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    SubjectRe: [PATCH jiffies] Avoid undefined behavior from signed overflow
    Quoting "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>:

    > According to the C standard 3.4.3p3, overflow of a signed integer results
    > in undefined behavior. This commit therefore changes the definitions
    > of time_after() and time_after_eq() to avoid this undefined behavior.
    > The trick is that the subtraction is done using unsigned arithmetic,
    > which according to 6.2.5p9 cannot overflow because it is defined as
    > modulo arithmetic. This has the added (though admittedly quite small)
    > benefit of shortening two lines of code by four characters each.
    >
    > Note that the C standard considers the cast from signed to
    > unsigned to be implementation-defined, see 6.3.1.3p3. However, on a
    > two-complement system, an implementation that defines anything other
    > than a reinterpretation of the bits is free come to me, and I will be
    > happy to act as a witness for its being committed to an insane asylum.
    > (Although I have nothing against saturating arithmetic or signals in
    > some cases, these things really should not be the default.)

    Don't worry, the case from signed to unsigned is actually well-defined -
    the relevant part is 6.3.1.3p2 (in C99):

    > Otherwise, if the new type is unsigned, the value is converted by
    > repeatedly adding or subtracting one more than the maximum value that
    > can be represented in the new type until the value is in the range of
    > the new type.

    ...which ends up just being reinterpretation of the bits on a two's
    complement system, as you'd hope (after sign-extension to the width of
    the target unsigned type, that is). This actually means if you were
    mad enough to implement C on a sign-magnitude system, you'd be forced to
    do a non-trivial conversion in this case.

    - Kevin

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