[patch, fortran] Create temporary variables for matmul

[Thomas Koenig]

Hi Jerry,

> OK. Could you post some before and after performance results. Intuitively this
> makes sense to do, but empirically does it pay off.

Thanks for the reviews!

Here is a quick&dirty benchmark, with the patch, with
-finline-matmul-limit=0 used to inhibit matmul inlining
(to simulate the behavior without).

$ cat bench.f90
program main
   implicit none
   integer, parameter :: dp = selected_real_kind(15)
   integer, parameter :: n1 = 10000
   integer, parameter :: n2 = 1000
   integer :: i,j,ind
   real(dp), dimension(3,3*n2) :: a, b
   real(dp), dimension(3,3) :: c
   real(dp) :: s
   s = 0
   do i=1,n1
      call random_number(a)
      call random_number(b)
      do j=1,n2
         ind = 3*j-2
         s = s + sum(matmul(a(:,ind:ind+2),b(:,ind:ind+2)))
      end do
   end do
   print *,s
end program main
$ gfortran -Ofast  -finline-matmul-limit=0 bench.f90 && time ./a.out
    67501526.340284660

real    0m4,856s
user    0m4,852s
sys     0m0,000s
$ gfortran -Ofast bench.f90 && time ./a.out
    67502996.159002081

real    0m2,583s
user    0m2,580s
sys     0m0,000s

Also interesting, a version which actually uses allocatable variables
as temporaries because we don't to value propagation in the front end:

program main
   implicit none
   integer, parameter :: dp = selected_real_kind(15)
   integer, parameter :: n1 = 10000
   integer, parameter :: n2 = 1000
   integer :: i,j,ind, ind2
   real(dp), dimension(3,3*n2) :: a, b
   real(dp), dimension(3,3) :: c
   real(dp) :: s
   s = 0
   do i=1,n1
      call random_number(a)
      call random_number(b)
      do j=1,n2
         ind = 3*j-2
         ind2 = ind + 2
         s = s + sum(matmul(a(:,ind:ind2),b(:,ind:ind2)))
      end do
   end do
   print *,s
end program main
$ gfortran -Ofast  -finline-matmul-limit=0 bench2.f90 && time ./a.out
    67500364.375209823

real    0m5,480s
user    0m5,472s
sys     0m0,000s
$ gfortran -Ofast bench2.f90 && time ./a.out
    67498343.869140804

real    0m3,475s
user    0m3,476s
sys     0m0,000s

Regards

	Thomas