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Erica FischerandCursor
3f526ccad7
Profiling tl_2022_us_county.json (sample(1) on Apple Silicon) showed
~38% of parse time in allocator work and ~14% in std::string::push_back
during string-token construction. These changes target the low-hanging
fruit from that profile:
- Pre-reserve 2 slots in json_array and 4 slots in json_hash so
coordinate `[x, y]` pairs and typical GeoJSON property maps avoid
the 0 -> 1 -> 2 -> 4 vector-growth chain (and the shared_ptr copies
it incurs).
- Reuse a parser-wide std::string buffer for JSON_STRING tokens
instead of constructing a fresh local std::string per token. The
buffer is cleared (capacity preserved) at the start of each token
and copied into the final json_string, so once it has grown to the
longest string seen it stops reallocating entirely.
- std::move the freshly-created container shared_ptr into the parser
container stack in the `[` and `{` handlers, and move it out of the
frame on the matching `]` / `}`. Each move skips one atomic
inc/dec round-trip per container open and close.
On a tl_2022_us_county.json benchmark (4-iter user-time mean, Apple
Silicon, /usr/bin/time):
- main baseline: ~8.17s
- jsonpull-cpp before these changes: ~10.90s (+33%)
- jsonpull-cpp with these changes: ~9.33s (+14%)
So this commit recovers roughly half of the post-port regression.
The remaining gap is dominated by shared_ptr atomic refcount traffic
on the parse tree and per-node heap allocations, which would require
the larger unique_ptr/arena reworks to address.
Co-authored-by: Cursor <cursoragent@cursor.com>