The following code generates suboptimal code on i686-pc-linux-gnu:

with Interfaces; use Interfaces;
with System.Storage_Elements;
package A is
   Var : unsigned_8;
   for Var'Address use System.Storage_Elements.To_Address (1000);
   pragma Volatile (Var);
end A;
with A;
with Interfaces; use Interfaces;
procedure U is
begin
   A.Var := A.Var or 1;
   A.Var := A.Var or 2;
end U;

With -O3 -fomit-frame-pointer, we get:

_ada_u:
        movl    a__var, %edx     <=== indirection instead of...
        movzbl  (%edx), %eax     <=== ...direct access here
        orl     $1, %eax
        movb    %al, (%edx)
        movl    a__var, %edx     <=== useless reloading of a__var
        movzbl  (%edx), %eax
        orl     $2, %eax
        movb    %al, (%edx)
        ret

If the variable is declared in the same compilation unit, we get:

_ada_u:
        movzbl  1000, %eax
        orl     $1, %eax
        movb    %al, 1000
        movzbl  1000, %eax
        orl     $2, %eax
        movb    %al, 1000
        ret

We should probably get the same code in both cases.