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Scan json_view strings with SIMD and index large objects

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Speed up json_view's parser with SIMD scanning and a hash table
for large objects.

Long runs of string bytes are scanned 16 bytes at a time with NEON
(AArch64, GCC and Clang) and SSE2 (x86-64), both baseline
instruction sets. Keys keep 16 table checks before the vector
loop, because their lengths repeat from record to record; string
values get 8, because their lengths vary more. Non-ASCII text is
validated 16 bytes at a time with simdjson's "lookup4" check
(Keiser and Lemire, 2021), with NEON on AArch64 and, on x86-64,
with SSSE3. SSSE3 is not part of baseline x86-64, so the check is
compiled for SSSE3 with a function attribute and used only where
CPUID reports it, which all x86-64 CPUs since about 2011 do; the
answer is cached in a statically initialized atomic, so there is
no guard of a local static and no global constructor. The same
input is accepted either way. JSON_VIEW_NO_SIMD selects the
portable code.

On x86-64, string runs are now checked vector-first: one SSE2
compare from the first byte finds the end of most keys and short
values, instead of a branch per byte for the first 8-16 bytes.
AArch64 keeps the byte-wise steps, where a NEON mask costs more and
the branches predict well. Entering an object or array no longer
stalls: open() stores the parent's frame field by field instead of
building it on the stack and reading it back with wider loads,
which waited for the narrower stores to retire.

Objects with 128 members or more get an open-addressing hash table
built when the object closes, so operator[], at(), find(),
contains(), count(), value(), and JSON pointers take constant time
on average in such objects; of duplicate keys, the first is kept,
as for the linear search. The idea comes from Boost.JSON.

simdjson is credited in simd.hpp's SPDX block, the README, and
license.md.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-06 10:48:07 +02:00
1 parent d26cbef976
commit 6ee5e89803
28 files changed
+1493 -47
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include/nlohmann/detail/view/builder.hpp
+39 -16
View File
@@ -116,6 +116,13 @@ class builder
frame shallow[64]; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays): not initialized on purpose; filled as containers open
std::vector<frame> deep{};
/// remember an object to index after parsing (out of line, so that the
/// parse loop only has a call for it)
NLOHMANN_VIEW_NOINLINE void note_large_object(std::uint32_t idx)
{
doc.large_objects.push_back(idx);
}
NLOHMANN_VIEW_NOINLINE bool fail(error_code c, const unsigned char* at) noexcept
{
m_failure.code = c;
@@ -501,7 +508,7 @@ 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); \
@@ -585,7 +592,7 @@ obj_key:
{
return fail(error_code::expected_key);
}
if (NLOHMANN_VIEW_UNLIKELY(!string()))
if (NLOHMANN_VIEW_UNLIKELY(!string<false>()))
{
return false;
}
@@ -626,19 +633,27 @@ obj_next:
if (enabled(TrailingCommas) && cur() == '}')
{
++p;
goto close_container;
goto close_object;
}
goto obj_key;
}
if (cur() == '}')
{
++p;
goto close_container;
goto close_object;
}
return fail(error_code::expected_object_end);
#undef NLOHMANN_VIEW_VALUE
close_object:
// a large object gets a hash index (objects only, so that closing
// an array pays nothing for this)
if (NLOHMANN_VIEW_UNLIKELY(cur_count >= document_data::index_min_members))
{
cold.note_large_object(cur_idx);
}
close_container:
close();
if (NLOHMANN_VIEW_UNLIKELY(depth == 0))
@@ -688,7 +703,7 @@ 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':
@@ -811,14 +826,19 @@ indent_done:
const auto idx = static_cast<std::uint32_t>(emit(k, 0, 0, static_cast<std::size_t>(p - b), 0) - base);
if (depth != 0)
{
const frame f = {cur_idx, cur_count, cur_is_object};
if (NLOHMANN_VIEW_LIKELY(depth <= 64))
{
cold.shallow[depth - 1] = f;
// field by field: a frame put together on the stack and
// copied would be read back wider than it was written,
// and that load waits until the stores are done
frame& f = cold.shallow[depth - 1];
f.idx = cur_idx;
f.count = cur_count;
f.is_object = cur_is_object;
}
else
{
cold.deep.push_back(f);
cold.deep.push_back(frame{cur_idx, cur_count, cur_is_object});
}
}
++depth;
@@ -834,19 +854,21 @@ indent_done:
n.next = static_cast<std::uint32_t>(out - base) - cur_idx;
if (--depth != 0)
{
frame f{};
if (NLOHMANN_VIEW_LIKELY(depth <= 64))
{
f = cold.shallow[depth - 1];
const frame& f = cold.shallow[depth - 1];
cur_idx = f.idx;
cur_count = f.count;
cur_is_object = f.is_object;
}
else
{
f = cold.deep.back();
const frame f = cold.deep.back();
cold.deep.pop_back();
cur_idx = f.idx;
cur_count = f.count;
cur_is_object = f.is_object;
}
cur_idx = f.idx;
cur_count = f.count;
cur_is_object = f.is_object;
}
}
@@ -975,12 +997,13 @@ 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));