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json_view: keep GCC's inlining of view walks with the diagnostics macros

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Measured on GCC 13 (x86-64) with JSON_DIAGNOSTICS or JSON_DIAGNOSTIC_POSITIONS,
a recursive walk over a view took 2.7 to 2.9 times the instructions: GCC no
longer inlined the walk into itself. The cause was not the walk's code but
that the path and range code of errors.hpp was instantiated in every
translation unit (the non-template throw helpers referenced it), which
changed GCC's early inlining of unrelated view functions. diagnostics() and
the throw helpers that take an error_site are now templates, so that code is
instantiated only where an exception can be thrown; the walks have the
instruction counts of develop again.

Iterators no longer keep their container: with it, one walk was still 1.1 %
slower. key() of an iterator over an array or a primitive value
(invalid_iterator.207) is therefore thrown without a context, as documented.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-30 22:46:49 +01:00
1 parent 8ac510b8d3
commit 429afd4fc0
7 files changed
+73 -81
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include/nlohmann/detail/view/errors.hpp
+21 -9
View File
@@ -238,7 +238,15 @@ void value_range(const document_data& d, const node* n, std::size_t& start, std:
/// a value: "(/path) " with JSON_DIAGNOSTICS (none for the root), then
/// "(bytes a-b) " with JSON_DIAGNOSTIC_POSITIONS (with JSON_DIAGNOSTICS, only
/// if there is a path)
inline std::string diagnostics(const error_site site)
///
/// This and the throw helpers that take an error_site are templates, so that
/// the path and range code is instantiated only where an exception can be
/// thrown: instantiated in every translation unit (as a non-template inline
/// function makes it), it changed GCC's early inlining of unrelated view
/// functions, so that GCC 13 no longer inlined a recursive walk over a view
/// into itself (2.7 times the instructions).
template<typename Unused = void>
std::string diagnostics(const error_site site)
{
#if JSON_DIAGNOSTICS || JSON_DIAGNOSTIC_POSITIONS
if (site.doc() == nullptr || site.value() == nullptr)
@@ -291,7 +299,7 @@ inline std::string diagnostics(const error_site site)
template<typename View>
std::string diagnostics_of(const View& v)
{
return diagnostics(site_of(v));
return diagnostics<>(site_of(v));
}
// Exceptions are thrown out of line, so that the accessors that may throw stay
@@ -318,19 +326,22 @@ std::string diagnostics_of(const View& v)
NLOHMANN_VIEW_THROW(invalid_iterator::create(id, msg, nullptr));
}
[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_type_error(const error_site site, exception_id id, const char* prefix, const char* type)
template<typename Unused = void>
[[noreturn]] NLOHMANN_VIEW_NOINLINE void throw_type_error(const error_site site, exception_id id, const char* prefix, const char* type)
{
NLOHMANN_VIEW_THROW(type_error::create(id, concat(diagnostics(site), prefix, type), nullptr));
NLOHMANN_VIEW_THROW(type_error::create(id, concat(diagnostics<>(site), prefix, type), nullptr));
}
[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_out_of_range(const error_site site, exception_id id, const std::string& msg)
template<typename Unused = void>
[[noreturn]] NLOHMANN_VIEW_NOINLINE void throw_out_of_range(const error_site site, exception_id id, const std::string& msg)
{
NLOHMANN_VIEW_THROW(out_of_range::create(id, concat(diagnostics(site), msg), nullptr));
NLOHMANN_VIEW_THROW(out_of_range::create(id, concat(diagnostics<>(site), msg), nullptr));
}
[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_invalid_iterator(const error_site site, exception_id id, const char* msg)
template<typename Unused = void>
[[noreturn]] NLOHMANN_VIEW_NOINLINE void throw_invalid_iterator(const error_site site, exception_id id, const char* msg)
{
NLOHMANN_VIEW_THROW(invalid_iterator::create(id, concat(diagnostics(site), msg), nullptr));
