Re: [PATCH 3/4] xdiff: use stronger hash function internally
- From
Stefan Beller <sbeller@google.com>
- Date
- Oct 24, 2017, 20:46 UTC
- Message-ID
- <CAGZ79kZF+kL0uCFpVE0o9cU2zgjqm=yzeCZSEWtsC1=q+8N3Yg@mail.gmail.com>
- In-Reply-To
- <16718615-c5df-24dd-fa62-2b42f5d83a02@web.de>
On Tue, Oct 24, 2017 at 1:23 PM, René Scharfe <l.s.r@web.de> wrote:
Show 8 quoted lines
> 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.
Show 23 quoted lines
> >> >> 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.
Show 24 quoted lines
>
>>
>>
>> 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