[Mesa-dev] [PATCH v3 25/25] configure.ac: Add required LLVM versions to the top

[Emil Velikov]

On 14 October 2016 at 01:45, Michel Dänzer <michel at daenzer.net> wrote:
> On 13/10/16 07:14 PM, Emil Velikov wrote:
>> On 13 October 2016 at 04:07, Michel Dänzer <michel at daenzer.net> wrote:
>>> On 13/10/16 03:37 AM, Tobias Droste wrote:
>>>> Am Mittwoch, 12. Oktober 2016, 11:53:50 CEST schrieb Emil Velikov:
>>>>>>
>>>>>> +LLVM_VERSION_REQUIRED_OPENCL=3.6.0
>>>>>> +LLVM_VERSION_REQUIRED_R600=3.6.0
>>>>>> +LLVM_VERSION_REQUIRED_RADEONSI=3.6.0
>>>>>
>>>>> There's a small related gotcha: as-is at build time we get the
>>>>> different codepaths thus, as people build against shared LLVM (hello
>>>>> Archlinux, I'm looking at you) and update their LLVM without
>>>>> rebuilding mesa (Arch I'm looking at you again) things go funny.
>>>
>>> What exactly happened there? LLVM upstream generates shared libraries
>>> named libLLVM-<major>.<minor>.so*, so it shouldn't be possible for a
>>> simple LLVM package update to break Mesa, unless Arch did something
>>> really stupid.
>>>
>> The issue is not specific to Arch, but anyone who links against shared LLVM.
>>
>> Here is another take on it:
>> At least swr and r600/radeonsi depend at _build_ time on the _patch_
>> version of LLVM. The latter of which is not part of the DSO name. Thus
>> at runtime as you change your LLVM (3.9.x>3.9.y) you'll execute the
>> "3.9.x" codepath even though you are be using "3.9.y" LLVM.
>
> That should be fine, since 3.9.y is backwards compatible with 3.9.x.
>
> Debian doesn't automatically recompile Mesa in such cases either, and I
> haven't seen any problems there because of that.
>
> So, what exactly was the problem?
>
Just grep through for LLVM_.*PATCH and you'll see it. Portable code
should not check that at compile time.

>>>>> Tl;Dr; We really want to enable static linking by default and prod
>>>>> distros to use it.
>>>>
>>>> I'm all in favor of statically linking LLVM (that's the way I'm doing this on
>>>> my pc).
>>>> I think the only reason this is not done is because people (also here on the
>>>> list) don't want any static linkg of external libraries because of size or
>>>> whatever.
>>>> So changing the default to static is easy, but I doubt it will make everyone
>>>> happy ;-)
>>>
>>> Indeed, it'd probably make many distro packagers unhappy, because
>>> they'll just have to re-enable shared linking, because packaging
>>> policies generally strongly discourage if not outright forbid static
>>> linking.
>>>
>> The toggle is there and is not going away, afaict. If people are going
>> to get upset that the default does not meet their policy... just
>> toggle and get on with other things ;-)
>
> The question is if it makes sense for the default to be different from
> what the majority of users end up using. It doesn't to me.
>
This might be better answered once you see what I mean above ;-)

Emil