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
