The original fixme #2699 was removed back in PR #6053.

I cannot tell whether the original comment was unsure about the
arithmetic calculations, or if it was unsure about the assumptions
being made about the alignment of the current allocation pointer.

The arithmetic calculation looks fine to me, though.  This technique
is documented e.g. in Henry Warren's "Hacker's Delight" (section 3-1).

(I am sure one can find it elsewhere too, its not an obscure
property.)