libexpr: Structural sharing of attrsets

This changes the implementation of Bindings to allow
for a more space-efficient implementation of attribute
set merges. This is accomplished by "layering" over the "base" Bindings.
The top "layer" is naturally the right-hand-side of the update operator //.

Such an implementation leads to significantly better memory usage on
something like nixpkgs:

nix-env --query --available --out-path --file ../nixpkgs --eval-system x86_64-linux > /dev/null

Comparison against 2b0fd88324 for x86_64-linux on nixpkgs f06c7c3b6f5074dbffcf02542fb86af3a5526afa:

| metric                 | mean_before     | mean_after      | mean_diff       | mean_%_change | p_value | t_stat  |
| -                      | -               | -               | -               | -             | -       | -       |
| cpuTime                | 21.1520         | 21.3414         | 0.1894          | 0.7784        | 0.3190  | 1.0219  |
| envs.bytes             | 461451951.6190  | 461451951.6190  | -               | -             | -       | -       |
| envs.elements          | 34344544.8571   | 34344544.8571   | -               | -             | -       | -       |
| envs.number            | 23336949.0952   | 23336949.0952   | -               | -             | -       | -       |
| gc.cycles              | 7.5238          | 7.2857          | -0.2381         | -4.6825       | 0.0565  | -2.0244 |
| gc.heapSize            | 1777848124.9524 | 1252162023.6190 | -525686101.3333 | -29.9472      | 0.0000  | -8.7041 |
| gc.totalBytes          | 3102787383.6190 | 2498431578.6667 | -604355804.9524 | -19.7704      | 0.0000  | -9.3502 |
| list.bytes             | 59928225.9048   | 59928225.9048   | -               | -             | -       | -       |
| list.concats           | 1240028.2857    | 1240028.2857    | -               | -             | -       | -       |
| list.elements          | 7491028.2381    | 7491028.2381    | -               | -             | -       | -       |
| nrAvoided              | 28165342.2381   | 28165342.2381   | -               | -             | -       | -       |
| nrExprs                | 1577412.9524    | 1577412.9524    | -               | -             | -       | -       |
| nrFunctionCalls        | 20970743.4286   | 20970743.4286   | -               | -             | -       | -       |
| nrLookups              | 10867306.0952   | 10867306.0952   | -               | -             | -       | -       |
| nrOpUpdateValuesCopied | 61206062.0000   | 25748169.5238   | -35457892.4762  | -58.8145      | 0.0000  | -8.9189 |
| nrOpUpdates            | 2167097.4286    | 2167097.4286    | -               | -             | -       | -       |
| nrPrimOpCalls          | 12337423.4286   | 12337423.4286   | -               | -             | -       | -       |
| nrThunks               | 29361806.7619   | 29361806.7619   | -               | -             | -       | -       |
| sets.bytes             | 1393822818.6667 | 897587655.2381  | -496235163.4286 | -36.7168      | 0.0000  | -9.1115 |
| sets.elements          | 84504465.3333   | 48270845.9524   | -36233619.3810  | -43.8698      | 0.0000  | -8.9181 |
| sets.number            | 5218921.6667    | 5218921.6667    | -               | -             | -       | -       |
| sizes.Attr             | 16.0000         | 16.0000         | -               | -             | -       | -       |
| sizes.Bindings         | 8.0000          | 24.0000         | 16.0000         | 200.0000      | -       | inf     |
| sizes.Env              | 8.0000          | 8.0000          | -               | -             | -       | -       |
| sizes.Value            | 16.0000         | 16.0000         | -               | -             | -       | -       |
| symbols.bytes          | 1368494.0952    | 1368494.0952    | -               | -             | -       | -       |
| symbols.number         | 109147.1905     | 109147.1905     | -               | -             | -       | -       |
| time.cpu               | 21.1520         | 21.3414         | 0.1894          | 0.7784        | 0.3190  | 1.0219  |
| time.gc                | 1.6011          | 0.8508          | -0.7503         | -37.1507      | 0.0017  | -3.6328 |
| time.gcFraction        | 0.0849          | 0.0399          | -0.0450         | -37.4504      | 0.0035  | -3.3116 |
| values.bytes           | 615968144.7619  | 615968144.7619  | -               | -             | -       | -       |
| values.number          | 38498009.0476   | 38498009.0476   | -               | -             | -       | -       |

