Niels Lohmann cf2af0396d Scan strings vector-first on x86-64 and avoid a stall when nesting ...

- With SSE2, string runs are checked 16 bytes at a time from their first
  byte: one compare finds the end of most keys and short values, faster on
  x86-64 than a branch per byte. AArch64 keeps the byte steps (there, a
  NEON mask costs more and the branches predict well; vector-first was 20%
  slower on Apple M1).
- open() stores the parent's frame field by field. Built on the stack and
  copied, it was read back by loads wider than its stores, which waited for
  them (store forwarding fails): 18% of the time on citm_catalog.json.

Parse on x86-64 (GCC 13 / Clang 18, us): twitter 352 -> 306 / 316 -> 288,
citm_catalog 1000 -> 770 / 843 -> 692, canada 1941 -> 1687 / 1978 -> 1722.
Unchanged on Apple M1.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-06 07:22:15 +02:00

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nlohmann

Scan strings vector-first on x86-64 and avoid a stall when nesting

2026-10-06 07:22:15 +02:00