[Openmp-dev] Bug in clang/openmp: max-reduction fails if data type is "long double"

[Stefan Illy via Openmp-dev]

Thanks for the reply,

in fact "long double" is the x86 extended precision which needs 80 bits 
(10 bytes) only. It is not a full 128 bit floating point data type (as 
the quad precision provided by gcc and the Intel compiler).
Nevertheless, sizeof(long double) gives a value of 16 (due to alignment 
reasons, I assume); 6 bytes are unused.

-- Stefan

Am 15.03.2016 um 10:43 schrieb Cownie, James H:
> This looks like an inconsistency in our knowledge of LLVM/Clang.
>
> Historically LLVM did not support 16 byte floating point, so the OpenMP runtime does not have support for them when compiled with clang (because the necessary routines couldn't be compiled!).
>
> If your code really is using 16 byte floating point numbers, then the tests in the runtime that don't compile those routines need to be enabled.
>
> It'd be worth checking, though, that the "long double" really is 16 bytes, so maybe you could just check sizeof(real_t).
>
> p.s. You don't need to initialize max_val; whatever value you put there will be ignored anyway, since the OpenMP standard says that the per-thread reduction values are initialized with the most negative value of the type.
Thanks for the hint - I was not aware of that!

>
> -- Jim
>
> James Cownie <james.h.cownie at intel.com>
> SSG/DPD/TCAR (Technical Computing, Analyzers and Runtimes)
> Tel: +44 117 9071438
>
> -----Original Message-----
> From: Openmp-dev [mailto:openmp-dev-bounces at lists.llvm.org] On Behalf Of Stefan Illy via Openmp-dev
> Sent: Friday, March 11, 2016 7:36 AM
> To: openmp-dev at lists.llvm.org
> Subject: [Openmp-dev] Bug in clang/openmp: max-reduction fails if data type is "long double"
>
> Hello everybody,
>
> When I compile the simple test program attached below I get the
> following error message:
>
> openmpbug.cc:(.text+0x2de): undefined reference to
> `__sync_val_compare_and_swap_16'
> openmpbug.cc:(.text+0x3a7): undefined reference to
> `__sync_val_compare_and_swap_16'
> clang-3.8: error: linker command failed with exit code 1 (use -v to see
> invocation)
>
> This is caused by the OMP max-reduction statement. It also fails if I
> switch from "max" to "min".
> I use version 3.8 of clang on an Ubuntu 14.04 LTS system. It also fails
> with version 3.9 (trunk, self-compiled).
> If I switch the floating point data type (real_t) from "long double" to
> "double" or "float", the code compiles and runs without problems.
>
> I hope this helps to make clang+openmp even better!
>
>
> Here comes the simple test program:
> ---------------------------------------SNIP-------------------------------------------
> #include <iostream>
>
> using namespace std;
>
> //typedef float real_t;
> //typedef double real_t;
> typedef long double real_t;
>
> int
> main()
> {
>     real_t maxval = -1.0e-10;
> #pragma omp parallel for reduction(max: maxval)
>     for (int i = 1; i <= 1000; i++) maxval = max(maxval, real_t(i));
>     cout << maxval << endl;
>     return 0;
> }
> ------------------------------------SNIP------------------------------------------------
>
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