Overall this does slow down the evaluator slightly (no more than ~10% in most cases),
but this seems like a very decent tradeoff for shaving off 33% of memory usage.

src/libexpr-c/nix_api_value.cc

@@ -371,13 +371,24 @@ bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalS
     NIXC_CATCH_ERRS_RES(false);
 }
 
-nix_value * nix_get_attr_byidx(
+static void collapse_attrset_layer_chain_if_needed(nix::Value & v, EvalState * state)
-    nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i, const char ** name)
+{
+    auto & attrs = *v.attrs();
+    if (attrs.isLayered()) {
+        auto bindings = state->state.buildBindings(attrs.size());
+        std::ranges::copy(attrs, std::back_inserter(bindings));
+        v.mkAttrs(bindings);
+    }
+}
+
+nix_value *
+nix_get_attr_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_in(value);
+        collapse_attrset_layer_chain_if_needed(v, state);
         const nix::Attr & a = (*v.attrs())[i];
         *name = state->state.symbols[a.name].c_str();
         nix_gc_incref(nullptr, a.value);
@@ -387,13 +398,13 @@ nix_value * nix_get_attr_byidx(
     NIXC_CATCH_ERRS_NULL
 }
 
-const char *
+const char * nix_get_attr_name_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i)
-nix_get_attr_name_byidx(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i)
 {
     if (context)
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_in(value);
+        collapse_attrset_layer_chain_if_needed(v, state);
         const nix::Attr & a = (*v.attrs())[i];
         return state->state.symbols[a.name].c_str();
     }

src/libexpr-c/nix_api_value.h

@@ -297,8 +297,8 @@ bool nix_has_attr_byname(nix_c_context * context, const nix_value * value, EvalS
  * @param[out] name will store a pointer to the attribute name
  * @return value, NULL in case of errors
  */
-nix_value * nix_get_attr_byidx(
+nix_value *
-    nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i, const char ** name);
+nix_get_attr_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i, const char ** name);
 
 /** @brief Get an attribute name by index in the sorted bindings
  *
@@ -311,8 +311,7 @@ nix_value * nix_get_attr_byidx(
  * @param[in] i attribute index
  * @return name, NULL in case of errors
  */
-const char *
+const char * nix_get_attr_name_byidx(nix_c_context * context, nix_value * value, EvalState * state, unsigned int i);
-nix_get_attr_name_byidx(nix_c_context * context, const nix_value * value, EvalState * state, unsigned int i);
 
 /**@}*/
 /** @name Initializers

src/libexpr-tests/nix_api_expr.cc

@@ -437,4 +437,31 @@ TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
     assert_ctx_ok();
     ASSERT_EQ(3, rInt);
 }
+
+TEST_F(nix_api_expr_test, nix_expr_attrset_update)
+{
+    nix_expr_eval_from_string(ctx, state, "{ a = 0; b = 2; } // { a = 1; b = 3; } // { a = 2; }", ".", value);
+    assert_ctx_ok();
+
+    ASSERT_EQ(nix_get_attrs_size(ctx, value), 2);
+    assert_ctx_ok();
+    std::array<std::pair<std::string_view, nix_value *>, 2> values;
+    for (unsigned int i = 0; i < 2; ++i) {
+        const char * name;
+        values[i].second = nix_get_attr_byidx(ctx, value, state, i, &name);
+        assert_ctx_ok();
+        values[i].first = name;
+    }
+    std::sort(values.begin(), values.end(), [](const auto & lhs, const auto & rhs) { return lhs.first < rhs.first; });
+
+    nix_value * a = values[0].second;
+    ASSERT_EQ("a", values[0].first);
+    ASSERT_EQ(nix_get_int(ctx, a), 2);
+    assert_ctx_ok();
+    nix_value * b = values[1].second;
+    ASSERT_EQ("b", values[1].first);
+    ASSERT_EQ(nix_get_int(ctx, b), 3);
+    assert_ctx_ok();
+}
+
 } // namespace nixC

src/libexpr/attr-set.cc

@@ -14,7 +14,7 @@ Bindings * EvalState::allocBindings(size_t capacity)
 {
     if (capacity == 0)
         return &Bindings::emptyBindings;
-    if (capacity > std::numeric_limits<Bindings::size_t>::max())
+    if (capacity > std::numeric_limits<Bindings::size_type>::max())
         throw Error("attribute set of size %d is too big", capacity);
     nrAttrsets++;
     nrAttrsInAttrsets += capacity;
@@ -35,7 +35,7 @@ Value & BindingsBuilder::alloc(std::string_view name, PosIdx pos)
 
 void Bindings::sort()
 {
-    std::sort(attrs, attrs + size_);
+    std::sort(attrs, attrs + numAttrs);
 }
 
