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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/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
};
/*!