Re: [PATCH jiffies] Avoid undefined behavior from signed overflow
From: Paul E. McKenney
Date: Mon Jul 29 2013 - 09:54:41 EST
On Mon, Jul 29, 2013 at 03:30:35PM +1000, caf@xxxxxxxxxxx wrote:
> Quoting "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>:
> >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 126.96.36.199p3. 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 188.8.131.52p2 (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.
Yep, but we are going in the other direction, from unsigned to signed.
> ...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.
Fortunately, I never used signed-magnitude systems. And even when I used
ones-complement systems back in my misguided youth, I didn't write C
programs for them. ;-)
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