[Tarantool-patches] [PATCH v3 2/2] box: refactor tuple_field_raw to omit path checks

[Serge Petrenko <sergepetrenko@tarantool.org>]

tuple_field_raw is an alias to tuple_field_raw_by_path with zero path.
This involves multiple path != NULL checks which aren't needed for tuple
field access by field number. The checks make this function rather slow
compared to its 1.10 counterpart (see results below).

In order to fix perf problems when JSON path indices aren't involved,
factor out the part of tuple_field_raw_by_path which is responsible for
direct field access by number and place it in tuple_field_raw.

This patch was tested by snapshot recovery part involving secondary
index building for a 1.5G snapshot with
one space and one secondary index over 4 integer and one string field.
Comparison table is below:

    Version    | time(seconds)  | Change relative to 1.10
---------------|----------------|------------------------
1.10           |      2:24      |           -/-
2.x(unpatched) |      3:03      |          + 27%
2.x (patched)  |      2:10      |          - 10%

Numbers below show cumulative time spent in tuple_compare_slowpath,
for 1.10 / 2.x(unpatched) / 2.x(patched) for 15, 19 and 14 second
profiles respectively: 13.9 / 17.8 / 12.5.

tuple_field_raw() isn't measured directly, since it's inlined, but all
its uses come from tuple_compare_slowpath.

As the results show, we manage to be even faster, than 1.10 used to be
in this test. This must be due to tuple comparison hints, which are
present only in 2.x.

Closes #4774
---
 src/box/tuple.h | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/src/box/tuple.h b/src/box/tuple.h
index 755aee506..fd373fdbf 100644
--- a/src/box/tuple.h
+++ b/src/box/tuple.h
@@ -697,8 +697,33 @@ static inline const char *
 tuple_field_raw(struct tuple_format *format, const char *tuple,
 		const uint32_t *field_map, uint32_t field_no)
 {
-	return tuple_field_raw_by_path(format, tuple, field_map, field_no,
-				       NULL, 0, NULL, MULTIKEY_NONE);
+	if (likely(field_no < format->index_field_count)) {
+		int32_t offset_slot;
+		uint32_t offset = 0;
+		struct tuple_field *field;
+		if (field_no == 0) {
+			mp_decode_array(&tuple);
+			return tuple;
+		}
+		struct json_token *token = format->fields.root.children[field_no];
+		field = json_tree_entry(token, struct tuple_field, token);
+		offset_slot = field->offset_slot;
+		if (offset_slot == TUPLE_OFFSET_SLOT_NIL)
+			goto parse;
+		offset = field_map_get_offset(field_map, offset_slot, MULTIKEY_NONE);
+		if (offset == 0)
+			return NULL;
+		tuple += offset;
+	} else {
+parse:
+		ERROR_INJECT(ERRINJ_TUPLE_FIELD, return NULL);
+		uint32_t field_count = mp_decode_array(&tuple);
+		if (unlikely(field_no >= field_count))
+			return NULL;
+		for ( ; field_no > 0; field_no--)
+			mp_next(&tuple);
+	}
+	return tuple;
 }
 
 /**
-- 
2.24.3 (Apple Git-128)