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Niels Lohmann
58e0d6d96e
key_hash() mixed a key eight bytes at a time and then packed the remaining
0-7 bytes one by one, in a loop. The last word of a key of eight or more
bytes is now read as the last eight bytes of the key (overlapping the word
before), and a key of 2-7 bytes as two overlapping words of four or two
bytes. A product only moves bits upwards, and the slot is taken from the low
bits: the high bits of the last word are folded down before the
multiplication, since keys that differ only in their last characters ("user_17",
"user_18") would otherwise share slots. The hash is not stored, so its values
may change.
Looking up every key of an object of 1000 random keys, Apple M1 instructions per 1000 lookups:
keys of 5 bytes 175 K -> 93 K, hex strings of 8 bytes 177 K -> 168 K, keys of 12 bytes 196 K -> 186 K
Linux x86-64 Ir (whole run), GCC 13 / Clang 18: 5-byte keys -4.7 % / -8.8 %, 12-byte keys -3.3 % / -7.4 %.
Building the tables while parsing such objects is cheaper as well (5-byte keys: 13.3 us -> 10.1 us).
Belongs to the PR "json_view: read-side speedups".
Signed-off-by: Niels Lohmann <mail@nlohmann.me>