> From: Linus Torvalds > Sent: 03 November 2015 20:45 > On Tue, Nov 3, 2015 at 12:05 PM, Linus Torvalds > wrote: > > result = add_overflow( > > mul_overflow(sec, SEC_CONVERSION, &overflow), > > mul_overflow(nsec, NSEC_CONVERSION, &overflow), > > &overflow); > > > > return overflow ? MAX_JIFFIES : result; > > Thinking more about this example, I think the gcc interface for > multiplication overflow is fine. > > It would end up something like > > if (mul_overflow(sec, SEC_CONVERSION, &sec)) > return MAX_JIFFY_OFFSET; > if (mul_overflow(nsec, NSEC_CONVERSION, &nsec)) > return MAX_JIFFY_OFFSET; > sum = sec + nsec; > if (sum < sec || sum > MAX_JIFFY_OFFSET) > return MAX_JIFFY_OFFSET; > return sum; > > and that doesn't look horribly ugly to me. If mul_overflow() is a real function you've just forced some of the values out to memory, generating a 'clobber' for all memory (unless 'strict-aliasing' is enabled) and making a mess of other optimisations. (If it is a static inline that might not happen.) If you assume that no one is stupid enough to multiply very large values by 1 and not get an error you could have mul_overflow() return the largest prime if the multiply overflowed. David {.n++%ݶw{.n+{G{ayʇڙ,jfhz_(階ݢj"mG?&~iOzv^m ?I