[cfe-dev] [libc++] debug mode

[Jean-Daniel Dupas]

Le 19 sept. 2011 à 09:25, Ruben Van Boxem a écrit :

> Op 19 sep. 2011 05:13 schreef "M.E. O'Neill" <oneill at cs.hmc.edu> het volgende:
> >
> > Howard Hinnant wrote:
> > >>> There isn't a lot there yet.  I've tried to get vector working (just the primary, not vector<bool> so far), and I believe I have done so.  Though I don't have tests for it yet (I've just been spot checking).  At this point I'm simply exploring whether the basic design is viable or not.
> > >>>
> > >>> A major goal of the design of this debug mode is to keep the ABI stable as debug mode is turned on/off.
> >
> > Sounds good, but it'd be useful to have a more detailed design document somewhere so that people can see where you're going and how.
> >
> > ... and Christopher Jefferson replied:
> > >> I have often seen interest on a "cheap" debugging mode, which would add checks which were possible without breaking complexity requirements, or "excessive" cost.
> >
> > Without seeing Howard's design, I don't know how "excessive" the costs of his design might be, but in principle, you can do some checks like iterator invalidation very cheaply -- if you have the right design.
> >
> > This may be obvious (in which case I apologize), it may even be what Howard has already done (in which case, oops, and, uh, cool), but in case it isn't, let me outline how:
> >
> > - For every container, associate a 64-bit tag (a.k.a. version stamp).
> >
> > - For every iterator, also associate a 64-bit tag.
> >
> > - When you create a new container, just pick a random 64-bit value.
> >
> > - When you create an iterator, copy the 64-bit tag from the associated container.  This represents the container/version the iterator belongs to.   Any access via the iterator checks that the tag of the iterator matches the tag of the container.  If it doesn't, BANG!
> >
> > - When iterators are invalidated, generate a new tag for container. (For speed, you could just increment it, but the important point is that the value is a new and different one)
> >
> > This method is probabilistic -- there is a 1 in 18446744073709551616 chance that it won't catch an invalid access, but personally I like those odds.  It also imposes very very little in runtime overhead -- you can invalidate N iterators in O(1) time.
> 
> Why not just use the cool c++11 random number generators, if performance would be an issue, time to optimise those ;-) I don't like the idea of probabilistic odds, especially in hugely iterated and containered program this would make debugging useless (especially for long-running server-like programs that would need to be debugged)
> 

if I correctly understand the issue, I would say this is because a random generator would not increase the number of possible values, and would instead increase the chance of collision.

With 64 bit counter, even if you increase it 1 billion times per second, you need more than 500 years to overflow.

-- Jean-Daniel




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