Re: [PATCH 1/2] patience diff: remove unnecessary string comparisons

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: May 5, 2021, 14:58 UTC
Message-ID: <nycvar.QRO.7.76.6.2105051649280.50@tvgsbejvaqbjf.bet>
In-Reply-To: <xmqqpmy658e1.fsf@gitster.g>

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,
>