[Bug lto/69564] New: lto makes scimark2 LU slower

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69564

            Bug ID: 69564
           Summary: lto makes scimark2 LU slower
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincenzo.innocente at cern dot ch
  Target Milestone: ---

mkdir scimark2; cd scimark2
wget http://math.nist.gov/scimark2/scimark2_1c.zip
unzip scimark2_1c.zip
c++ -Ofast *.c; ./a.out
c++ -Ofast *.c -flto; ./a.out


with gcc 4.9.3
gcc version 4.9.3 (GCC) 

c++ -Ofast *.c; ./a.out
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:         2462.90
FFT             Mflops:  2070.32    (N=1024)
SOR             Mflops:  1661.17    (100 x 100)
MonteCarlo:     Mflops:   813.44
Sparse matmult  Mflops:  2978.91    (N=1000, nz=5000)
LU              Mflops:  4790.64    (M=100, N=100)
[innocent@vinavx3 scimark2]$ c++ -Ofast *.c -flto; ./a.out
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:         2582.94
FFT             Mflops:  2064.19    (N=1024)
SOR             Mflops:  1654.04    (100 x 100)
MonteCarlo:     Mflops:  1426.90
Sparse matmult  Mflops:  2978.91    (N=1000, nz=5000)
LU              Mflops:  4790.64    (M=100, N=100)


with latest build
gcc version 6.0.0 20160129 (experimental) (GCC) 

[innocent@vinavx3 scimark2]$ c++ -Ofast *.c; ./a.out
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:         2377.18
FFT             Mflops:  1970.89    (N=1024)
SOR             Mflops:  1654.04    (100 x 100)
MonteCarlo:     Mflops:   810.37
Sparse matmult  Mflops:  3328.81    (N=1000, nz=5000)
LU              Mflops:  4121.76    (M=100, N=100)
[innocent@vinavx3 scimark2]$ c++ -Ofast *.c -flto; ./a.out
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:         2136.23
FFT             Mflops:  2076.48    (N=1024)
SOR             Mflops:  1654.04    (100 x 100)
MonteCarlo:     Mflops:  1533.92
Sparse matmult  Mflops:  3266.59    (N=1000, nz=5000)
LU              Mflops:  2150.13    (M=100, N=100)