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

From: Dave Abrahams (abrahams_at_[hidden])
Date: 1999-12-26 18:34:17

> 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?

Andy's analysis notwithstanding, my statement will always be true unless 2
threads assign to the same smart_ptr simultaneously.

-Dave