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

[Jack Howarth]

On Fri, May 1, 2015 at 4:45 AM, Andrey Bokhanko <andreybokhanko at gmail.com>
wrote:

> Chandler,
>
> Thanks for the reply -- I always included you in libiomp supporters camp;
> it is good to see I wasn't mistaken! ;-)
>
> On Fri, May 1, 2015 at 12:51 AM, Chandler Carruth <chandlerc at google.com>
> wrote:
>
>> 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. =]
>>
>
> No, sorry. libgomp doesn't support Intel API and clang generates Intel API
> calls only -- as simple as that. Someday someone may implement generation
> of GNU API calls as well, but this is a separate big task that, IMHO,
> doesn't serve any real purpose -- and potentially introduces nasty
> GPL-related legal issues.
>
> There is an option to choose what library clang links
> (-fopenmp={libiomp|libgomp}), though.
>
>
>> 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.
>>
>
> This really belongs to a separate thread (
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-April/085037.html); see
> my answer there in a couple of minutes.
>
>
>> - 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?
>>
>
> Adding openmp-dev list (in retrospect, should have been done at the very
> start...), Jim Cownie and Andrey Churbanov.
>
>
>> 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.
>>
>
> Cool! Thank you!
>
>
>> 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.
>>
>
> This is very tricky for me -- I'm employed by a CPU vendor (Intel), and we
> have very strict rules and long processes for publishing benchmark results.
> I simply can't run a benchmark and say: "hey! clang has this number and gcc
> has that number".
>
> The only thing I can share is that we do tested SPEC OMP2012 (
> https://www.spec.org/omp2012/), which is the industry standard for OMP
> benchmarks, on a non-server class Darwin machine, and the results are quite
> good and comparable with other compilers.
>
> Speaking on Phoronix, two benchmarks where clang always lose due to lack
> of OpenMP are "John the Ripper" (
> http://www.phoronix.com/scan.php?page=article&item=clang-gcc-broadwell&num=3)
> and ImageMagick -- though latter is not included in most recent "clang vs
> gcc" comparison.
>

Unfortunately, the current llvm 3.7svn and available OPENMP patches don't
allow for imageMagick-6.8.2 to compile. The build fails on the compiler
error...

magick/attribute.c:331:12: error: loop iteration variable in the associated
loop of 'omp parallel for' directive may not be threadprivate or thread
local, predetermined as
      private
      for (i=0; i < (ssize_t) image->colors; i++)
           ^
magick/attribute.c:325:9: note: loop iteration variable is predetermined as
linear
        i;
        ^
magick/attribute.c:978:12: error: loop iteration variable in the associated
loop of 'omp parallel for' directive may not be threadprivate or thread
local, predetermined as
      private
      for (i=0; i < (ssize_t) image->colors; i++)
           ^
magick/attribute.c:972:9: note: loop iteration variable is predetermined as
linear
        i;
        ^
2 errors generated.

I get the same error when I try to build the latest ImageMagick 6.9.1-2
sources with clang 3.7svn using -fopenmp=libiomp5 -Xclang -fopenmp=libiomp5.


> Is there a generous soul (not employed by a CPU vendor :-)) willing to run
> "John the Ripper" with "clang -fopenmp=libiomp5 -Xclang -fopenmp=libiomp5
> -lm -O3" and compare results with "clang -O3"?
>
> Also, Jack Howarth did testing with some other benchmarks, and it is nice
> to see that clang + libiomp compare quite well (to say it mildly ;-)) with
> gcc + libgomp!
>
> Andrey
>
>
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