optimization/7799: [3.2/3.3 regression] Loop bug with optimization flag -Os in gcc

[Robert Dewar]

> It can't wrap around there, but if pointers are signed, it might straddle 0
> nonetheless.  This requires, of course, for an aligned start address
> that the address 0 is not the same as NULL, which AFAICR is something gcc
> doesn't currently support.

Note that addresses *are* considered signed on the INMOS Transputer as a
proof of existence :-)