Opening and closing an array or object cost the DOM parser more than before
the exact-size array change, which showed on documents of many small
objects (jeopardy.json: +1.4 % Ir with GCC). This takes it back:
- the stack of open containers keeps its first eight entries in the parser
object instead of in a vector, so it needs no capacity check beyond the
push itself and no allocation for typical documents;
- a container is created in the place it ends up in, instead of as a
temporary that is then moved there;
- a new object member, which is null, is constructed over the null instead
of being swapped with it (only with the default base class of basic_json);
- the key index that big ordered_json objects use is only a part of the
stack entries of objects that can have one;
- an array that outgrows the stack reserves twice what it has so far, as a
vector that grew by itself would have.
DOM parse Ir of the series up to here against develop, Linux GCC / Clang:
canada -6.6 % / -7.3 %, citm +1.2 % / +0.2 %, jeopardy +0.4 % / -0.1 %,
124-byte object -6.5 % / -6.2 %. (twitter measures +6.8 % / +6.6 % at this
point in steady state, almost all of it in glibc's malloc/consolidate for the
new allocation sizes; with the last commit of the series it is -0.1 % / 0.0 %.)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
ffa9babc8d