Why do base objects appear behind the delta in packs?

From: Shawn Pearce <spearce@spearce.org>
Date: Aug 29, 2006, 13:42 UTC
Message-ID: <20060829134233.GA21335@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;
 }