debug mode performance patch

[Jonathan Wakely]

On 10 November 2010 22:02, Jonathan Wakely wrote:
>
> e.g. this fixes both deadlocks
>
> void swap(A& a, A& b)
> {
>  std::mutex& m1 = get_mutex(&a);
>  std::mutex& m2 = get_mutex(&b);
>  if (&m1 < &m2)

Doh, that should be &m1 == &m2, sorry.