From: Junio C Hamano Date: Mon, 15 Apr 2013 01:32:42 GMT Subject: Re: [PATCH 3/3] commit-slab: introduce a macro to define a slab for new type Message-ID: <7vppxw4lz9.fsf@alter.siamese.dyndns.org> In-Reply-To: <20130414184121.GA1621@sigill.intra.peff.net> Jeff King writes: > On Sat, Apr 13, 2013 at 11:04:49PM -0700, Junio C Hamano wrote: > >> Suppose you want to give one bit per existing ref and paint commits >> down to find which refs are descendants of each commit. You find >> that you have 320 refs only at runtime. >> >> The code can declare a commit slab "struct flagbits" >> >> define_commit_slab(flagbits, unsigned char); >> struct flagbits flags; >> >> and initialize it by: >> >> nrefs = ... count number of refs that returns say 320 ... >> init_flagbits_with_stride(&flags, (nrefs + 7) / 8); >> >> so that >> >> unsigned char *fp = flagbits_at(&flags, commit); >> >> will return a pointer pointing at an array of 40 "unsigned char"s >> associated with the commit. > > Thanks, I was thinking originally that we would want to break it down > into "unsigned long" or something, but there is probably no real > performance advantage to doing that over bytes. The 320 came from writing "an array of 5 unsigned long long" in the first draft ;-) > I'd probably further wrap it with a flagbit_set and flagbit_tst to wrap > the "figure out which byte, then which bit of that byte" logic, but that > would be a wrapper around flagbits_at, anyway. It can come later. Exactly. At that point, it is not about "what you could use commit slab for" but is about "how you would implement unbounded set of flag bits". > We'd probably want the hot path of this (returning the actual pointer) > to be inline, but not necessarily the parts about growing,... Yeah, this was just a technology demonstration as your original.