Iterator: test for objects before arrays

#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>

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -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))
+        // objects are tested before arrays here and in the other operators:
-        {
+        // with arrays first, walking a document of objects such as
-            JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
+        // twitter.json took 14% more cycles than with 3.12.0's switch; with
-            return *m_it.array_iterator;
+        // 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);
     }

single_include/nlohmann/json.hpp

@@ -24506,18 +24506,22 @@ 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))
+        // objects are tested before arrays here and in the other operators:
-        {
+        // with arrays first, walking a document of objects such as
-            JSON_ASSERT(m_it.array_iterator != m_object->m_data.m_value.array->end());
+        // twitter.json took 14% more cycles than with 3.12.0's switch; with
-            return *m_it.array_iterator;
+        // 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()))
         {
@@ -24535,18 +24539,18 @@ 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()))
         {
@@ -24575,18 +24579,18 @@ 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;
@@ -24611,18 +24615,18 @@ 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;
@@ -24644,16 +24648,16 @@ 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);
     }