Re: [PATCH 3/4] xdiff: use stronger hash function internally

[Stefan Beller <sbeller@google.com>]

On Tue, Oct 24, 2017 at 1:23 PM, René Scharfe <l.s.r@web.de> wrote:
> Am 24.10.2017 um 20:59 schrieb Stefan Beller:
>> Instead of using the hash seeded with 5381, and updated via
>> `(hash << 5) ^ new_byte`, use the FNV-1 primitives as offered by
>> hashmap.h, which is seeded with 0x811c9dc5 and computed as
>> `(hash * 0x01000193) ^ new_byte`.
>
> The hash function you're replacing is called DJB2; I think that's worth
> mentioning.

I was not aware of the name. I'll look it up; thanks!

>
> Performance test results would be nice.  No idea how to find edge cases,
> though, or better: demonstrate a lack thereof.

My reasoning, though not in the commit message, is that the operations
are essentially equal, just with different numeric values, hence no impact.

I can look at the assembly and measure, too.

>
>>
>> Signed-off-by: Stefan Beller <sbeller@google.com>
>> ---
>>   xdiff/xutils.c | 19 ++++++++-----------
>>   1 file changed, 8 insertions(+), 11 deletions(-)
>>
>> diff --git a/xdiff/xutils.c b/xdiff/xutils.c
>> index 04d7b32e4e..a58a28c687 100644
>> --- a/xdiff/xutils.c
>> +++ b/xdiff/xutils.c
>> @@ -24,7 +24,8 @@
>>   #include <assert.h>
>>   #include "xinclude.h"
>>
>> -
>> +#include "cache.h"
>> +#include "hashmap.h"
>
> Ouch.  Defining FNV32_BASE and FNV32_PRIME here would be much easier
> overall.  And if that's too much duplication then those definitions
> could be extracted into a new header file (fnv32.h?) included by both
> hashmap.h and xutils.c.

I guess fnv32.h would do; it would contain the defines as well as the
static inline
function to be used in the inner loop of patch 1.

>
>>
>>
>>   long xdl_bogosqrt(long n) {
>> @@ -228,7 +229,7 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
>>
>>   static unsigned long xdl_hash_record_with_whitespace(char const **data,
>>               char const *top, long flags) {
>> -     unsigned long ha = 5381;
>> +     unsigned long ha = memhash(NULL, 0);
>>       char const *ptr = *data;
>>
>>       for (; ptr < top && *ptr != '\n'; ptr++) {
>> @@ -243,21 +244,18 @@ static unsigned long xdl_hash_record_with_whitespace(char const **data,
>>                               ; /* already handled */
>>                       else if (flags & XDF_IGNORE_WHITESPACE_CHANGE
>>                                && !at_eol) {
>> -                             ha += (ha << 5);
>> -                             ha ^= (unsigned long) ' ';
>> +                             ha = memhash_feed(ha, (unsigned char) ' ');
>
> All the memhash_feed() callers in this file cast to unsigned char.  A
> macro or a function (possibly inline) defined at the top could do
> that for them.

That would go away when using fnv32.h

Thanks for the review!
Stefan