[llvm-dev] Ubuntu LLVM packages incompatible with clang built projects?

[Alastair Murray via llvm-dev]

Hi Hans,

No we didn't surface this beyond submitting the patch.  As Reid pointed 
out these same ifdefs have already been removed/re-added a few times in 
2018, so while the patch works for us it isn't suitable for back-porting 
until it has been merged to trunk and the build-bots have given it a try.

It did not occur to me that it would not be possible to apply the change 
to 7.0.1, but your point make sense.

We didn't notice this during release-testing in the sense of building an 
LLVM package, but rather when linking some configurations of our own 
code against an LLVM built from the release branch.

Regards,
Alastair Murray.


On 29/09/18 09:09, Hans Wennborg wrote:
> Trunk still has the different gcc and clang versions.
>
> What's worse, the 7.0.0 release has them too :-( I completely missed
> this and we can't fix it for 7.0.1 since that would also be an ABI
> break.
>
> Alastair, if you noticed this during the release testing, did you
> surface it anywhere?
>
>
> On Fri, Sep 28, 2018 at 6:07 PM, Reid Kleckner <rnk at google.com> wrote:
>> Just be aware that those ifdefs were recommitted and reverted several times,
>> so I'm not sure what the state is.
>>
>> On Fri, Sep 28, 2018 at 8:48 AM Alastair Murray via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>> Hi Kern,
>>>
>>> We also had issues with mixing GCC/Clang builds when testing the 7.0
>>> release branch.
>>>
>>> My colleague submitted a patch that fixed the issue for us:
>>> * https://reviews.llvm.org/D50710
>>>
>>> I no longer have a copy of the error we were seeing to see if it was the
>>> same, but the fix is to llvm::OptionalStorage and your error message
>>> involves llvm::Optional.  The patch removes a GCC specific ifdef, and
>>> hopefully fixes the issue that made that necessary in the first place, but
>>> we never really knew how the GCC miscompilation was expressed.
>>>
>>> It would be interesting to know whether this resolves your issue, but
>>> regardless we should give the patch a prod.
>>>
>>> Regards,
>>> Alastair Murray.
>>>
>>>
>>> On 27/09/18 08:46, Kern Handa via llvm-dev wrote:
>>>
>>> Hi folks,
>>>
>>> Not sure if this is the right mailing list target, but I'm trying out the
>>> new LLVM 7.0 packages found at http://apt.llvm.org by porting over an
>>> existing LLVM 6.0 project of ours to the new version. In doing so, I found
>>> that the executable always segfaulted at the same spot with no explanation:
>>>
>>> 0x0000000000fefe33 in
>>> llvm::RegisterTargetMachine<llvm::X86TargetMachine>::Allocator(llvm::Target
>>> const&, llvm::Triple const&, llvm::StringRef, llvm::StringRef,
>>> llvm::TargetOptions const&, llvm::Optional<llvm::R
>>> eloc::Model>, llvm::Optional<llvm::CodeModel::Model>,
>>> llvm::CodeGenOpt::Level, bool) ()
>>>
>>> This happens if I build my project and link it against LLVM 7.0 with
>>> either clang++ 6.1 or clang++ 7.0. This does not happen if I build my
>>> project with g++ 8.
>>>
>>> I have also confirmed that building LLVM 7.0 with clang++ and then using
>>> that private build of the libraries fixes the issue.
>>>
>>> To summarize, it seems that there's an ABI incompatibility introduced
>>> somewhere, which means LLVM 7.0 compiled with gcc can only be used by other
>>> projects also built with gcc. Is this something that's being investigated
>>> and to be resolved as a fix in 7.1? If not, is there any official release
>>> note that's remarking on this incompatibility?
>>>
>>> Thanks,
>>> Kern

-- 
Alastair Murray
Principal Software Engineer, Compilers
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