gfortran and ifort gives different results - fortran

Question

I am making a small code that will read numbers from 2.txt which is bunch of random real numbers, save it in a 2d array and pass it on the information to a subroutine. In the subroutine, I am assigning a particular dimension of this array to another matrix to save this for future calculation.
implicit none
double precision, allocatable :: x1(:,:)
integer :: nstep, nheatoms, k, i, j
open(unit=999,file="2.txt",status="old")
nstep = 5
nheatoms = 2
allocate(x1(3,nheatoms))
do k = 1, nstep
do i = 1, 3
do j = 1, nheatoms
read(999,*) x1(i,j)
end do
read(999,*)
end do
call diffusion(nstep,nheatoms,x1)
end do
stop
end
subroutine diffusion(nstep,nheatoms,qhecent)
implicit none
integer, intent (in) :: nheatoms, nstep
double precision :: qhecent(3,nheatoms)
integer :: j
double precision, allocatable :: qhestep(:,:)
integer :: nstepdiff, ncross
integer, save :: l = 0
double precision :: diff
l = l + 1
allocate(qhestep(nstep,nheatoms))
do j = 1, nheatoms
qhestep(l,j) = qhecent(3,j)
end do
if (l .gt. 1) then
do j = 1, nheatoms
write(*,*)qhestep(l,j), qhestep(l-1,j)
end do
end if
end subroutine
The 2.txt file is as follows:
1.0
2.1
3.2
-1.1
-2.2
-3.3
5.0
3.5
4.4
1.9
2.1
1.5
6.0
3.5
4.4
1.9
2.8
2.5
6.0
3.5
4.4
1.9
2.1
3.2
6.0
3.5
4.4
1.9
-4.3
7.9
Now if I compile with gfortran, most of the time I obtain the output as:
2.1000000000000001 -2.2000000000000002
1.5000000000000000 -3.2999999999999998
2.7999999999999998 2.1000000000000001
2.5000000000000000 1.5000000000000000
2.1000000000000001 2.7999999999999998
3.2000000000000002 2.5000000000000000
-4.2999999999999998 2.1000000000000001
7.9000000000000004 3.2000000000000002
which is expected. But if I run the code several times, an unknown number appear in the output, such as the example:
2.1000000000000001 -2.2000000000000002
1.5000000000000000 -3.2999999999999998
2.7999999999999998 1.2882297539194267E-231
2.5000000000000000 0.0000000000000000
2.1000000000000001 2.7999999999999998
3.2000000000000002 2.5000000000000000
-4.2999999999999998 2.1000000000000001
7.9000000000000004 3.2000000000000002
If I compile with ifort, then I obtain the output that have two zeros, which is wrong.
2.10000000000000 -2.20000000000000
1.50000000000000 -3.30000000000000
2.80000000000000 0.000000000000000E+000
2.50000000000000 0.000000000000000E+000
2.10000000000000 2.80000000000000
3.20000000000000 2.50000000000000
-4.30000000000000 2.10000000000000
7.90000000000000 3.20000000000000
I must stress that compiling on ifort most of the time gives correct results as in the case for gfortran. I am on a Mac OS High Sierra.
Thanks in advance.

Answer 1

Both compilers should give garbage results, as you are allocating qhestep in diffusion for each DO k loop. On exiting diffusion, qhestep is deallocated, so the accumulated information should be lost. That ifort returns the same heap address for qhestep on each call is misleading.
To overcome the problem, you should use allocate (qhestep(nstep,nheatoms)) before entering the DO k loop, then transfer the accumulation array in call diffusion.
That ifort gives your expected result is unfortunate, as it disguises a significant logic error. Each ALLOCATE basically provides a new memory location for the array.
You also provide no error tests when reading the file. I would recommend using iostat= for the reads and also report the data as it is read.
In summary, your approach gives the wrong result, which both compilers provide. The ifort results you provided are also wrong in some cases, but correct in others, which is misleading.