Re: [PATCH 1/2] patience diff: remove unnecessary string comparisons
Hi Junio,
On Wed, 5 May 2021, Junio C Hamano wrote:
Show 30 quoted lines
> "Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> > From: Phillip Wood <phillip.wood@dunelm.org.uk>
> >
> > xdl_prepare_env() calls xdl_classify_record() which arranges for the
> > hashes of non-matching lines to be different so lines can be tested
> > for equality by comparing just their hashes.
>
> Hmph, that is a bit different from what I read from the comment in
> the post context of the first hunk, though.
>
> /*
> * After xdl_prepare_env() (or more precisely, due to
> * xdl_classify_record()), the "ha" member of the records (AKA lines)
> * is _not_ the hash anymore, but a linearized version of it. In
> * other words, the "ha" member is guaranteed to start with 0 and
> * the second record's ha can only be 0 or 1, etc.
> *
> * So we multiply ha by 2 in the hope that the hashing was
> * "unique enough".
> */
>
> The words "home" and "enough" hints to me that the "ha" member is
> not hash, but "lineralized version of it" (whatever it means) does
> not guarantee that two records with the same "ha" are identical, or
> does it?
>
> Well, I should just go read xdl_classify_record() to see what it
> really does, but if it eliminates collisions, then the patch is a
> clear and obvious improvement.
Right. I had the same concern. But it does look as if `xdl_classify_record()` replaced the possibly non-unique hash values to unique sequential identifiers.
I have to admit that the code is unnecessarily hard to read for me: https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L110-L157
But I do gather that the loop at https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L119-L123 is called for every line, that it does compare it to every seen line with the same hash, and that it exits the loop early if the contents disagree:
for (rcrec = cf->rchash[hi]; rcrec; rcrec = rcrec->next)
if (rcrec->ha == rec->ha &&
xdl_recmatch(rcrec->line, rcrec->size,
rec->ptr, rec->size, cf->flags))
break;
Since naming is hard (and you can easily err on saving space at the expense of costing readers' time, as libxdiff proves), and since I am running out of review time, I'll have to assume that https://github.com/git/git/blob/v2.31.1/xdiff/xprepare.c#L150-L154 means that indeed, the `ha` field is set to a counter that uniquely identifies the line contents:
rec->ha = (unsigned long) rcrec->idx;
hi = (long) XDL_HASHLONG(rec->ha, hbits);
rec->next = rhash[hi];
rhash[hi] = rec;
So I am fairly confident that the patch is good, and the performance win is nice.
Thanks! Dscho
Show 40 quoted lines
>
> Thanks.
>
> > diff --git a/xdiff/xpatience.c b/xdiff/xpatience.c
> > index 20699a6f6054..db2d53e89cb0 100644
> > --- a/xdiff/xpatience.c
> > +++ b/xdiff/xpatience.c
> > @@ -90,7 +90,7 @@ static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
> > {
> > xrecord_t **records = pass == 1 ?
> > map->env->xdf1.recs : map->env->xdf2.recs;
> > - xrecord_t *record = records[line - 1], *other;
> > + xrecord_t *record = records[line - 1];
> > /*
> > * After xdl_prepare_env() (or more precisely, due to
> > * xdl_classify_record()), the "ha" member of the records (AKA lines)
> > @@ -104,11 +104,7 @@ static void insert_record(xpparam_t const *xpp, int line, struct hashmap *map,
> > int index = (int)((record->ha << 1) % map->alloc);
> >
> > while (map->entries[index].line1) {
> > - other = map->env->xdf1.recs[map->entries[index].line1 - 1];
> > - if (map->entries[index].hash != record->ha ||
> > - !xdl_recmatch(record->ptr, record->size,
> > - other->ptr, other->size,
> > - map->xpp->flags)) {
> > + if (map->entries[index].hash != record->ha) {
> > if (++index >= map->alloc)
> > index = 0;
> > continue;
> > @@ -253,8 +249,7 @@ static int match(struct hashmap *map, int line1, int line2)
> > {
> > xrecord_t *record1 = map->env->xdf1.recs[line1 - 1];
> > xrecord_t *record2 = map->env->xdf2.recs[line2 - 1];
> > - return xdl_recmatch(record1->ptr, record1->size,
> > - record2->ptr, record2->size, map->xpp->flags);
> > + return record1->ha == record2->ha;
> > }
> >
> > static int patience_diff(mmfile_t *file1, mmfile_t *file2,
>
