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From: "xin.liu@compiler-dev.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/97063] New: [ MATMUL intrinsic] The value of result is
 wrong when vector (step size is negative) * matrix
Date: Wed, 16 Sep 2020 01:00:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97063

            Bug ID: 97063
           Summary: [ MATMUL intrinsic] The value of result is wrong when
                    vector (step size is negative) * matrix
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xin.liu@compiler-dev.com
  Target Milestone: ---

Consider the following Fortran program:
program p
    implicit none
    real(kind=3D16), dimension(6) :: arr1
    real(kind=3D16), dimension(6, 10) :: arr2
    real(kind=3D16), dimension(10) :: arr3, arr6
    real(kind=3D16), dimension(3) :: arr4
    real(kind=3D16), dimension(3, 10) :: arr5
    integer :: i, j
    data arr1 /0.0_16, 1.0_16, 2.0_16, 3.0_16, 4.0_16, 5.0_16/
    do i =3D 1, 6
       do j =3D 1,10
          arr2(i, j) =3D (i + j) * 1.0_16
       end do
    end do
    write(*,*) "--arr3--:"
    arr3 =3D matmul(arr1(5:1:-2),arr2(5:1:-2, :))
    write(*,'(5F5.1)') arr3
    write(*,*) "--arr4--:"
    arr4 =3D arr1(5:1:-2)
    write(*,'(5F5.1)') arr4
    write(*,*) "--arr6--:"
    arr6 =3D matmul(arr4, arr2(5:1:-2, :))
    write(*,'(5F5.1)') arr6
end program

Compiled with gfortran 10.1.0, the program prints:

 --arr3--:
 44.0 53.0 62.0 71.0 80.0
 89.0 98.0107.0116.0125.0
 --arr4--:
  4.0  2.0  0.0
 --arr6--:
 32.0 38.0 44.0 50.0 56.0
 62.0 68.0 74.0 80.0 86.0

As you can see,the value of arr3 is differnet with arr6.=