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Date: Sun, 05 Apr 2015 13:48:00 -0000
From: Thomas Koenig <tkoenig@netcologne.de>
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To: Jerry DeLisle <jvdelisle@charter.net>,  "fortran@gcc.gnu.org" <fortran@gcc.gnu.org>, gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [patch, fortran, RFC] First steps towards inlining matmul
References: <55212B5D.6010309@netcologne.de> <552136A1.1020706@charter.net>
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Hi Jerry,

> I am curious about what performance gain results from this? I can see
> saving a library call to our runtime libraries.  Do you have some timing
> results?

The speedup can be quite drastic for small matrices which can be
completely unrolled by -O3:

b1.f90:

program main
  use b2
  implicit none
  real, dimension(3,3) :: a, b, c
  integer :: i

  call random_number(a)
  call random_number(b)
  do i=1,10**8
     c = matmul(a,b)
     call bar(b,c)
  end do
end program main

b2.f90:

module b2
contains
  subroutine bar(b,c)
    real, dimension(3,3) :: b,c
  end subroutine bar
end module b2

ig25@linux-fd1f:~/Krempel/Matmul> gfortran -O3 -fno-frontend-optimize
b2.f90 b1.f90 && time ./a.out

real    0m15.411s
user    0m15.404s
sys     0m0.001s

ig25@linux-fd1f:~/Krempel/Matmul> gfortran -O3 b2.f90 b1.f90 && time ./a.out

real    0m1.736s
user    0m1.735s
sys     0m0.001s