On 9/6/2019 1:04 PM, Jeff King wrote: > On Fri, Sep 06, 2019 at 12:48:05PM -0400, Derrick Stolee wrote: > >>> diff --git a/revision.h b/revision.h >>> index 4134dc6029..5c0b831b37 100644 >>> --- a/revision.h >>> +++ b/revision.h >>> @@ -33,7 +33,7 @@ >>> #define ALL_REV_FLAGS (((1u<<11)-1) | NOT_USER_GIVEN | TRACK_LINEAR) >>> >>> #define TOPO_WALK_EXPLORED (1u<<27) >>> -#define TOPO_WALK_INDEGREE (1u<<28) >>> +#define TOPO_WALK_INDEGREE (1u<<24) >> >> As an aside, these flag bit modifications look fine, but would need to >> be explained. I'm guessing that since you are adding a bit of data >> to struct object you want to avoid increasing the struct size across >> a 32-bit boundary. Are we sure that bit 24 is not used anywhere else? >> (My search for "1u<<24" found nothing, and "1 << 24" found a bit in >> the cache-entry flags, so this seems safe.) > > Yeah, I'd definitely break this up into several commits with explanation > (though see an alternate I posted that just uses the parsed flag without > any new bits). > > Bit 24 isn't used according to the table in objects.h, which is > _supposed_ to be the source of truth, though of course there's no > compiler-level checking. (One aside: is there a reason TOPO_WALK_* isn't > part of ALL_REV_FLAGS?). > > And yes, the goal was to keep things to the 32-bit boundary. But in the > course of this, I discovered something interesting: 64-bit systems are > now padding this up to the 8-byte boundary! > > The culprit is the switch of GIT_MAX_RAWSZ for sha256. Before then, our > object_id was 20 bytes for sha1. Adding 4 bytes of flags still left us > at 24 bytes, which is both 4- and 8-byte aligned. > > With the switch to sha256, object_id is now 32 bytes. Adding 4 bytes > takes us to 36, and then 8-byte aligning the struct takes us to 40 > bytes, with 4 bytes of wasted padding. > > I'm sorely tempted to use this as an opportunity to move commit->index > into "struct object". That would actually shrink commit object sizes by > 4 bytes, and would let all object types do the commit-slab trick to > store object data with constant-time lookup. This would make it possible > to migrate some uses of flags to per-operation bitfields (so e.g., two > traversals would have their _own_ flag data, and wouldn't risk stomping > on each other's bits). This reminds me that I'm hoping to eventually get around to moving "generation" into a commit slab. That would reduce the space for people still working without a commit-graph, and would allow updating to generation number v2 (which needs 64 bits of data). -Stolee
