7.6.2.4 Aliasing By Overindexing

Compile the following example with -xalias=overindex

integer a,z
common // a(100),z
z = 1
call sub(a)
print*, z
subroutine sub(x)
  integer x(10)
  x(101) = 2
The compiler may assume that the call to sub may write to z
The program prints 2, and not 1, when compiled with -xalias=overindex

Overindexing appears in many legacy Fortran 77 programs and should be avoided. In many cases the result will be unpredictable. To insure correctness, programs should be compiled and tested with the -C (runtime array bounds checking) option to flag any array subscripting problems.

In general, the overindex flag should only be used with legacy Fortran 77 programs. -xalias=overindex does not apply to array syntax expressions, array sections, WHERE, and FORALL statements.

Fortran 95 programs should always conform to the subscripting rules in the Fortran standard to insure correctness of the generated code. For example, the following example uses ambiguous subscripting in an array syntax expression that will always produce an incorrect result due to the overindexing of the array:

This example of array syntax overindexing DOES NOT GIVE CORRECT RESULTS!

   parameter (n=10)
   integer a(n),b(n)
   common /qq/a,b
   integer c(n)
   integer m, k
   a = (/ (i,i=1,n) /)
   b = a
   c(1) = 1
   c(2:n) = (/ (i,i=1,n-1) /)

   m = n
   k = n + n
C
C   the reference to a is actually a reference into b
C   so this should really be  b(2:n) = b(1:n-1)
C
   a(m+2:k) = b(1:n-1)

C  or doing it in reverse
   a(k:m+2:-1) = b(n-1:1:-1)

Intuitively the user might expect array b to now look like array c, but the result is unpredictable

The xalias=overindex flag will not help in this situation since the overindex flag does not extend to array syntax expressions. The example compiles, but will not give the correct results. Rewriting this example by replacing the array syntax with the equivalent DO loop will work when compiled with -xalias=overindex. But this kind of programming practice should be avoided entirely.