 Value & Value::mkAttrs(BindingsBuilder & bindings)

src/libexpr/eval.cc

@@ -1873,37 +1873,71 @@ void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 
     state.nrOpUpdates++;
 
-    if (v1.attrs()->size() == 0) {
+    const Bindings & bindings1 = *v1.attrs();
+    if (bindings1.empty()) {
         v = v2;
         return;
     }
-    if (v2.attrs()->size() == 0) {
+
+    const Bindings & bindings2 = *v2.attrs();
+    if (bindings2.empty()) {
         v = v1;
         return;
     }
 
-    auto attrs = state.buildBindings(v1.attrs()->size() + v2.attrs()->size());
+    /* Simple heuristic for determining whether attrs2 should be "layered" on top of
+       attrs1 instead of copying to a new Bindings. */
+    bool shouldLayer = [&]() -> bool {
+        if (bindings1.isLayerListFull())
+            return false;
+
+        if (bindings2.size() > state.settings.bindingsUpdateLayerRhsSizeThreshold)
+            return false;
+
+        return true;
+    }();
+
+    if (shouldLayer) {
+        auto attrs = state.buildBindings(bindings2.size());
+        attrs.layerOnTopOf(bindings1);
+
+        std::ranges::copy(bindings2, std::back_inserter(attrs));
+        v.mkAttrs(attrs.alreadySorted());
+
+        state.nrOpUpdateValuesCopied += bindings2.size();
+        return;
+    }
+
+    auto attrs = state.buildBindings(bindings1.size() + bindings2.size());
 
     /* Merge the sets, preferring values from the second set.  Make
        sure to keep the resulting vector in sorted order. */
-    auto i = v1.attrs()->begin();
+    auto i = bindings1.begin();
-    auto j = v2.attrs()->begin();
+    auto j = bindings2.begin();
 
-    while (i != v1.attrs()->end() && j != v2.attrs()->end()) {
+    while (i != bindings1.end() && j != bindings2.end()) {
         if (i->name == j->name) {
             attrs.insert(*j);
             ++i;
             ++j;
-        } else if (i->name < j->name)
+        } else if (i->name < j->name) {
-            attrs.insert(*i++);
+            attrs.insert(*i);
-        else
+            ++i;
-            attrs.insert(*j++);
+        } else {
+            attrs.insert(*j);
+            ++j;
+        }
     }
 
-    while (i != v1.attrs()->end())
+    while (i != bindings1.end()) {
-        attrs.insert(*i++);
+        attrs.insert(*i);
-    while (j != v2.attrs()->end())
+        ++i;
-        attrs.insert(*j++);
+    }
+
+    while (j != bindings2.end()) {
+        attrs.insert(*j);
+        ++j;
+    }
 
     v.mkAttrs(attrs.alreadySorted());
 

src/libexpr/include/nix/expr/attr-set.hh

@@ -4,9 +4,12 @@
 #include "nix/expr/nixexpr.hh"
 #include "nix/expr/symbol-table.hh"
 
+#include <boost/container/static_vector.hpp>
+
 #include <algorithm>
 #include <functional>
-#include <concepts>
+#include <ranges>
+#include <optional>
 
 namespace nix {
 
@@ -48,11 +51,18 @@ static_assert(
  * by its size and its capacity, the capacity being the number of Attr
  * elements allocated after this structure, while the size corresponds to
  * the number of elements already inserted in this structure.
+ *
+ * Bindings can be efficiently `//`-composed into an intrusive linked list of "layers"
+ * that saves on copies and allocations. Each lookup (@see Bindings::get) traverses
+ * this linked list until a matching attribute is found (thus overlays earlier in
+ * the list take precedence). For iteration over the whole Bindings, an on-the-fly
+ * k-way merge is performed by Bindings::iterator class.
  */
 class Bindings
 {
 public:
-    typedef uint32_t size_t;
+    using size_type = uint32_t;
+
     PosIdx pos;
 
