[Python-Dev] Memory size overflows

[Gerald S. Williams]

Actually, my original solution doesn't seem to stand up
to comprehensive testing, either. Essentially all it was
doing was checking to see if the sum of the high-order
bit positions was less than the number of bits. I created
a Python equivalent for smaller width numbers. At a width
of 6 bits, the following combinations aren't reported as
overflows but should be:

(6, 13) (6, 14) (6, 15) (7, 11)
(7, 12) (7, 13) (7, 14) (7, 15)

Perhaps there is some property of the false negatives
that can be exploited.

I don't know if it helps, but there are certainly a number
of ways to narrow the field:

  if (src1 & HIMASK) && (src2 & HIMASK), overflow=True
  if !(src1 & HIMASK) && !(src2 & HIMASK), overflow=False
  if !src1 || !src2, overflow=False
  if dest < MAX(src1,src2), overflow=True
  if log2(src1) + log2(src2) >= width, overflow=True
  (the integer form of log2 can be done quite cheaply by
   exploiting known properties of src1 and src2 at this
   point)

But you'd still need to filter out false negatives from
that field. Perhaps there's a way, but divides would have
to be really expensive for this to be worth it.

-Jerry