Daniel Gröber (dxld) authored Jul 21, 2019 and Marge Bot committed Feb 17, 2021

Simon's concern in the old comment, specifically:

    So all of the calls to traverseMaybeInitClosureData() here are
    initialising retainer sets with the wrong flip.

Is actually exactly what the code was intended to do. It makes the closure
data valid, then at the beginning of the traversal the flip bit is flipped
resetting all closures across the heap to invalid.

Now it used to be that the profiling code using the traversal has it's own
sense of valid vs. invalid beyond what the traversal code does and indeed
the retainer profiler still does this, there a getClosureData of NULL is
considered an invalid retainer set.

So in effect there wasn't any difference in invalidating closure data
rather than just resetting it to a valid zero, which might be what confused
Simon at the time.

As the code is now it actually uses the value of the valid/invalid bit in
the form of the 'first_visit' argument to the 'visit' callback so there is
a difference.