Re: [RFC] COMDAT Safe Module Level Multi versioning

[Sriraman Tallam <tmsriram at google dot com>]

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?

* 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

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