Yes, there would be an allocation race, so if you cannot accept allocating unnecessarily in a discardable/recoverable way (but also one that preserves the semantics of the singleton: one and only one instance returned over its life), you would still need a flag. Though you might instead use an "incomplete"/place-holder reference value to simulate the flag and prevent duplication of effort/wasting resources while the allocation happens - all in the space of the pointer.

Right. I was still thinking in terms of my thread-local-less model posted before when I wrote the above, which took for granted those barriers once past init.

If the pointer is 0 through a normal test, you would allocate the resource and do a compare-and-swap to record the allocation if the pointer is still 0. As noted beside this comment, you might wind up allocating several times in parallel (which may be undesirable too), but you would never clobber an existing allocation or change the instance that is dispensed.