     /**
@@ -62,7 +72,32 @@ public:
     static Bindings emptyBindings;
 
 private:
-    size_t size_ = 0;
+    /**
+     * Number of attributes in the attrs FAM (Flexible Array Member).
+     */
+    size_type numAttrs = 0;
+
+    /**
+     * Number of attributes with unique names in the layer chain.
+     *
+     * This is the *real* user-facing size of bindings, whereas @ref numAttrs is
+     * an implementation detail of the data structure.
+     */
+    size_type numAttrsInChain = 0;
+
+    /**
+     * Length of the layers list.
+     */
+    uint32_t numLayers = 1;
+
+    /**
+     * Bindings that this attrset is "layered" on top of.
+     */
+    const Bindings * baseLayer = nullptr;
+
+    /**
+     * Flexible array member of attributes.
+     */
     Attr attrs[0];
 
     Bindings() = default;
@@ -71,15 +106,22 @@ private:
     Bindings & operator=(const Bindings &) = delete;
     Bindings & operator=(Bindings &&) = delete;
 
+    friend class BindingsBuilder;
+
+    /**
+     * Maximum length of the Bindings layer chains.
+     */
+    static constexpr unsigned maxLayers = 8;
+
 public:
-    size_t size() const
+    size_type size() const
     {
-        return size_;
+        return numAttrsInChain;
     }
 
     bool empty() const
     {
-        return !size_;
+        return size() == 0;
     }
 
     class iterator
@@ -94,77 +136,276 @@ public:
         friend class Bindings;
 
     private:
-        pointer ptr = nullptr;
+        struct BindingsCursor
-
-        explicit iterator(pointer ptr)
-            : ptr(ptr)
         {
+            /**
+             * Attr that the cursor currently points to.
+             */
+            pointer current;
+
+            /**
+             * One past the end pointer to the contiguous buffer of Attrs.
+             */
+            pointer end;
+
+            /**
+             * Priority of the value. Lesser values have more priority (i.e. they override
+             * attributes that appear later in the linked list of Bindings).
+             */
+            uint32_t priority;
+
+            pointer operator->() const noexcept
+            {
+                return current;
+            }
+
+            reference get() const noexcept
+            {
+                return *current;
+            }
+
+            bool empty() const noexcept
+            {
+                return current == end;
+            }
+
+            void increment() noexcept
+            {
+                ++current;
+            }
+
+            void consume(Symbol name) noexcept
+            {
+                while (!empty() && current->name <= name)
+                    ++current;
+            }
+
+            GENERATE_CMP(BindingsCursor, me->current->name, me->priority)
+        };
+
+        using QueueStorageType = boost::container::static_vector<BindingsCursor, maxLayers>;
+
+        /**
+         * Comparator implementing the override priority / name ordering
+         * for BindingsCursor.
+         */
+        static constexpr auto comp = std::greater<BindingsCursor>();
+
+        /**
+         * A priority queue used to implement an on-the-fly k-way merge.
+         */
+        QueueStorageType cursorHeap;
+
+        /**
+         * The attribute the iterator currently points to.
+         */
+        pointer current = nullptr;
+
+        /**
+         * Whether iterating over a single attribute and not a merge chain.
+         */
+        bool doMerge = true;
+
+        void push(BindingsCursor cursor) noexcept
+        {
+            cursorHeap.push_back(cursor);
+            std::ranges::make_heap(cursorHeap, comp);
+        }
+
+        [[nodiscard]] BindingsCursor pop() noexcept
+        {
+            std::ranges::pop_heap(cursorHeap, comp);
+            auto cursor = cursorHeap.back();
+            cursorHeap.pop_back();
+            return cursor;
+        }
+
+        iterator & finished() noexcept
+        {
+            current = nullptr;
+            return *this;
+        }
+
+        void next(BindingsCursor cursor) noexcept
+        {
+            current = &cursor.get();
+            cursor.increment();
+
+            if (!cursor.empty())
+                push(cursor);
+        }
+
+        std::optional<BindingsCursor> consumeAllUntilCurrentName() noexcept
+        {
+            auto cursor = pop();
+            Symbol lastHandledName = current->name;
+
+            while (cursor->name <= lastHandledName) {
+                cursor.consume(lastHandledName);
+                if (!cursor.empty())
+                    push(cursor);
+
+                if (cursorHeap.empty())
+                    return std::nullopt;
+
+                cursor = pop();
+            }
+
+            return cursor;
+        }
+
+        explicit iterator(const Bindings & attrs) noexcept
+            : doMerge(attrs.baseLayer)
+        {
+            auto pushBindings = [this, priority = unsigned{0}](const Bindings & layer) mutable {
+                auto first = layer.attrs;
+                push(
+                    BindingsCursor{
+                        .current = first,
+                        .end = first + layer.numAttrs,
+                        .priority = priority++,
+                    });
+            };
+
+            if (!doMerge) {
+                if (attrs.empty())
+                    return;
+
+                current = attrs.attrs;
+                pushBindings(attrs);
+
+                return;
+            }
+
+            const Bindings * layer = &attrs;
+            while (layer) {
+                if (layer->numAttrs != 0)
+                    pushBindings(*layer);
+                layer = layer->baseLayer;
+            }
+
+            if (cursorHeap.empty())
+                return;
+
+            next(pop());
         }
 
