[Bug fortran/68600] New: Inlined MATMUL is too slow.

["dominiq at lps dot ens.fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68600

            Bug ID: 68600
           Summary: Inlined MATMUL is too slow.
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dominiq at lps dot ens.fr
  Target Milestone: ---

Expected results:
(1) to be at least as fast as the MATMUL from the library when compiled 
with the same options (-O2 -ftree-vectorize -funroll-loops);
(2) to be at least as fast as dgemm from lapack when compiled 
with the same options.

Options tested
(a) -O2 -ftree-vectorize -funroll-loops -fno-frontend-optimize, i.e., MATMUL
from the library;
(b) -O2 -ftree-vectorize -funroll-loops, i.e., inlined MATMUL;
(c) -Ofast -march=native -funroll-loops;
(d) -O2 -ftree-vectorize.

Timings in Gflops/s on a Corei7 2.8Ghz (turbo 3.8Ghz) show that neither (1) nor
(2) are true (comparing columns 4 and 6 gives an idea of timings accuracy).

                        (a)                 (b)
 Size     Loops   Matmul   dgemm      Matmul   dgemm
 ===================================================
    2    200000    0.360   0.218      0.723   0.221
    4    200000    1.246   0.959      1.379   0.969
    8    200000    2.098   2.396      2.186   2.385
   16    200000    3.748   3.648      2.920   3.645
   32    200000    5.386   5.406      3.096   5.418
   64     30757    6.364   6.385      3.220   6.494
  128      3829    6.362   6.760      3.256   6.702
  256       477    6.515   6.527      3.164   6.444
  512        59    6.313   6.634      3.189   6.675
 1024         7    4.796   4.842      2.935   4.853
 2048         1    4.026   4.032      2.824   3.996
 4096         1    3.355   3.467      2.652   3.475

                        (c)                (d)
 Size     Loops   Matmul   dgemm     Matmul   dgemm
 ========================================================
    2    200000    0.403   0.172      0.919   0.204
    4    200000    0.956   0.799      1.668   1.104
    8    200000    1.796   2.089      2.060   2.310
   16    200000    2.948   4.297      2.253   3.475
   32    200000    4.119   6.219      2.049   4.229
   64     30757    5.174   7.652      2.268   4.464
  128      3829    5.042   6.985      2.371   4.353
  256       477    5.052   6.492      2.423   4.696
  512        59    5.136   6.738      2.421   4.704
 1024         7    3.978   5.075      2.361   4.012
 2048         1    3.476   4.304      2.372   3.543
 4096         1    2.966   3.307      2.370   3.333