Re: [PATCH] sha1_name: use bsearch_hash() for abbreviations

[Jonathan Tan <jonathantanmy@google.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.