     public:
         iterator() = default;
 
-        reference operator*() const
+        reference operator*() const noexcept
         {
-            return *ptr;
+            return *current;
         }
 
-        const value_type * operator->() const
+        pointer operator->() const noexcept
         {
-            return ptr;
+            return current;
         }
 
-        iterator & operator++()
+        iterator & operator++() noexcept
         {
-            ++ptr;
+            if (!doMerge) {
+                ++current;
+                if (current == cursorHeap.front().end)
+                    return finished();
+                return *this;
+            }
+
+            if (cursorHeap.empty())
+                return finished();
+
+            auto cursor = consumeAllUntilCurrentName();
+            if (!cursor)
+                return finished();
+
+            next(*cursor);
             return *this;
         }
 
-        iterator operator++(int)
+        iterator operator++(int) noexcept
         {
-            pointer tmp = ptr;
+            iterator tmp = *this;
             ++*this;
-            return iterator(tmp);
+            return tmp;
         }
 
-        bool operator==(const iterator & rhs) const = default;
+        bool operator==(const iterator & rhs) const noexcept
+        {
+            return current == rhs.current;
+        }
     };
 
     using const_iterator = iterator;
 
     void push_back(const Attr & attr)
     {
-        attrs[size_++] = attr;
+        attrs[numAttrs++] = attr;
+        numAttrsInChain = numAttrs;
     }
 
-    const Attr * get(Symbol name) const
+    /**
+     * Get attribute by name or nullptr if no such attribute exists.
+     */
+    const Attr * get(Symbol name) const noexcept
     {
-        Attr key(name, 0);
+        auto getInChunk = [key = Attr{name, nullptr}](const Bindings & chunk) -> const Attr * {
-        auto first = attrs;
+            auto first = chunk.attrs;
-        auto last = attrs + size_;
+            auto last = first + chunk.numAttrs;
-        const Attr * i = std::lower_bound(first, last, key);
+            const Attr * i = std::lower_bound(first, last, key);
-        if (i != last && i->name == name)
+            if (i != last && i->name == key.name)
-            return i;
+                return i;
+            return nullptr;
+        };
+
+        const Bindings * currentChunk = this;
+        while (currentChunk) {
+            const Attr * maybeAttr = getInChunk(*currentChunk);
+            if (maybeAttr)
+                return maybeAttr;
+            currentChunk = currentChunk->baseLayer;
+        }
+
         return nullptr;
     }
 
+    /**
+     * Check if the layer chain is full.
+     */
+    bool isLayerListFull() const noexcept
+    {
+        return numLayers == Bindings::maxLayers;
+    }
+
+    /**
+     * Test if the length of the linked list of layers is greater than 1.
+     */
+    bool isLayered() const noexcept
+    {
+        return numLayers > 1;
+    }
+
     const_iterator begin() const
     {
-        return const_iterator(attrs);
+        return const_iterator(*this);
     }
 
     const_iterator end() const
     {
-        return const_iterator(attrs + size_);
+        return const_iterator();
     }
 
