Signed overflow optimization hazards in the kernel [LWN.net]

Signed overflow optimization hazards in the kernel

Posted Aug 16, 2012 21:10 UTC (Thu) by daney (guest, #24551)
In reply to: Signed overflow optimization hazards in the kernel by jmspeex
Parent article: Signed overflow optimization hazards in the kernel

> Actually, signed-integer overflow was undefined long before C11.
> It was definitely undefined in C99 and I'm pretty sure it also was in C89.

The first edition of K & R explicitly states the signed-integer overflow is undefined. This has carried through to the present.

to post comments

Signed overflow optimization hazards in the kernel

Posted Aug 16, 2012 22:48 UTC (Thu) by PaulMcKenney (✭ supporter ✭, #9624) [Link] (2 responses)

Understood. And when it was suggested within the C11 Standards committee that signed-integer overflow be given twos-complement semantics, the discussion was both emphatic and short. ;-)

Signed overflow optimization hazards in the kernel

Posted Aug 17, 2012 16:07 UTC (Fri) by josh (subscriber, #17465) [Link] (1 responses)

What did the arguments against it say, other than "that would remove compiler optimization possibilities"?

Signed overflow optimization hazards in the kernel

Posted Aug 18, 2012 18:19 UTC (Sat) by PaulMcKenney (✭ supporter ✭, #9624) [Link]

One objection was that there really are still non-twos-complement machines in common use. As was noted by the comment to this article discussing saturating adders, where 32767+1==32767. But this would be addressed by "implementation defined" rather than "undefined".

Another objection was that there are systems still in common use that trap on signed integer overflow. If the C standard required wrapping, compilers for such systems would require special edge-case checks on pretty much any signed integer operation.

And there was of course also the objection that signed integer overflow always has been undefined. ;-)