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Keep the descent bookkeeping in one place

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Copying carried a depth count, a depth limit and a guard of its own, and
the comparison in the follow-up added a second set beside them. Neither
operation needs its own: they are never nested inside one another by the
library - copying a value does not compare one, and comparing two values
does not copy them - and where user code nests them anyway, sharing the
count only ends a descent sooner than it had to.

So there is now one nesting_depth(), one nesting_depth_limit(), one
nesting_depth_exhausted() and one nesting_depth_guard, which the follow-up
uses instead of adding its own. Inverting the test in copy_structured
leaves the too-deep case and the no-thread-local case as the same code.

The guard takes the count where the caller has already looked it up to
test it, and looks it up itself where the caller cannot - the comparison
operators are written as a macro, and a macro cannot use the preprocessor.
Whether an operator descends at all is passed to nesting_depth_exhausted()
rather than tested at the call site, where the constant makes MSVC report
C4127.

The switch that copies the value of anything that is not an object or an
array was written twice - once in the copy constructor, once in
copy_shallow - so that adding a value_t meant editing both, and missing
one would have been silent. It is copy_leaf_value now, and inlined: both
callers have already sorted the containers out, and folding that test into
the switch is what keeps a value made mostly of numbers copying as fast as
it did.

Copying canada.json, citm_catalog.json and twitter.json is within 0.6% of
what it was before, measured as a paired ratio over 18 interleaved rounds
against a run-to-run spread of 0.3%.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-21 08:49:54 +02:00
parent e66a55066e
commit ad3b07f7e5
2 changed files with 274 additions and 214 deletions
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include/nlohmann/json.hpp
+137 -107
View File
@@ -821,47 +821,106 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
return j;
}
#ifndef JSON_NO_THREAD_LOCAL
/// the number of levels the copy constructor descends into before it
/// finishes the value below without the call stack
static constexpr std::size_t copy_depth_limit()
/// the number of levels an operation descends into before it finishes the
/// value below it without the call stack
static constexpr std::size_t nesting_depth_limit()
{
return 128;
}
/// @brief how many levels the copy going on in this thread has descended into
static std::size_t& copy_depth() noexcept
#ifndef JSON_NO_THREAD_LOCAL
/*!
@brief how many levels the operation going on in this thread has descended into
Copying a value and comparing two values share this count. The library never
nests one inside the other - copying a value does not compare one, and
comparing two values does not copy them - and where user code nests them
anyway, sharing the count only ends a descent sooner than it had to, which
costs a little speed and is never wrong.
A byte is enough: the count never exceeds the limit by more than the single
level that notices the limit has been reached.
*/
static std::size_t& nesting_depth() noexcept
{
static thread_local std::size_t depth = 0; // NOLINT(misc-use-internal-linkage)
return depth;
}
#endif
#ifndef JSON_NO_THREAD_LOCAL
/// @brief counts one level of @ref copy_structured for as long as it runs
class copy_depth_guard
/*!
@brief whether a descent must stop here and finish without the call stack
@a may_descend says whether the operation descends at all; it is a constant
at every call site, and is passed rather than tested by the caller so that
the test does not become a constant condition there, which MSVC reports as
C4127.
*/
static bool nesting_depth_exhausted(bool may_descend = true) noexcept
{
#ifdef JSON_NO_THREAD_LOCAL
// without a count of its own per thread, a descent cannot be bounded
// without racing another one, so none is made
static_cast<void>(may_descend);
return true;
#else
return !may_descend || nesting_depth() >= nesting_depth_limit();
#endif
}
/*!
@brief counts one level of a bounded descent for as long as it runs
The constructor taking the count is for callers that have looked it up
already to test it: reaching thread-local storage is not free, and the path
that is taken almost every time should reach it once rather than twice. The
other is for callers that cannot look it up - the comparison operators are
written as a macro, and a macro cannot use the preprocessor.
*/
class nesting_depth_guard
{
public:
explicit copy_depth_guard(std::size_t& depth) noexcept
: m_depth(depth)
explicit nesting_depth_guard(std::size_t& depth) noexcept
: m_depth(&depth)
{
++m_depth;
++*m_depth;
}
~copy_depth_guard() noexcept
nesting_depth_guard() noexcept
: m_depth(countable())
{
--m_depth;
if (m_depth != nullptr)
{
++*m_depth;
}
}
copy_depth_guard(const copy_depth_guard&) = delete;
copy_depth_guard& operator=(const copy_depth_guard&) = delete;
copy_depth_guard(copy_depth_guard&&) = delete;
copy_depth_guard& operator=(copy_depth_guard&&) = delete;
~nesting_depth_guard()
{
if (m_depth != nullptr)
{
--*m_depth;
}
}
nesting_depth_guard(const nesting_depth_guard&) = delete;
nesting_depth_guard& operator=(const nesting_depth_guard&) = delete;
nesting_depth_guard(nesting_depth_guard&&) = delete;
nesting_depth_guard& operator=(nesting_depth_guard&&) = delete;
private:
std::size_t& m_depth;
};
/// @brief the count to keep, or nullptr where there is none to keep
static std::size_t* countable() noexcept
{
#ifdef JSON_NO_THREAD_LOCAL
return nullptr;
#else
return &nesting_depth();
#endif
}
std::size_t* m_depth;
};
/// an entry of the iterative deep copy's worklist: a structured value and
/// the value that is to become its copy
@@ -887,27 +946,21 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}
/*!
@brief copy everything of @a src into the null value @a dst but the children
@brief copy the value of @a src into @a dst, which must not be structured
