The result of to_cbor() and its siblings reserved the lower bound of
binary_reserve_hint(), for a small object a few bytes, and the first write of
the sink replaced it by an allocation of 64 bytes, as an empty or too small
vector is grown to at least that. A small document therefore cost two
allocations and a free.
The result now reserves at least min_vector_sink_capacity (64) characters at
once. binary_reserve_hint() itself, a lower bound that is tested as such, is
unchanged.
Instructions to serialize a small object (six members, a nested array and object), Linux, GCC 13: to_cbor 1700 -> 1500, to_msgpack 1800 -> 1600, to_ubjson 1800 -> 1700, to_bson 2700 -> 2500. Documents larger than 64 bytes of hint are unchanged.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
ceb3c92709