David Li <davidxl@google.com> wrote:
>>>
>>> Hm.  But which options are unsafe?  Also wouldn't it be better to simply
>>> _not_ have unsafe options produce comdats but always make local clones
>>> for them (thus emit the comdat with "unsafe" flags dropped)?
>>
>> Always localize comdat functions may lead to text size increase. It
>> does not work if the comdat function is a virtual function for
>> instance.
>
> 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.
>
> To summarize, this is the intended usage of this option. Modules which
> use unsafe code options, like -m<isa> for multiversioning, to generate
> code that is meant to run only on a subset of CPUs can generate
> comdats with specialized instructions which when picked by the linker
> can get run unconditionally causing SIGILL on unsupported platforms.
> This flag hides these comdats to be local to these modules and not
> make them available publicly,  with the caveat that it does not apply
> to virtual comdats.
>
> Could you please review?

Ping.  This patch uses Richard's suggestion to localize comdat
functions with option -fno-weak-comdat-functions.  Comments?

* c-family/c.opt (fweak-comdat-functions): New option.
* cp/decl2.c (comdat_linkage): Implement new option.  Warn when
virtual comdat functions are seen.
* doc/invoke.texi: Document new option.
* testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.


>
> * c-family/c.opt (fweak-comdat-functions): New option.
> * cp/decl2.c (comdat_linkage): Implement new option.  Warn when
> virtual comdat functions are seen.
> * doc/invoke.texi: Document new option.
> * testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
>
>
> Thanks
> Sri
>
>
>>
>> David
>>
>>
>>>
>>> Richard.
>>>
>>>>
>>>> Thanks
>>>> Sri