From: Dmitry Ivankov Date: Sat, 20 Aug 2011 18:28:34 GMT Subject: Re: [PATCH v4] fast-import: do not write bad delta for replaced subtrees Message-ID: In-Reply-To: <20110820174812.GD15864@elie.gateway.2wire.net> On Sat, Aug 20, 2011 at 11:48 PM, Jonathan Nieder wrote: > Dmitry Ivankov wrote: > >> How about adding a new bit field "no_delta" instead? > > Currently the layout of "struct tree_entry_ms" is: > >        uint16_t mode;                  two bytes >        unsigned char sha1[20]          20 bytes > > which adds up to 22 bytes.  Here is "struct tree_entry": > >        struct tree_entry *tree;                one machine word >        struct atom_str *name;                  one machine word >        struct tree_entry_ms versions[2];       44 bytes > > Although it only looks like it adds one byte per tree entry, in > practice I suspect your patch adds four.  Is that worth it?  (The > answer might be yes.  I'm not sure.) Hm, we can make mode 1 byte in fast-import (only 8 values are used). But not in this patch of course. > >> The patch is >> smaller this way. Also could 04000 theoretically be S_IFDIR on some >> platform? > > No, these modes are part of the format of objects as written on disk > and over the wire, so when we meet a platform with S_IFDIR != 040000, > there will have to be bigger changes (to distinguish between the > platform's idea of file status and git's idea of modes). > >> - switch to a separate no_delta bit in tree_entry > > If it doesn't cost too much, this is a good idea. > >> - when setting no_delta = 1 don't check for S_ISDIR(versions[0].mode), >>   this is a redundant check and logic duplication. Who knows, maybe some >>   day we'll want to delta a tree against blob. :) > > Why?  When versions[0] is not a tree, the hack is not needed, since > versions[0].mode and versions[0].sha1 accurately describe the delta > base and are not inconsistent with anything. Oh, right you are. The new check and one in store_tree look the same but the purpose differs. > > Thanks, that was helpful. >