-    Attr & operator[](size_t pos)
+    Attr & operator[](size_type pos)
     {
+        if (isLayered()) [[unlikely]]
+            unreachable();
         return attrs[pos];
     }
 
-    const Attr & operator[](size_t pos) const
+    const Attr & operator[](size_type pos) const
     {
+        if (isLayered()) [[unlikely]]
+            unreachable();
         return attrs[pos];
     }
 
@@ -176,10 +417,9 @@ public:
     std::vector<const Attr *> lexicographicOrder(const SymbolTable & symbols) const
     {
         std::vector<const Attr *> res;
-        res.reserve(size_);
+        res.reserve(size());
-        for (size_t n = 0; n < size_; n++)
+        std::ranges::transform(*this, std::back_inserter(res), [](const Attr & a) { return &a; });
-            res.emplace_back(&attrs[n]);
+        std::ranges::sort(res, [&](const Attr * a, const Attr * b) {
-        std::sort(res.begin(), res.end(), [&](const Attr * a, const Attr * b) {
             std::string_view sa = symbols[a->name], sb = symbols[b->name];
             return sa < sb;
         });
@@ -202,11 +442,11 @@ class BindingsBuilder final
 public:
     // needed by std::back_inserter
     using value_type = Attr;
-    using size_type = Bindings::size_t;
+    using size_type = Bindings::size_type;
 
 private:
     Bindings * bindings;
-    Bindings::size_t capacity_;
+    Bindings::size_type capacity_;
 
     friend class EvalState;
 
@@ -217,6 +457,19 @@ private:
     {
     }
 
+    bool hasBaseLayer() const noexcept
+    {
+        return bindings->baseLayer;
+    }
+
+    void finishSizeIfNecessary()
+    {
+        if (hasBaseLayer())
+            /* NOTE: Do not use std::ranges::distance, since Bindings is a sized
+               range, but we are calculating this size here. */
+            bindings->numAttrsInChain = std::distance(bindings->begin(), bindings->end());
+    }
+
 public:
     std::reference_wrapper<EvalState> state;
 
@@ -232,10 +485,26 @@ public:
 
     void push_back(const Attr & attr)
     {
-        assert(bindings->size() < capacity_);
+        assert(bindings->numAttrs < capacity_);
         bindings->push_back(attr);
     }
 
+    /**
+     * "Layer" the newly constructured Bindings on top of another attribute set.
+     *
+     * This effectively performs an attribute set merge, while giving preference
+     * to attributes from the newly constructed Bindings in case of duplicate attribute
+     * names.
+     *
+     * This operation amortizes the need to copy over all attributes and allows
+     * for efficient implementation of attribute set merges (ExprOpUpdate::eval).
+     */
+    void layerOnTopOf(const Bindings & base) noexcept
+    {
+        bindings->baseLayer = &base;
+        bindings->numLayers = base.numLayers + 1;
+    }
+
     Value & alloc(Symbol name, PosIdx pos = noPos);
 
     Value & alloc(std::string_view name, PosIdx pos = noPos);
@@ -243,11 +512,13 @@ public:
     Bindings * finish()
     {
         bindings->sort();
+        finishSizeIfNecessary();
         return bindings;
     }
 
     Bindings * alreadySorted()
     {
+        finishSizeIfNecessary();
         return bindings;
     }
 

src/libexpr/include/nix/expr/eval-settings.hh

@@ -342,6 +342,25 @@ struct EvalSettings : Config
           This is useful for improving code readability and making path literals
           more explicit.
     )"};
+
+    Setting<unsigned> bindingsUpdateLayerRhsSizeThreshold{
+        this,
+        sizeof(void *) == 4 ? 8192 : 16,
+        "eval-attrset-update-layer-rhs-threshold",
+        R"(
+          Tunes the maximum size of an attribute set that, when used
+          as a right operand in an [attribute set update expression](@docroot@/language/operators.md#update),
+          uses a more space-efficient linked-list representation of attribute sets.
+
+          Setting this to larger values generally leads to less memory allocations,
+          but may lead to worse evaluation performance.
+
+          A value of `0` disables this optimization completely.
+
+          This is an advanced performance tuning option and typically should not be changed.
+          The default value is chosen to balance performance and memory usage. On 32 bit systems
+          where memory is scarce, the default is a large value to reduce the amount of allocations.
+    )"};
 };
 
 /**