How to do compare and increment atomically?

Question

In my attempt to develope a thread-safe C++ weak pointer template class, I need to check a flag that indicating the object is still alive, if yes then increment the object's reference count and I need to do the both steps atomically.

I know the existance of intrinsics functions provided by the compiler, for instance _InterlockedCompareExchange() and _InterlockedIncrement(). But what I want is an interlockedCompareIncrement() function, is there an efficient way to simulate this intrinsic using other primitives, at least on the Windows x86 platform?

Answer 1

Suppose that value is your flag variable. It should be declared volatile.

long curvalue;
long newvalue;

do
{
    curvalue = value;
    newvalue = curvalue + 1;
}
while( _InterlockedCompareExchange( &value, newvalue, curvalue ) != curvalue );

As you see you can generalize this to whatever kind of arithmetic you need by changing the operations that are applied to calculate newvalue.

If you want to compare two values at the same time, your best bet is to pack both values into a single variable, and then operate on that single variable. Since you're using a flag combined with a reference count, I'd recommend using the lowest bit of value as the 'alive' flag, and then increment/decrement by 2 at a time. This allows you to encode both the flag and the reference count into a single 32-bit variable.