[Bug libgomp/35517] New: OpenMP: Incorrect result when run with two or more threads

[Bug libgomp/35517] New: OpenMP: Incorrect result when run with two or more threads

[bart dot vanassche at gmail dot com]

The attached program is a straightforward implementation of matrix inversion
via the Gauss-Jordan algorithm. The explicitly parallelized version (not
attached) produces exactly the same result for each run independent of the
number of threads. The version parallelized via OpenMP produces incorrect
results for sufficiently large matrices and two or more threads. An example:

$ OMP_NUM_THREADS=1 ./matinv_openmp 180
error = 4.75689e-14; epsilon = 2.22045e-16; error / (epsilon * n) = 1.19018
Error within bounds.
$ OMP_NUM_THREADS=2 ./matinv_openmp 180
error = nan; epsilon = 2.22045e-16; error / (epsilon * n) = nan
Error out of bounds.

I compiled this program as follows:
$ ~/gcc-4.2.3/bin/gcc -Wall -fopenmp -g matinv_openmp.c -static -o
matinv_openmp -lm
$ ~/gcc-4.2.3/bin/gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: /home/bart/software/gcc-4.2.3/configure --disable-nls
--enable-threads=posix --enable-tls --prefix=/home/bart/gcc-4.2.3
Thread model: posix
gcc version 4.2.3


-- 
           Summary: OpenMP: Incorrect result when run with two or more
                    threads
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: libgomp
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bart dot vanassche at gmail dot com
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35517

[Bug libgomp/35517] OpenMP: Incorrect result when run with two or more threads

[bart dot vanassche at gmail dot com]

------- Comment #1 from bart dot vanassche at gmail dot com  2008-03-09 19:58 -------
Created an attachment (id=15286)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15286&action=view)
Test program matinv_openmp.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35517

[Bug libgomp/35517] OpenMP: Incorrect result when run with two or more threads

[bart dot vanassche at gmail dot com]

------- Comment #2 from bart dot vanassche at gmail dot com  2008-03-10 18:32 -------
Everything works correctly after changing
#pragma omp parallel for
into
#pragma omp parallel for private(j, k)


-- 

bart dot vanassche at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35517