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

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- 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>
This commit is contained in:
Niels Lohmann
2026-10-06 07:22:15 +02:00
parent 44fba02ec2
commit cf2af0396d
3 changed files with 52 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/view/builder.hpp
+16 -9
View File
@@ -828,14 +828,19 @@ 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;
@@ -851,19 +856,21 @@ 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;
}
}
include/nlohmann/detail/view/scan.hpp
+10 -1
View File
@@ -67,13 +67,19 @@ NLOHMANN_VIEW_ALWAYS_INLINE std::uint16_t load16(const unsigned char* p) noexcep
/// in predicted branches: 16 for keys, whose lengths repeat from record to
/// record, and 8 for string values (Value) where a vector loop follows, as
/// their lengths vary more. Longer runs continue 16 bytes at a time with NEON
/// or SSE2, else eight bytes at a time.
/// or SSE2, else eight bytes at a time. With SSE2, the run is checked 16 bytes
/// at a time from its first byte instead: on x86-64, one compare that finds
/// the end of most keys and short values is faster than a branch per byte (on
/// AArch64, where a NEON mask costs more and branches predict well, slower).
template<bool Value = false>
NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned char* p, const unsigned char* e) noexcept
{
const std::uint8_t* plain = string_plain();
for (;;)
{
#if NLOHMANN_VIEW_SSE2
p = vector_plain_run(p, e);
#else
if (e - p >= 16)
{
#define NLOHMANN_VIEW_STEP(i) if (NLOHMANN_VIEW_LIKELY(plain[p[i]] != 0)) {} else { p += (i); goto stop; }
@@ -107,6 +113,7 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned
#endif
continue;
}
#endif
while (p != e && plain[*p] != 0)
{
++p;
@@ -115,7 +122,9 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned
{
return p;
}
#if !NLOHMANN_VIEW_SSE2
stop:
#endif
if (*p < 0x80)
{
return p; // quote, backslash, or control character