[Mesa-dev] [PATCH 7/7] configure.ac: static link LLVM by default

[Tom Stellard]

On 10/12/2017 07:14 AM, Emil Velikov wrote:
> On 5 October 2017 at 18:11, Tom Stellard <tstellar at redhat.com> wrote:
>> On 10/05/2017 08:40 AM, Emil Velikov wrote:
>>> On 5 October 2017 at 16:16, Tom Stellard <tstellar at redhat.com> wrote:
>>>> On 10/05/2017 06:33 AM, Michel Dänzer wrote:
>>>>> On 05/10/17 12:19 PM, Emil Velikov wrote:
>>>>>> From: Emil Velikov <emil.velikov at collabora.com>
>>>>>>
>>>>>> A while back Michel reported that LLVM has symbol versioning to avoid
>>>>>> symbol collisions. Based on observations LLVM 5.0 is the first upstream
>>>>>> version to actually has it.
>>>>>
>>>>> Not exactly. Adam Jackson originally added symbol versioning in LLVM 3.6
>>>>> (in SVN r214418), but it was only effective when LLVM was built with
>>>>> autotools. As of 5.0, it's effective with cmake as well.
>>>>>
>>>>>
>>>>>> Since symbol collisions do come up again and again (fortunately not so
>>>>>> often) let's flip the switch back to static.
>>>>>
>>>>> It seems a bit weird to make this change now, that LLVM is solving the
>>>>> issue for good. But I don't feel strongly about it.
>>>>>
>>>>>
>>>>
>>>> I agree with this, symbol versioning should solve the issues with  symbol
>>>> collisions, so why change this now?
>>>>
>>> LLVM with symbol versioning as not so widely used as I/we hope it was.
>>> See the list in my other reply.
>>>
>>
>> I looked at the list, but my preference is still that LLVM shared libraries
>> should remain the default.  What is motivating the change to static by
>> default?  Do the symbol collision problems affect most users?
>>
>> Static linking really just works around a bug/deficiency in older versions
>> of LLVM and I think this is something distros should be handling.
>>
>> static linking has the added downside of build breakage when LLVM changes
>> the component names for it's static libraries, which can be a pain.  Not
>> to mention the increase in library size.
>>
>> As a compromise, if shared libraries are really causing a lot of issues,
>> then maybe you could make static the default for for LLVM < 5.0, but I really
>> prefer using shared libraries for all versions.
>>
> Looking the whole thing from another angle:
> 
> I noticed that Fedora (RHEL?) has been using statlc libstdc++ for ~2 years.
> In the packaging [1] there is this comment:
> 
> # C++ note: we never say "catch" in the source.  we do say "typeid" once,
> # in an assert, which is patched out above.  LLVM doesn't use RTTI or throw.
> #
> # We do say 'catch' in the clover and d3d1x state trackers, but we're not
> # building those yet.
> 

These comments refer to the use of the -frtti -fexceptions compiler flags.
This doesn't really have anything to do the decision to statically or
dynamically link LLVM.

> d3d1x is long gone, but seemingly clover is build these days.
> Thus static linking LLVM is the way right approach in that case.
> 
> Of course - everyone who knows the pros/cons of the toggle can adjust
> it to their needs.
> 
> I hope that with these in mind I could get your blessing (ack) on the patch?
> 

I'm not a huge fan of having mesa work around bugs in other projects, but
it would be fine with me if we had static linking by default for llvm <= 4.0
and shared linking by default for llvm >= 5.0

-Tom

> Thanks
> Emil
> 
> [1] http://pkgs.fedoraproject.org/cgit/rpms/mesa.git/tree/mesa.spec#n387
>