Objects and arrays are not copied here; they are appended to @a worklist to
be created later by @ref copy_iteratively. Until that happens, @a dst remains
a null value, so that a partially built copy can be destroyed at any point
without ever violating the class invariants.
Objects and arrays are left alone: creating those is the one thing the copy
constructor and @ref copy_shallow do differently from one another, and it is
the reason copying a value can descend at all.
*/
static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
/// @note inlined on purpose: both callers have already told an object or an
/// array apart from the rest, and letting the compiler fold that test
/// into this switch is worth a few percent when copying a value made
/// mostly of numbers
JSON_HEDLEY_ALWAYS_INLINE
static void copy_leaf_value(const basic_json& src, basic_json& dst)
{
copy_metadata(src, dst);
switch (src.m_data.m_type)
{
case value_t::object:
case value_t::array:
{
// defer: dst stays a null value until its container exists
worklist.emplace_back(&src, &dst);
return;
}
case value_t::string:
{
dst.m_data.m_value = *src.m_data.m_value.string;
@@ -944,11 +997,35 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
break;
}
case value_t::object:
case value_t::array:
case value_t::null:
case value_t::discarded:
default:
break;
}
}
/*!
@brief copy everything of @a src into the null value @a dst but the children
Objects and arrays are not copied here; they are appended to @a worklist to
be created later by @ref copy_iteratively. Until that happens, @a dst remains
a null value, so that a partially built copy can be destroyed at any point
without ever violating the class invariants.
*/
static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
{
copy_metadata(src, dst);
if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
{
// defer: dst stays a null value until its container exists
worklist.emplace_back(&src, &dst);
return;
}
copy_leaf_value(src, dst);
// only now that the value exists may the type be set: had the creation
// of the value thrown, dst would have been left as a valid null value
@@ -1018,7 +1095,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
The values whose copy has not been created yet are kept on an explicit
worklist rather than on the call stack. This is only reached for values
nested deeper than @ref copy_depth_limit levels, which is why it copies
nested deeper than @ref nesting_depth_limit levels, which is why it copies
every container by hand instead of letting the container do it: the fast
ways of doing so would descend into the elements and defeat the purpose.
*/
@@ -1049,8 +1126,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
break;
}
src_value = worklist.back().first;
dst_value = worklist.back().second;
const auto& next = worklist.back();
src_value = next.first;
dst_value = next.second;
worklist.pop_back();
// the value stops being a null value exactly here
@@ -1058,7 +1136,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
}
}
#ifndef JSON_NO_THREAD_LOCAL
/*!
@brief copy one level of the object or array @a src into this value
@@ -1078,14 +1155,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
set_parents();
}
#endif
/*!
@brief deep-copy the object or array @a src into this value
Copying a container copies its elements, so a value nested deeply enough
used to exhaust the call stack. The descent is bounded here: the first
@ref copy_depth_limit levels are copied by the containers themselves, just
@ref nesting_depth_limit levels are copied by the containers themselves, just
as they always were, and anything below that is copied without the call
stack by @ref copy_iteratively. Copying a value can therefore no longer
exhaust the stack, however deeply it is nested, just like destroying one
@@ -1099,26 +1175,22 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
*/
void copy_structured(const basic_json& src)
{
#ifdef JSON_NO_THREAD_LOCAL
// without a counter of its own per thread, the descent cannot be
// bounded without racing another one, so none is made
copy_iteratively(src);
#else
std::size_t& depth = copy_depth();
#ifndef JSON_NO_THREAD_LOCAL
std::size_t& depth = nesting_depth();
if (JSON_HEDLEY_UNLIKELY(depth >= copy_depth_limit()))
if (JSON_HEDLEY_LIKELY(depth < nesting_depth_limit()))
{
// Finish this value without descending any further. It is completed
// before this returns, so a copy made by a custom base class - or by
// anything else that runs while a copy is going on - is unaffected
// by the copy it is nested in.
copy_iteratively(src);
const nesting_depth_guard guard(depth);
copy_level(src);
return;
}
const copy_depth_guard guard(depth);
copy_level(src);
#endif
// Finish this value without descending any further. It is completed
// before this returns, so a copy made by a custom base class - or by
// anything else that runs while a copy is going on - is unaffected by
// the copy it is nested in.
copy_iteratively(src);
}
@@ -1501,57 +1573,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
// check of passed value is valid
other.assert_invariant();
switch (m_data.m_type)
if (m_data.m_type == value_t::object || m_data.m_type == value_t::array)
{
case value_t::object:
case value_t::array:
{
// copying the container directly would call this constructor
// again for every element, once per nesting level
copy_structured(other);
break;
}
case value_t::string:
{
m_data.m_value = *other.m_data.m_value.string;
break;
}
case value_t::boolean:
{
m_data.m_value = other.m_data.m_value.boolean;
break;
}
case value_t::number_integer:
{
m_data.m_value = other.m_data.m_value.number_integer;
break;
}
case value_t::number_unsigned:
{
m_data.m_value = other.m_data.m_value.number_unsigned;
break;
}
case value_t::number_float:
{
m_data.m_value = other.m_data.m_value.number_float;
break;
}
case value_t::binary:
{
m_data.m_value = *other.m_data.m_value.binary;
break;
}
case value_t::null:
case value_t::discarded:
default:
break;
// copying the container directly would call this constructor again
// for every element, once per nesting level
copy_structured(other);
}
else
{
copy_leaf_value(other, *this);
}
set_parents();