From: Jeff King <peff@peff.net>
To: Michael Haggerty <mhagger@alum.mit.edu>
Cc: Johan Herland <johan@herland.net>,
Junio C Hamano <gitster@pobox.com>,
git@vger.kernel.org
Subject: Re: [RFC 16/17] object_array_entry: copy name before storing in name field
Date: Mon, 20 May 2013 12:44:58 -0400 [thread overview]
Message-ID: <20130520164458.GA27788@sigill.intra.peff.net> (raw)
In-Reply-To: <519A365E.6020807@alum.mit.edu>
On Mon, May 20, 2013 at 04:42:38PM +0200, Michael Haggerty wrote:
> >> * Many callers store the empty string ("") as the name; for example,
> >> most of the entries created during a run of rev-list have "" as
> >> their name. This means that lots of needless copies of "" are being
> >> made. I think that the best solution to this problem would be to
> >> store NULL rather than "" for such entries, but I haven't figured
> >> out all of the places where the name is used.
> >
> > Use strbufs?
> >
> > No allocation (except for the strbuf object itself) is needed for
> > empty strings, and string ownership and be transferred to and from it
> > to prevent extra copies.
>
> That would cost two extra size_t per object_array_entry. I have the
> feeling that this structure is used often enough that the extra overhead
> would be a disadvantage, but I'm not sure.
>
> The obvious alternative would be to teach users to deal with NULL and
> either add another constructor alternative that transfers string
> ownership or *always* transfer string ownership and change the callers
> to call xstrdup() if they don't already own the name string. I think I
> will try that approach first.
You could use the same trick that strbuf does: instead of NULL, point to
a well-known empty string literal. Readers do not have to care about
this optimization at all; only writers need to recognize the well-known
pointer value. And since we do not update in place but only eventually
free, it really is just that anyone calling free() would do "if (name !=
well_known_empty_string)".
-Peff
next prev parent reply other threads:[~2013-05-20 16:45 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-19 20:26 [PATCH 00/17] Remove assumptions about refname lifetimes Michael Haggerty
2013-05-19 20:26 ` [PATCH 01/17] describe: make own copy of refname Michael Haggerty
2013-05-19 20:26 ` [PATCH 02/17] fetch: make own copies of refnames Michael Haggerty
2013-05-19 20:26 ` [PATCH 03/17] add_rev_cmdline(): make a copy of the name argument Michael Haggerty
2013-05-19 20:26 ` [PATCH 04/17] builtin_diff_tree(): make it obvious that function wants two entries Michael Haggerty
2013-05-21 17:27 ` Junio C Hamano
2013-05-23 7:19 ` Michael Haggerty
2013-05-19 20:27 ` [PATCH 05/17] cmd_diff(): use an object_array for holding trees Michael Haggerty
2013-05-21 17:30 ` Junio C Hamano
2013-05-23 7:21 ` Michael Haggerty
2013-05-19 20:27 ` [PATCH 06/17] cmd_diff(): rename local variable "list" -> "entry" Michael Haggerty
2013-05-19 20:27 ` [PATCH 07/17] cmd_diff(): make it obvious which cases are exclusive of each other Michael Haggerty
2013-05-19 20:27 ` [PATCH 08/17] revision: split some overly-long lines Michael Haggerty
2013-05-21 17:34 ` Junio C Hamano
2013-05-23 6:27 ` Michael Haggerty
2013-05-23 17:08 ` Junio C Hamano
2013-05-19 20:27 ` [PATCH 09/17] gc_boundary(): move the check "alloc <= nr" to caller Michael Haggerty
2013-05-21 17:49 ` Junio C Hamano
2013-05-23 7:09 ` Michael Haggerty
2013-05-23 18:02 ` Junio C Hamano
2013-05-19 20:27 ` [PATCH 10/17] get_revision_internal(): make check less mysterious Michael Haggerty
2013-05-21 17:38 ` Junio C Hamano
2013-05-23 6:39 ` Michael Haggerty
2013-05-19 20:27 ` [PATCH 11/17] object_array: add function object_array_filter() Michael Haggerty
2013-05-19 20:27 ` [PATCH 12/17] object_array_remove_duplicates(): rewrite to reduce copying Michael Haggerty
2013-05-19 20:27 ` [PATCH 13/17] fsck: don't put a void*-shaped peg in a char*-shaped hole Michael Haggerty
2013-05-19 20:27 ` [PATCH 14/17] find_first_merges(): initialize merges variable using initializer Michael Haggerty
2013-05-19 20:27 ` [PATCH 15/17] find_first_merges(): remove unnecessary code Michael Haggerty
2013-05-19 20:27 ` [RFC 16/17] object_array_entry: copy name before storing in name field Michael Haggerty
2013-05-20 10:33 ` Johan Herland
2013-05-20 14:42 ` Michael Haggerty
2013-05-20 16:44 ` Jeff King [this message]
2013-05-20 21:34 ` Michael Haggerty
2013-05-19 20:27 ` [RFC 17/17] refs: document the lifetime of the refname passed to each_ref_fn Michael Haggerty
2013-05-20 10:28 ` [PATCH 00/17] Remove assumptions about refname lifetimes Johan Herland
2013-05-20 12:15 ` Michael Haggerty
2013-05-20 16:37 ` Junio C Hamano
2013-05-20 16:59 ` Jeff King
2013-05-20 17:08 ` Johan Herland
2013-05-20 18:03 ` Junio C Hamano
2013-05-20 17:03 ` Johan Herland
2013-05-21 18:39 ` Junio C Hamano
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