I suppose this is the simplest version of the test.
program test
implicit none
integer :: i
real :: x(5)
x = 1.0E37 ! All elements of x are set to the same value.
print *, 'Before division, x = ', x
print *, 'All entries of x are identical?', all(x == x(1))
x = x / 1.0E38 ! OR: x = x / x(1)
print *, 'After division, x = ', x
do i = 1, size(x)
print *, 'i = ', i, 'x(1) == x(i)?', x(1) == x(i)
print "(*(b0,1x))", x(1), x(i)
if (x(1) /= x(i)) then
print *, ''
print *, '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'
print *, '!!!!!!!!!!! Surprise! x(1) is different from x(i) !!!!!!!!!'
print *, '!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!'
print *, ''
end if
end do
end program test
Result:
$ uname -a && ifort --version && ifort test_div.f90 && ./a.out
Linux zX10 5.15.0-56-generic #62-Ubuntu SMP Tue Nov 22 19:54:14 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
ifort (IFORT) 2021.8.0 20221119
Copyright (C) 1985-2022 Intel Corporation. All rights reserved.
Before division, x = 9.9999999E+36 9.9999999E+36 9.9999999E+36
9.9999999E+36 9.9999999E+36
All entries of x are identical? T
After division, x = 0.0000000E+00 0.0000000E+00 0.0000000E+00 0.0000000E+00 0.1000000
i = 1 x(1) == x(i)? T
0 0
i = 2 x(1) == x(i)? T
0 0
i = 3 x(1) == x(i)? T
0 0
i = 4 x(1) == x(i)? T
0 0
i = 5 x(1) == x(i)? F
0 111101110011001100110011001101
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!! Surprise! x(1) is different from x(i) !!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The behavior of floating-point numbers is arbitrary without options like -fp-model=srict, isn’t it?