Issue with libarcher in the libomp-10-dev package

[Joachim Protze]

Dear LLVM package maintainers,

I'm glad, that libarcher made it into a debian package. But 
unfortunately it doesn't work.

The idea of Archer is to complement ThreadSanitizer and provide the 
synchronization semantics for OpenMP. In a vanilla build of LLVM with 
openmp, libarcher is loaded by the OpenMP runtime (libomp.so) 
automatically and checks whether the TSan library is present in the 
execution:

$ export ARCHER_OPTIONS=verbose=1
$ clang -fopenmp -fsanitize=thread red-norace.c -g
$ OMP_NUM_THREADS=2 ./a.out
Archer detected OpenMP application with TSan, supplying OpenMP 
synchronization semantics
Sum: 4999950000

Using clang-10 from the package, there are several issues. First, the 
archer library is not found as the library is not in the dl search path
$ clang-10 -fopenmp -fsanitize=thread red-norace.c -g
$ OMP_NUM_THREADS=2 ./a.out
... WARNING: ThreadSanitizer: data race ...

Creating a link in /lib/x86_64-linux-gnu should help for this.

The next issue (after I added the link):
$ OMP_NUM_THREADS=2 ./a.out
Archer detected OpenMP application without TSan stopping operation
... WARNING: ThreadSanitizer: data race ...

I could not figure out, why the archer library does not use the exported 
dynamic symbols from the tsan runtime, which is statically linked into 
the application:

$ readelf --dyn-syms a.out | grep RunningOnValgrind
    529: 000000000048f6f0    18 FUNC    GLOBAL DEFAULT   13 
RunningOnValgrind
$ readelf --dyn-syms /usr/lib/llvm-10/lib/libarcher.so | grep 
RunningOnValgrind
     61: 0000000000002770    10 FUNC    WEAK   DEFAULT   12 
RunningOnValgrind
$ readelf --dyn-syms /home/.../clang/10.0/lib/libarcher.so | grep 
RunningOnValgrind
     64: 0000000000002590    10 FUNC    WEAK   DEFAULT   12 
RunningOnValgrind

I can successfully use clang-10 with my vanilla build of libarcher. So, 
it seems like there is something funny going on with libarcher from the 
deb package.

Finally, I'm not sure why the library should not be installed if 
compiler and OpenMP runtime are installed. I can understand that 
developers packages are not installed by default, but a compiler is 
typically installed by developers?

Thanks in advance for looking into this.

Best
Joachim

-- 
Dipl.-Inf. Joachim Protze

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering
RWTH Aachen University
Seffenter Weg 23
D 52074  Aachen (Germany)
Tel: +49 241 80- 24765
Fax: +49 241 80-624765
protze at itc.rwth-aachen.de
www.itc.rwth-aachen.de
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