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

[Jack Howarth]

On Thu, Apr 30, 2015 at 5:51 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.
> - 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.
>

Another set of benchmark numbers are attached in
openmpbench_C_v3_clang-3.7.log and openmpbench_C_v3_gcc_5.1.log using the
schedbench, syncbench and taskbench compiled with the default -O1
optimization using the clang 3.7svn trunk with the two remaining OPENMP
patches from cfe-commits and the gcc 5.1.0 release compiler. The
OMP_NUM_THREADS=4 is set before each set of runs which were performed on
x86_64-apple-darwin14 using a early 2008 MacPro with dual 2.8 GHz quad-core
Xeon processors and 10 Gb of memory. The benchmark is detailed at
https://www.epcc.ed.ac.uk/research/computing/performance-characterisation-and-benchmarking/epcc-openmp-micro-benchmark-suite
.
>
>
>
>>
>> Yours,
>> Andrey Bokhanko
>> ==============
>> Software Engineer
>> Intel Compiler Team
>> Intel
>>
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>>
>
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