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From: Jonathan Tan <jonathantanmy@google.com>
To: Derrick Stolee <dstolee@microsoft.com>
Cc: git@vger.kernel.org, stolee@gmail.com
Subject: Re: [PATCH] sha1_name: use bsearch_hash() for abbreviations
Date: Tue, 20 Mar 2018 15:25:05 -0700	[thread overview]
Message-ID: <20180320152505.bd66f0deaecf6d92fa6d62de@google.com> (raw)
In-Reply-To: <20180320200325.168147-1-dstolee@microsoft.com>

On Tue, 20 Mar 2018 16:03:25 -0400
Derrick Stolee <dstolee@microsoft.com> wrote:

> This patch updates the abbreviation code to use bsearch_hash() as defined
> in [1]. It gets a nice speedup since the old implementation did not use
> the fanout table at all.

You can refer to the patch as:

  b4e00f7306a1 ("packfile: refactor hash search with fanout table",
  2018-02-15)

Also, might be worth noting that this patch builds on
jt/binsearch-with-fanout.

> One caveat about the patch: there is a place where I cast a sha1 hash
> into a struct object_id pointer. This is because the abbreviation code
> still uses 'const unsigned char *' instead of structs. I wanted to avoid
> a hashcpy() in these calls, but perhaps that is not too heavy a cost.

I recall a discussion that there were alignment issues with doing this,
but I might have be remembering wrongly - in my limited knowledge of C
alignment, both "unsigned char *" and "struct object_id *" have the same
constraints, but I'm not sure.

> +	const unsigned char *index_fanout = p->index_data;
[snip]
> +	return bsearch_hash(oid->hash, (const uint32_t*)index_fanout,
> +			    index_lookup, index_lookup_width, result);

This cast to "const uint32_t *" is safe, because p->index_data points to
a mmap-ed region (which has very good alignment, as far as I know). I
wonder if we should document alignment guarantees on p->index_data, and
if yes, what guarantees to declare.

  reply	other threads:[~2018-03-20 22:25 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-20 20:03 [PATCH] sha1_name: use bsearch_hash() for abbreviations Derrick Stolee
2018-03-20 22:25 ` Jonathan Tan [this message]
2018-03-21 13:24   ` Derrick Stolee
2018-03-21 22:42     ` brian m. carlson
2018-03-22 17:40       ` [PATCH v2 0/3] Use " Derrick Stolee
2018-03-22 17:40         ` [PATCH v2 1/3] sha1_name: convert struct min_abbrev_data to object_id Derrick Stolee
2018-03-22 17:40         ` [PATCH v2 2/3] packfile: define and use bsearch_pack() Derrick Stolee
2018-03-22 17:40         ` [PATCH v2 3/3] sha1_name: use bsearch_pack() for abbreviations Derrick Stolee
2018-03-24 16:41         ` [PATCH 4/3] sha1_name: use bsearch_pack() in unique_in_pack() René Scharfe
2018-03-25 16:19           ` Junio C Hamano
2018-03-25 16:32             ` René Scharfe
2018-03-25 18:21           ` Derrick Stolee

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