NLOHMANN_VIEW_THROW(invalid_iterator::create(id, concat(diagnostics<>(site), msg), nullptr));
}
/// a parse error without a position (as those of json_pointer)
@@ -347,7 +358,8 @@ template<typename Index>
}
/// out_of_range.402: the JSON Pointer token '-' of an array of @a size elements
[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_array_index_past_end(const error_site site, std::size_t size)
template<typename Unused = void>
[[noreturn]] NLOHMANN_VIEW_NOINLINE void throw_array_index_past_end(const error_site site, std::size_t size)
{
throw_out_of_range(site, exception_id::pointer_past_the_end_index, concat("array index '-' (", std::to_string(size), ") is out of range"));
}
include/nlohmann/detail/view/iterator.hpp
+7 -25
View File
@@ -67,17 +67,9 @@ class view_iterator
/// @param[in] pos the element, or the key of the member
/// @param[in] object whether pos is a key (its value is the next node)
/// @param[in] container the value iterated over (the context of the
/// exceptions, kept only with JSON_DIAGNOSTICS or
/// JSON_DIAGNOSTIC_POSITIONS)
view_iterator(const document_data* d, const node* pos, bool object, const node* container) noexcept
view_iterator(const document_data* d, const node* pos, bool object) noexcept
: m_doc(d), m_pos(pos), m_value_offset(object ? 1 : 0)
#if JSON_DIAGNOSTICS || JSON_DIAGNOSTIC_POSITIONS
, m_container(container)
#endif
{
static_cast<void>(container);
}
{}
NLOHMANN_VIEW_ALWAYS_INLINE View operator*() const noexcept
{
@@ -113,12 +105,15 @@ class view_iterator
}
/// the key of the current object member; throws invalid_iterator.207 for
/// other iterators, like basic_json's iterators
/// other iterators, like basic_json's iterators (but without the context
/// JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS add: the iterator does not
/// know the value it iterates over, and keeping it would make iterators
/// larger, which keeps GCC from inlining recursive walks)
string_view_t key() const
{
if (NLOHMANN_VIEW_UNLIKELY(m_value_offset == 0))
{
throw_invalid_iterator(site(), exception_id::iterator_key_not_object, "cannot use key() for non-object iterators");
throw_invalid_iterator(exception_id::iterator_key_not_object, "cannot use key() for non-object iterators");
}
return string_view_t(m_doc->str(*m_pos), m_pos->len);
}
@@ -135,22 +130,9 @@ class view_iterator
}
private:
/// the value iterated over, as the context of an exception
error_site site() const noexcept
{
#if JSON_DIAGNOSTICS || JSON_DIAGNOSTIC_POSITIONS
return error_site(m_doc, m_container);
#else
return error_site();
#endif
}
const document_data* m_doc = nullptr;
const node* m_pos = nullptr;
std::size_t m_value_offset = 0; ///< 1 for objects: the value follows its key
#if JSON_DIAGNOSTICS || JSON_DIAGNOSTIC_POSITIONS
const node* m_container = nullptr;
#endif
};
/*!
include/nlohmann/json_view.hpp
+5 -5
View File
@@ -481,7 +481,7 @@ class basic_json_view
return end();
}
const node* const k = find_key(key);
return k != nullptr ? iterator(m_doc, k, true, m_node) : end();
return k != nullptr ? iterator(m_doc, k, true) : end();
}
iterator find(const char* key) const
@@ -544,18 +544,18 @@ class basic_json_view
{
if (NLOHMANN_VIEW_LIKELY(is_structured()))
{
return iterator(m_doc, navigation::first(*m_doc, m_node), is_object(), m_node);
return iterator(m_doc, navigation::first(*m_doc, m_node), is_object());
}
return iterator(m_doc, m_node, false, m_node);
return iterator(m_doc, m_node, false);
}
NLOHMANN_VIEW_ALWAYS_INLINE iterator end() const noexcept
{
if (NLOHMANN_VIEW_LIKELY(is_structured()))
{
return iterator(m_doc, navigation::end(*m_doc, m_node), is_object(), m_node);
return iterator(m_doc, navigation::end(*m_doc, m_node), is_object());
}
return iterator(m_doc, (is_null() || is_discarded()) ? m_node : m_node + 1, false, m_node);
return iterator(m_doc, (is_null() || is_discarded()) ? m_node : m_node + 1, false);
}
iterator cbegin() const noexcept