[Llvm-bgq-discuss] Status of OpenMP support in BG/Q

[Jeff Hammond]

>> mpibgclang++/*: -std=c++11 -stdlib=libc++  -O3 -DNDEBUG -mtune=native
>> -fvectorize -fslp-vectorize
>
> -mtune=native seems wrong in general here: you're cross compiling! (I don't think it does anything with bgclang, but if it did, if would cause you to optimize for the P7 and not the A2, and the instruction scheduling is very different for those two cores).

I would argue that mpibgclang should provide native=a2qpx not
power7vsx because the mpi/bg compilers are only for cross-compiling
anyways.

Does LLVM support -march= such that the code will not run on the wrong
processor?  That would prevent certain accidental errors.

> In general, I don't recommend that anyone switch without benchmarking ;)

Sure, but for C++11 codes, LLVM is infinitely faster than XLC :-)

 "The best performance improvement is the transition from the
nonworking state to the working state." - John Osterhout

> In my experience, the performance of bgclang vs. xl has been steadily improving, but there is substantial variance. xl's performance has also improved substantially over the last year or so on some benchmarks (and decreased on others -- they also have a large variance).

Indeed, no one should make decisions based upon measurements with the
2012 XLC, for example.

Best,

Jeff

-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at anl.gov / jhammond at uchicago.edu / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond