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Signed overflow optimization hazards in the kernelSigned overflow optimization hazards in the kernelPosted Aug 16, 2012 21:10 UTC (Thu) by daney (subscriber, #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.
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Signed overflow optimization hazards in the kernel Posted Aug 16, 2012 22:48 UTC (Thu) by PaulMcKenney (subscriber, #9624) [Link]
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]
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 (subscriber, #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. ;-)
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