15 Aug
2025
15 Aug
'25
9:04 p.m.
Idle thought; There's been mention of 1's complement. If overflow is
UB because of that possibility, maybe ==0 should be, too!
Doug
On Fri, Aug 15, 2025 at 2:44 PM John Levine <johnl(a)taugh.com> wrote:
It appears that Luther Johnson <luther.johnson(a)makerlisp.com> said:
-=-=-=-=-=-
I hear and understand what you're saying. I think what I'm trying to
point out, is that in C, as it was originally implemented, in
expressions "a + b", "a >> 1", "++a", C "does
what the machine does".
We just had the same argument in comp.arch and came to largely the same
conclusion. While overflow behavior on any particular machine may be
predictable, there's no consistency from one machine to another,
particularly back when there were still one's complement machines
where people compiled C code (some of the Univac mainframes.)
It isn't all that predictable even on a single machine. I know several
where overflow might or might not trap depending on a program-settable
status bit.
R's,
John