A string that contains an escape is decoded by builder::slow_string(). After
each escape it scanned the next run of plain bytes (8 or 16 byte-by-byte
steps, then a vector loop), and then copied that run to the decode arena in a
second pass. The run is now copied 16 bytes at a time while it is scanned
(one vector load, a store of the same 16 bytes, and the mask of the bytes
that end the run), and the write position lives in a local variable, not in
the builder, since a store of a character may alias the builder's members and
made the compiler reload it after every escape. A run that ends at a quote
needs no scan, and non-ASCII text goes through the UTF-8 check as before;
the portable code is unchanged.
Parsing, Apple M1 instructions: jeopardy.json 304.0 M -> 293.8 M (-3.4 %), 3000 strings with
escapes 1.72 M -> 1.54 M (-10 %), twitter -0.6 %.
Linux x86-64 Ir, GCC 13 / Clang 18: escaped strings -7.6 % / -11.4 %, twitter 0.0 % / 0.0 %.
Documents and error messages are identical on 900 k random strings of escapes, UTF-8 and control
characters (also under ASan), and on mutants of the test corpus.
Belongs to the PR "json_view: read-side speedups".
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
5cfff60577