[Bug fortran/37131] New: inline matmul for small matrix sizes

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

Ouch.

This is a factor of 20 for this simple test case on my computer.

$ cat foo.f90
program main
  real, dimension(3,3) :: a,b,c
  call random_number(a)
  call random_number(b)
  do i=1,10**8
    c = matmul(a,b)
    a(1,1) = a(1,1) + b(1,1) - c(1,1)
  end do
  print *,c
end program main

$ gfortran -O3 foo.f90
$ time ./a.out
  0.34224379      0.27477881      0.48155165      0.76788843      0.65491939   
   1.2103429      0.38770726      0.38460296      0.87301219

real    0m20.733s
user    0m19.585s
sys     0m0.000s
$ cat bar.f90
program main
  real, dimension(3,3) :: a,b,c
  call random_number(a)
  call random_number(b)
  do i=1,10**8
    forall (i=1:3)
      forall (j=1:3)
        c(i,j) = sum(a(i,:) * b(:,j))
      end forall
    end forall
    a(1,1) = a(1,1) + b(1,1) - c(1,1)
  end do
  print *,c
end program main

$ gfortran -O3 bar.f90
$ time ./a.out
  0.34224379      0.27477881      0.48155165      0.76788843      0.65491939   
   1.2103429      0.38770726      0.38460296      0.87301219

real    0m1.075s
user    0m1.060s
sys     0m0.000s
$


-- 
           Summary: inline matmul for small matrix sizes
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tkoenig at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37131