https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61000
--- Comment #7 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
When tackling this in the front end, one problem is when
_not_ to do loop interchange.
Consider
do i=1,10
do j=1,10
call foo(a(i,j))
end do
end do
where we have no idea what foo does, and if it depends on
the order it is called (unless it is pure).
Also, if you look at
program main
integer, dimension(3,3) :: a
integer, dimension(9) :: b
a = reshape([(4**(i-1),i=1,9)],shape(a))
b = [(2*4**(i-1),i=1,9)]
k = 0
do i=1,3
do j=1,3
k = k + 1
a(i,j) = a(i,j) + b(k)
end do
end do
print *,a
a = reshape([(4**(i-1),i=1,9)],shape(a))
k = 0
do j=1,3
do i=1,3
k = k + 1
a(i,j) = a(i,j) + b(k)
end do
end do
print *,a
end program main
you see that loop reordering can introduce wrong code quite
easily.
