dec_and_test

Decrements the value of atomic by 1.

Think of this operation as an atomic version of `{ *atomic -= 1; return (*atomic == 0); }`.

This call acts as a full compiler and hardware memory barrier.

While atomic has a `volatile` qualifier, this is a historical artifact and the pointer passed to it should not be `volatile`.

Parameters:

atomic

a pointer to a int or uint

Returns:

true if the resultant value is zero