Re: MATMUL broken with frontend optimization.

[Steve Kargl <sgk@troutmask.apl.washington.edu>]

On Thu, Mar 18, 2021 at 09:55:27PM +0100, Thomas Koenig wrote:
> 
> > I haven't checked.  If so, how about disabling
> > in-lining MATMUL for 11.1;
> 
> Absolutely not for the general case. This would cause a huge regression
> in execution time for 2*2 matrices, and also for small matrix-vector
> multiplications.
> 
> What we could do is only to enable the inlining for vector*matrix
> at -O2 or higher. Again, this will mean a penalty for smaller loops,
> but at less than -O2, people probably don't care too much.
> 

On my old core2 cpu, a quick test with N=1000 and NxN matrix
suggest a cross over near N=1000 for REAL(4).  This cpu doesn't
have any AVX* instruction, so YMMV.  Program follows .sig

-- 
Steve

program t

   implicit none

   character(len=10) str
   integer i, j
   integer, parameter :: &
   &  n(10) = [100, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 10000]
   real t0, t1, t3, t4
   real, allocatable :: a(:), b(:,:), c(:)
   !
   ! Loop over n(j) array.  Run each test 5 times and average.
   !
   do j = 1, 10

      allocate(a(n(j)), b(n(j),n(j)), c(n(j)))

      a = 1
      b = 1

      t3 = 0
      do i = 1, 5
         call cpu_time(t0)
         c = matmul(a, b)
         call cpu_time(t1)
         t3 = t3 + (t1 - t0)
         if (c(1) /= n(j)) stop 1
      end do
 
      t4 = 0
      do i = 1, 5
         call cpu_time(t0)
         c = matmul(b, a)
         call cpu_time(t1)
         t4 = t4 + (t1 - t0)
         if (c(1) /= n(j)) stop 2
      end do

      print '(I5,1X,2(F8.4,1X))', n(j), (t3/5) * 1000, (t4/5) * 1000

      deallocate(a, b, c)
   end do

end program t