[Bug fortran/97063] [ MATMUL intrinsic] The value of result is wrong when vector (step size is negative) * matrix

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97063

--- Comment #4 from anlauf at gcc dot gnu.org ---
Having looked at the tree-dump, I am not sure if it is a FE or a library issue.

I appears that the negative stride miscalculates the base or offset of
the array probably by one:

program p
  implicit none
  integer               :: i, j
  real, dimension(6)    :: arr1 = [(i,i=0,5)]
  real, dimension(6,10) :: arr2 = reshape ([(((i+j),i=0,5),j=0,9)],[6,10])
  real, dimension(10)   :: arr3, arr6
  real, dimension(3)    :: arr4
  arr4 = arr1(5:1:-2)
  write(*,*) "--arr6--:"
  arr6 = matmul (arr4        , arr2(5:1:-2, :))
  write(*,'(10F6.1)') arr6
  arr4 = arr1(6:2:-2)
  write(*,*) "--arr6-- (with arr1 off by one):"
  arr6 = matmul (arr4        , arr2(5:1:-2, :))
  write(*,'(10F6.1)') arr6
  arr3 = matmul (arr1(5:1:-2), arr2(5:1:-2, :))
  write(*,*) "--arr3--:"
  write(*,'(10F6.1)') arr3
end program

This prints:

 --arr6--:
  20.0  26.0  32.0  38.0  44.0  50.0  56.0  62.0  68.0  74.0
 --arr6-- (with arr1 off by one):
  26.0  35.0  44.0  53.0  62.0  71.0  80.0  89.0  98.0 107.0
 --arr3--:
  26.0  35.0  44.0  53.0  62.0  71.0  80.0  89.0  98.0 107.0