[Bug c++/69564] lto and/or C++ make scimark2 LU slower

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |lto, missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-02-01
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
          Component|lto                         |c++
            Summary|lto makes scimark2 LU       |lto and/or C++ make
                   |slower                      |scimark2 LU slower
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Any reason you are using the c++ driver here?

I get

> gcc-6 -Ofast -flto *.c -lm -B /abuild/rguenther/trunk3-g/gcc
> ./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:         1729.02
FFT             Mflops:  1247.04    (N=1024)
SOR             Mflops:  1537.70    (100 x 100)
MonteCarlo:     Mflops:   842.21
Sparse matmult  Mflops:  1657.86    (N=1000, nz=5000)
LU              Mflops:  3360.29    (M=100, N=100)

> gcc-6 -Ofast  *.c -lm -B /abuild/rguenther/trunk3-g/gcc
> ./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:         1645.94
FFT             Mflops:  1288.61    (N=1024)
SOR             Mflops:  1471.29    (100 x 100)
MonteCarlo:     Mflops:   459.90
Sparse matmult  Mflops:  1665.91    (N=1000, nz=5000)
LU              Mflops:  3343.98    (M=100, N=100)


Ok, when using g++ to compile things I _do_ get

> g++-6 -Ofast -flto *.c -lm -B /abuild/rguenther/trunk3-g/gcc
> ./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:         1321.43
FFT             Mflops:  1261.86    (N=1024)
SOR             Mflops:  1533.77    (100 x 100)
MonteCarlo:     Mflops:   850.69
Sparse matmult  Mflops:  1669.90    (N=1000, nz=5000)
LU              Mflops:  1290.93    (M=100, N=100)

> g++-6 -Ofast  *.c -lm -B /abuild/rguenther/trunk3-g/gcc
> ./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:         1492.12
FFT             Mflops:  1279.12    (N=1024)
SOR             Mflops:  1479.86    (100 x 100)
MonteCarlo:     Mflops:   433.83
Sparse matmult  Mflops:  1637.11    (N=1000, nz=5000)
LU              Mflops:  2630.71    (M=100, N=100)

So even without LTO I get a hit in using C++ to compile LU.  Interesting.