[LLVMdev] libiomp, not libgomp as default library linked with -fopenmp

[Richard Smith]

On Thu, Apr 30, 2015 at 2:51 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> On Thu, Apr 30, 2015 at 6:52 AM Andrey Bokhanko <andreybokhanko at gmail.com>
> wrote:
>
>> All,
>>
>> I'd like to resurrect the discussion on replacing libgomp with libiomp as
>> the default OpenMP runtime library linked with -fopenmp.
>>
>
> Just for the record, I'm really excited to see this. =]
>
>
>> We are very close to getting *full* OpenMP 3.1 specification supported in
>> clang (only one (!) clause is not implemented yet, and the patch is already
>> sent for review today: http://reviews.llvm.org/D9370). This
>> implementation generates Intel API library calls; thus, it can't be used
>> with libgomp and it is simply logical to link a compatible runtime
>> (libiomp) instead.
>>
>
> Is there no way to support libgomp here as well? I don't say this to hold
> up changing the defaults in any way, just curious. =]
>
> Also, for the record, I'm really excited to see the progress here as well.
>
>
>> Chandler?
>>
>
> Hi! ;]
>
> I totally agree, I think things are way better now. I generally support
> the direction. I think there are a few things I'd suggest we do as part of
> the process, but I think these are really small and just about "how" we
> switch.
>
> 1) I completely agree with the comments some others have made about us
> needing to make it clear that this isn't some Intel-only thing, its the
> LLVM OpenMP runtime. Some suggestions that I think would make sense to help
> here:
> - I agree with finding some non-Intel folks to add as explicit code
> owners. I don't know who has been sufficiently involved, but if Hal makes
> sense, awesome.
> - Clearly updating the readme and such would be appropriate.
> - I suspect we should change the name of the installed library. 'libiomp'
> is pretty clearly the Intel library. We could continue in the grand
> tradition of LLVM naming conventions and use 'libllomp'? Of course, we
> should install symlinks under the name 'libiomp' if needed for existing
> users to not be broken.
>

Just some bikeshed-painting: if we're expecting each compiler that uses the
library to distribute a separate copy as part of that compiler's runtime,
then I think the best name for clang to use for the library would probably
be libclang_rt.omp-<arch> or libclang_rt.openmp-<arch> (as we do for all
our other runtime libraries). If we expect this to be installed somewhere
central on the system and shared by different compilers and different
versions of the same compiler, then libllomp or similar seems reasonable to
me. What's the intended distribution model here? How stable is the ABI?

- Any other changes?
>
> 2) I think we need to update the instructions for checking out LLVM and
> all the tools to include checking out the openmp project. I'm planning to
> try it out in a bit.
>
> 3) It would be nice to get at least one boring benchmark into the
> test-suite that uses OpenMP just so there's more coverage that the basic
> stuff all works. In particular, if we could get the benchmark that Phoronix
> and others keep pointing at, that'd be nice.
>
>
> Speaking of which, have you checked the performance of some of the basic
> benchmarks using OpenMP with the two runtimes? Or looked at Clang vs GCC
> there? I'd be interested to see the numbers.
>
>
>>
>> Yours,
>> Andrey Bokhanko
>> ==============
>> Software Engineer
>> Intel Compiler Team
>> Intel
>>
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