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Dear DDJ,

Mark Mitchell's article "Type-Based Alias Analysis" (DDJ, Oct.2000) was refreshing reading after having programmed for some 12 years in an aliasing-free concurrent language. However, I have some points to add:

  1. Mitchell does not directly state that aliasing can also happen in pointer-less languages. When an object in such a language is used by its name only it may still be treated by reference by the compiler, thus opening for aliasing errors.
  2. Aliasing-free code could be fast, but aliasing of objects could be fatal! In the Java example below x=x+(x-x) may correctly be x but fatally zero when c1 is aliased.
    class thing { // "{}" became ? in layout in magazine
      // By Peter Welch, Univ. of Kent at Canterbury, UK
      int x;
      public thing (int v) {x = v;}
      public static void harmless (thing t1, thing t2) {
        t1.x = t1.x + t2.x;
        t1.x = t1.x - t2.x;
    class alias {
      public static void main (String args[]) {
        thing c1 = new thing(10);
        thing c2 = new thing(20);
        thing.harmless (c1,c2); // c1.x=10 c2.x=20
        thing.harmless (c1,c1); // c1.x=0 has become 0!
  3. Aliasing may also be a wanted trait. Passing by reference and doubly-linked lists are examples.
  4. Mitchell mentions that the C9X "restrict" does not prohibit the programmer from making aliasing errors. With occam the compiler takes me at the subtlest places, where I would not have imagined problems - but then there is no "restrict"-type decoration, ALL variables are verified for aliasing errors, and the compiler can therefore always assume aliasing-free code.
  5. I have just recently (At a SIG at the CPA 2000 conference: learned that if one could control aliasing in an object-oriented language, one could also control garbage collection. An object free of aliasing can be de-allocated in unit-time once it goes out of scope for the single only referencer of that particular object.

Oyvind Teig
New address 2001

End of Letter.
Read more: Mission impossible? Encapsulate that aliased alien!