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From: Jonathan Tan <jonathantanmy@google.com>
To: christian.couder@gmail.com
Cc: git@vger.kernel.org, gitster@pobox.com, peff@peff.net,
	chriscool@tuxfamily.org, ramsay@ramsayjones.plus.com,
	Jonathan Tan <jonathantanmy@google.com>
Subject: Re: [RFC PATCH 04/10] ewah/bitmap: always allocate 2 more words
Date: Thu, 10 Oct 2019 16:40:40 -0700	[thread overview]
Message-ID: <20191010234040.168894-1-jonathantanmy@google.com> (raw)
In-Reply-To: <20190913130226.7449-5-chriscool@tuxfamily.org>

> From: Jeff King <peff@peff.net>
> 
> In a following patch we will allocate a variable number
> of words in some bitmaps. When iterating over the words we
> will need a mark to tell us when to stop iterating. Let's
> always allocate 2 more words, that will always contain 0,
> as that mark.

[snip]

>  	if (block >= self->word_alloc) {
>  		size_t old_size = self->word_alloc;
> -		self->word_alloc = block * 2;
> +		self->word_alloc = (block + 1) * 2;
>  		REALLOC_ARRAY(self->words, self->word_alloc);
>  		memset(self->words + old_size, 0x0,
>  			(self->word_alloc - old_size) * sizeof(eword_t));

This patch set was mentioned as needing more thorough review in "What's
Cooking" [1], so I thought I'd give it a try. As Peff said [2], the
justification in the commit message looks incorrect. He suggests that it
is most likely because "block" might be 0 (which is possible because a
previous patch eliminated the minimum of 32), which makes sense to me.

In any case, the next patch does not use 0 as a sentinel mark. Iteration
stops when word_alloc is reached anyway, and since this is a regular
bitmap, 0 is a valid word and cannot be used as a sentinel. (Maybe 0 is
a valid word in a compressed EWAH bitmap too...not sure about that.)

I think this should be squashed with patch 3, adding to that commit
message "since word_alloc might be 0, we need to change the growth
function". (Or just make the minimum word_alloc be 1 or 32 or something
positive, if that's possible.)

[1] https://public-inbox.org/git/xmqq36g5444k.fsf@gitster-ct.c.googlers.com/
[2] https://public-inbox.org/git/20191002155721.GD6116@sigill.intra.peff.net/

  reply	other threads:[~2019-10-10 23:40 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-13 13:02 [RFC PATCH 00/10] Rewrite packfile reuse code Christian Couder
2019-09-13 13:02 ` [RFC PATCH 01/10] builtin/pack-objects: report reused packfile objects Christian Couder
2019-09-13 13:02 ` [RFC PATCH 02/10] packfile: expose get_delta_base() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 03/10] ewah/bitmap: introduce bitmap_word_alloc() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 04/10] ewah/bitmap: always allocate 2 more words Christian Couder
2019-10-10 23:40   ` Jonathan Tan [this message]
2019-10-11  7:49     ` Christian Couder
2019-10-11 18:05       ` Jeff King
2019-09-13 13:02 ` [RFC PATCH 05/10] pack-bitmap: don't rely on bitmap_git->reuse_objects Christian Couder
2019-10-10 23:44   ` Jonathan Tan
2019-10-11  7:50     ` Christian Couder
2019-09-13 13:02 ` [RFC PATCH 06/10] pack-bitmap: introduce bitmap_walk_contains() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 07/10] csum-file: introduce hashfile_total() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 08/10] pack-objects: introduce pack.allowPackReuse Christian Couder
2019-09-13 21:37   ` Junio C Hamano
2019-09-13 13:02 ` [RFC PATCH 09/10] builtin/pack-objects: introduce obj_is_packed() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 10/10] pack-objects: improve partial packfile reuse Christian Couder
2019-09-13 22:29   ` Junio C Hamano
2019-09-14  2:02     ` Jeff King
2019-09-14  3:06       ` Junio C Hamano
2019-10-02 15:57         ` Jeff King
2019-10-03  2:06           ` Junio C Hamano
2019-10-03  6:55             ` Christian Couder
2019-10-10 23:59   ` Jonathan Tan
2019-10-11  7:39     ` Christian Couder
2019-10-11 18:01     ` Jeff King
2019-10-11 21:04       ` Jonathan Tan
2019-10-12  0:04       ` Junio C Hamano
2019-10-13  7:38         ` Jeff King
2019-10-17  7:03           ` Junio C Hamano
2019-10-17  7:23             ` Jeff King

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