The sizes of a run of integers are hard to predict, and so is the type of
each element in an array that mixes negative (number_integer) and
non-negative (number_unsigned) values. Writing an integer branched on the
type, the sign, and the 1/2/4/8-byte size class, and the mispredictions
were the larger part of the time left on integer arrays.
- number_integer and number_unsigned share one case of write_cbor; the
major type and the argument are selected with a sign mask.
- An integer's head is assembled without branches: the size class comes
from a table indexed by count_leading_zeros(), and the first byte and
the argument in network order are stored as one byte and one 8-byte
word, of which only the head's length is kept. Heads of strings and
containers keep the branching write_cbor_head(), as their lengths are
mostly predictable.
- Arrays are written by write_cbor_array(), kept out of line so that the
registers its loop needs are not saved on every call of write_cbor.
Runs of integers are encoded straight into space reserved from the sink
(the new reserve()/commit() of both sinks), so the write pointer stays
in a register instead of being reloaded for every value.
The output is unchanged; the new test compares integer arrays at every
head-size boundary, in runs shorter and longer than one reservation and
broken up by other values, with an independent encoder through each sink.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
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