Why do base objects appear behind the delta in packs?

[Shawn Pearce <spearce@spearce.org>]

Sorry but this really is a pretty stupid question on my part:

In builtin-pack-objects.c write_one(), why is the base object written
behind the first delta that depends on it (if it hasn't been written
already) rather than BEFORE the first delta that depends on it?

If the base always had to appear before any delta that uses it then
unpack-objects wouldn't need to cache a delta in memory waiting
for the base to get unpacked.

>From a data locality perspective putting the base object before
or after the delta shouldn't matter, as either way the delta
is useless without the base.  So placing the base immediately
before the delta should perform just as well as placing it after.
Either way the OS should have the base in cache by the time the
delta is being accessed.

In other words, why not apply this patch and make it a requirement
of the pack file format?


diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
index 46f524d..5dd97b9 100644
--- a/builtin-pack-objects.c
+++ b/builtin-pack-objects.c
@@ -341,11 +341,11 @@ static unsigned long write_one(struct sh
 		 * if it is written already.
 		 */
 		return offset;
-	e->offset = offset;
-	offset += write_object(f, e);
 	/* if we are deltified, write out its base object. */
 	if (e->delta)
 		offset = write_one(f, e->delta, offset);
+	e->offset = offset;
+	offset += write_object(f, e);
 	return offset;
 }