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

> 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.