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Iterator: test for objects before arrays

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#5835 replaced the switch on the value type in operator*, operator->,
operator++, operator-- and operator== of iter_impl with an if chain that
tests for an array first. Every step over an object then pays for a
failed test and a taken branch: with the iterator inlined, walking
twitter.json took 14% more cycles than with 3.12.0, and citm_catalog.json
3.5% more.

Test for objects first. Walking every value of the three files now
takes fewer cycles than with 3.12.0 (Apple M1 Max, Apple clang 21):
citm_catalog.json 1.21x, twitter.json 1.04x, canada.json 1.17x as fast.
canada.json, which holds only arrays, gives back part of the gain of
#5835 (develop: 1.62x); the other orders tried (arrays first without
hints, objects first without hints, a switch) each left one of the
files slower than 3.12.0.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-11-07 23:44:11 +01:00
1 parent 7fc8312c64
commit 1a2828f36c
2 files changed
+66 -58
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include/nlohmann/detail/iterators/iter_impl.hpp
+33 -29
View File
@@ -277,18 +277,22 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return *m_it.array_iterator;
}
// objects are tested before arrays here and in the other operators:
// with arrays first, walking a document of objects such as
// twitter.json took 14% more cycles than with 3.12.0's switch; with
// objects first, no document walks slower than with 3.12.0
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
return m_it.object_iterator->second;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return *m_it.array_iterator;
}
// neither array nor object: a primitive value (null has no value to point to)
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type != value_t::null && m_it.primitive_iterator.is_begin()))
{
@@ -306,18 +310,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return &*m_it.array_iterator;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
JSON_ASSERT(m_it.object_iterator != m_object->m_data.m_value.object->end());
return &(m_it.object_iterator->second);
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
return &*m_it.array_iterator;
}
// neither array nor object: a primitive value
if (JSON_HEDLEY_LIKELY(m_it.primitive_iterator.is_begin()))
{
@@ -346,18 +350,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
++m_it.array_iterator;
return *this;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
std::advance(m_it.object_iterator, 1);
return *this;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
++m_it.array_iterator;
return *this;
}
// neither array nor object: a primitive value
++m_it.primitive_iterator;
return *this;
@@ -382,18 +386,18 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
{
JSON_ASSERT(m_object != nullptr);
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
--m_it.array_iterator;
return *this;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
std::advance(m_it.object_iterator, -1);
return *this;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
--m_it.array_iterator;
return *this;
}
// neither array nor object: a primitive value
--m_it.primitive_iterator;
return *this;
@@ -415,16 +419,16 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
return true;
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
return (m_it.array_iterator == other.m_it.array_iterator);
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::object))
{
return (m_it.object_iterator == other.m_it.object_iterator);
}
if (JSON_HEDLEY_LIKELY(m_object->m_data.m_type == value_t::array))
{
return (m_it.array_iterator == other.m_it.array_iterator);
}
// neither array nor object: a primitive value
return (m_it.primitive_iterator == other.m_it.primitive_iterator);
}