Re: MATMUL broken with frontend optimization.

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Mar 18, 2021 at 8:49 AM Steve Kargl via Fortran
<fortran@gcc.gnu.org> wrote:
>
> It seems that gfortran will inline MATMUL with optimization.
> This  produce very poor performance.  In fact, gfortran will
> inline MATMUL even if one specifies -fexternal-blas.  This is
> very bad.
>
> % cat a.f90
> program main
>
>    implicit none
>
>    integer, parameter :: imax = 20000, jmax = 10000
>    real, allocatable :: inVect(:), matrix(:,:), outVect(:)
>    real :: start, finish
>
>    allocate(invect(imax), matrix(imax,jmax), outvect(jmax))
>
>    call random_number(inVect)
>    call random_number(matrix)
>
>    call cpu_time(start)
>    outVect = matmul(inVect, matrix)
>    call cpu_time(finish)
>
>    print '("Time = ",f10.7," seconds. – First Value = ",f10.4)',finish-start,outVect(1)
> end program main
>
> % gfcx -o z -O0 a.f90 && ./z
> Time =  0.2234111 seconds. – First Value =  4982.6362
> % nm z | grep matmul
>                  U _gfortran_matmul_r4@@GFORTRAN_8
> % gfcx -o z -O1 a.f90 && ./z
> Time =  0.3295890 seconds. – First Value =  4971.0962
> % nm z | grep matmul
> % gfcx -o z -O2 a.f90 && ./z
> Time =  0.3299561 seconds. – First Value =  5025.4902
> % nm z | grep matmul
> % gfcx -o z -O2 -fexternal-blas a.f90 && ./z
> Time =  0.3295580 seconds. – First Value =  5022.8291
>
> This last one is definitely broken.  I did not link with
> an external BLAS library.  Please fix before 11.1 is
> released.

Since the libgfortran MATMUL should be vectorized
I think it's not reasonable to inline any but _very_ small
MATMUL at optimization levels that do not enable vectorization.

Richard.

>
> --
> Steve