[Octave-bug-tracker] [bug #54572] int64 does not saturate correctly in negative direction

[Rik]

Follow-up Comment #14, bug #54572 (project octave):

jwe's test case fails for -O2, but works for other optimization levels.  I
also verified that OCTAVE_HAVE_FAST_INT_OPS must be defined to show the bug. 
There are two places where OCTAVE_HAVE_FAST_INT_OPS is checked.  I ruled out
one with testing.  This leaves this bit of code as the problem:


  static T
  add (T x, T y)
  {
#if defined (OCTAVE_HAVE_FAST_INT_OPS)
    // The typecasts do nothing, but they are here to prevent an optimizing
    // compiler from interfering.  Also, the signed operations on small types
    // actually return int.
    T u = static_cast<UT> (x) + static_cast<UT> (y);
    T ux = u ^ x;
    T uy = u ^ y;
    if ((ux & uy) < 0)
      {
        u = octave_int_base<T>::max_val () + __signbit (~u);
      }
    return u;
#else


The comment sounds interesting.  It specifically says that the casts are there
to prevent an optimizing compiler from interfering, which it seems like it
is.



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