[Bug libgomp/61333] New: potential target specific performance issue with libgomp

[howarth.at.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61333

            Bug ID: 61333
           Summary: potential target specific performance issue with
                    libgomp
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: howarth.at.gcc at gmail dot com
                CC: jakub at gcc dot gnu.org

While benchmarking the new openmp/clang-omp support against gcc/libgomp on
x86_64-apple-darwin13, I noticed a potential performance issue in gcc/libgomp
on darwin. Using the attached heated_plate_openmp.c and heated_plate_gcc.sh
shell script, I am seeing reproducible timing on darwin that ratio to the
timing for one thread as..

gcc 4.8.3   1:1.90:3.31
gcc 4.9.0   1:1.90:3.30
clang 2.4   1:1.99:3.71

compared to Fedora 15 linux 

gcc 4.6.3   1:1.99:3.92

This is where the timings for the one, two and four OMP processes are
normalized to the timing for one OMP process.

Unfortunately I don't have access to gcc 4.8.3 or 4.9.0 to test on linux.
The timings are reproducible and suggest that darwin may have a performance
issue for higher number of OMP threads.