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Scan the strings of json_view with NEON and SSE2

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Long runs of string bytes are scanned 16 at a time with NEON (AArch64, with
GCC and Clang) and SSE2 (x86-64): both belong to the baseline instruction
sets. A signed compare with 0x20 finds control characters and non-ASCII
bytes at once. Keys keep 16 table checks before the vector loop (their
lengths repeat from record to record, so the branches predict well);
string values have 8, as their lengths vary more.

Non-ASCII text is validated 16 bytes at a time with the "lookup4" check of
simdjson (J. Keiser and D. Lemire, "Validating UTF-8 In Less Than One
Instruction Per Byte", 2021): with NEON, and on x86-64 with SSSE3 if
JSON_VIEW_USE_SSSE3 is defined (SSSE3 is not part of x86-64, and the code
must not depend on the flags of a translation unit). JSON_VIEW_NO_SIMD
selects the portable code. The vector code sits in
detail/view/simd.hpp; the same input is accepted either way.

json_document::parse, best of 7 runs in separate processes (M1 Max):
poet.json (CJK text) -72%, random.json -25%, twitter.json -22%,
gsoc-2018.json -20%, semanticscholar -19%, github_events -11%,
apache_builds -9.5%, canada/citm -5/-6%; lottie +4%, tree-pretty +2.5%.

Tests: every two-byte sequence and three- and four-byte sequences with
continuation bytes at the edges of their ranges, at every offset around
the vector blocks of keys and values, cut short, and long runs of text
with a damaged byte, against json::accept and json::parse. CMake builds
the parser tests again with JSON_VIEW_NO_SIMD, and on x86-64 with
JSON_VIEW_USE_SSSE3 and -mssse3; the macros are documented.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-29 14:03:06 +02:00
parent a491cc7663
commit ebfbbf29cb
15 changed files with 863 additions and 20 deletions
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include/nlohmann/detail/view/builder.hpp
+6 -5
View File
@@ -492,7 +492,7 @@ class builder
switch (cur()) \
{ \
case '"': \
if (NLOHMANN_VIEW_UNLIKELY(!string())) { return false; } \
if (NLOHMANN_VIEW_UNLIKELY(!string<true>())) { return false; } \
goto NEXT; \
case '{': \
open(value_t::object); \
@@ -576,7 +576,7 @@ obj_key:
{
return fail(error_code::expected_key);
}
if (NLOHMANN_VIEW_UNLIKELY(!string()))
if (NLOHMANN_VIEW_UNLIKELY(!string<false>()))
{
return false;
}
@@ -679,7 +679,7 @@ root_done:
switch (cur())
{
case '"':
return string();
return string<true>();
case 't':
return literal("true", 4, value_t::boolean, node_flags::is_true);
case 'f':
@@ -965,12 +965,13 @@ indent_done:
return true;
}
/// a string (value or key) at p
/// a string at p: a value (Value) or a key
template<bool Value>
NLOHMANN_VIEW_ALWAYS_INLINE bool string()
{
++p; // opening quote
const unsigned char* const s = p;
p = scan_string_run(p, e);
p = scan_string_run<Value>(p, e);
if (NLOHMANN_VIEW_LIKELY(p != e && *p == '"'))
{
emit(value_t::string, 0, 0, static_cast<std::size_t>(s - b), static_cast<std::uint64_t>(p - s));