[boost] Re: Smart Pointers - (many) thanks and (some) questions

From: Andy Glew (glew_at_[hidden])
Date: 1999-12-26 18:30:29

> >       if (--*pn == 0) { delete px; }
> 
> Well, if everything is OK up to that point, you'll have a leak, not a
> double-deletion, because there will never be fewer references than
> shared_ptrs needing to be destroyed. 

A compiler can legally implement this as (in pseudocode):

    treg1 := mem( *pn )
    treg2 := sub( treg1 - 1 )
    mem( *pn ) := treg2
    treg3 := mem( *pn )
    if( treg3 != 0 ) goto 1f
        ...delete px
    1h:

Note that if this is done, then 2 threads can both 
execute the decrement, and both arrive at the 
load into treg3 at the same time, leading to a double
decrement.

Now, most compilers will avoid the treg3 load,
using the treg2 value in the compare,
so that Dave's statement will be true on most systems.
But, it doesn't have to be so. Which is half of the fun
of multothreading, isn't it?