Re: PDD 4: Internal data types

[Hong Zhang]

> Looks like they do operations with 16-bit integers. I'd as soon go with
> 32-bit ones--wastes a little space, but should be faster. (Except where we
> should shift to 64-bit words)

Using 32/31-bit requires general support of 64-bit arithmetics, for shift
and multiply. Without it, we have to use some extremely complicated
code to deal with multiply/division/shift, and we will lose the speed.

Detecting overflow for 2's complement signed integer addtion/substract will
be easy:
  c = a + b;
  if (((c ^ a) & (c ^ b)) < 0) {
    /* overflow */
  }
The code is super-scalar and need only one branch. It is also fast to
generate boolean flag without using branch:
  int overflow = ((unsigned) ((c ^ a) & (c ^ b))) >> 31;

Hong