[cpp-threads] modes, interlude

[Peter Dimov]

Alexander Terekhov wrote:

> Ok, some "lambda"-like stuff added to atomics can probably
> solve the problem of exposing LR/SC and can also be used
> to achieve efficient barriers for control-dependent stuff as
> actually required (apart from implied cchsb).
>
> Must be a peanut for Peter. ;-)

Something like

T read_modify_write( T * addr, F f )
{
    for( ;; )
    {
        T r = *(T volatile *)addr; // nonatomic_load( addr );
        if( compare_and_exchange( addr, r, f(r) ) ) return r;
    }
}

It's not good for LR/SC, unfortunately, because f(r) can cause stores in the 
reservation granule at the compiler's whim. LR/SC will be left for real 
programmers.

void compare_and_execute( T const * addr, T value, F f );

can take advantage of cchlb/isync, but I'm not sure whether anyone will 
actually use it.