Tolkien stated that the semi-vowel [w] was vocalized to the vowel [u] when appearing before a consonant or finally. In similar conditions the semi-vowel [j] became [i] (SD/419).

There is some ambiguity of whether this w/y-vocalization happened before or after [ɣ] assimilated to preceding voiced consonants. Either way, the net result would have been the same. In the first case, ✶[wɣV/yɣV] would have become [uɣV/iɣV] and thereafter [uwV/uyV] when [ɣ] became [w], [j] between [u], [i] and another vowel. In the second case, the [ɣ] in ✶[wɣV/yɣV] would have assimilated to [wwV/yyV] before the vocalization could apply, and thereafter became [uwV/uyV]. The discussion here assumes w/y-vocalization happened first, since it simplifies some other rules. See the word ✶dāw’r for more evidence that this vocalization was early.