libexpr: Slim down Bindings to 8 bytes (on 64 bit systems)

Since the only construction and push_back() calls
to Bindings happen through the `BindingsBuilder` [1] we don't
need to keep `capacity` around on the heap anymore. This saves 8 bytes
(because of the member alignment padding)
per one Bindings allocation. This isn't that much, but it does
save significant memory.

This also shows that the Bindings don't necessarily have to
be mutable, which opens up opportunities for doing small bindings
optimization and storing a 1-element Bindings directly in Value.

For the following scenario:

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

(nixpkgs revision: ddcddd7b09a417ca9a88899f4bd43a8edb72308d)

This patch results in reduction of `sets.bytes` 13115104016 -> 12653087640,
which amounts to 462 MB less bytes allocated for Bindings.

[1]: Not actually, `getBuiltins` does mutate bindings, but this is pretty
     inconsequential and doesn't lead to problems.

src/libexpr-c/nix_api_value.cc

@@ -594,7 +594,7 @@ nix_err nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * b
         context->last_err_code = NIX_OK;
     try {
         auto & v = check_value_not_null(value);
-        nix::Symbol s = bb->builder.state.symbols.create(name);
+        nix::Symbol s = bb->builder.state.get().symbols.create(name);
         bb->builder.insert(s, &v);
     }
     NIXC_CATCH_ERRS

src/libexpr-tests/value/print.cc

@@ -61,7 +61,7 @@ TEST_F(ValuePrintingTests, tAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -196,11 +196,11 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builderEmpty(state, state.allocBindings(0));
+    BindingsBuilder builderEmpty = state.buildBindings(0);
     Value vAttrsEmpty;
     vAttrsEmpty.mkAttrs(builderEmpty.finish());
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
     builder.insert(state.symbols.create("nested"), &vAttrsEmpty);
@@ -208,7 +208,7 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
 
-    BindingsBuilder builder2(state, state.allocBindings(10));
+    BindingsBuilder builder2 = state.buildBindings(10);
     builder2.insert(state.symbols.create("one"), &vOne);
     builder2.insert(state.symbols.create("two"), &vTwo);
     builder2.insert(state.symbols.create("nested"), &vAttrs);
@@ -233,14 +233,14 @@ TEST_F(ValuePrintingTests, depthList)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
 
-    BindingsBuilder builder2(state, state.allocBindings(10));
+    BindingsBuilder builder2 = state.buildBindings(10);
     builder2.insert(state.symbols.create("one"), &vOne);
     builder2.insert(state.symbols.create("two"), &vTwo);
     builder2.insert(state.symbols.create("nested"), &vAttrs);
@@ -295,7 +295,7 @@ TEST_F(ValuePrintingTests, attrsTypeFirst)
     Value vApple;
     vApple.mkStringNoCopy("apple");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("type"), &vType);
     builder.insert(state.symbols.create("apple"), &vApple);
 
@@ -374,7 +374,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrs)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -392,7 +392,7 @@ TEST_F(ValuePrintingTests, ansiColorsDerivation)
     Value vDerivation;
     vDerivation.mkStringNoCopy("derivation");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.s.type, &vDerivation);
 
     Value vAttrs;
@@ -437,7 +437,7 @@ TEST_F(ValuePrintingTests, ansiColorsDerivationError)
     Value vDerivation;
     vDerivation.mkStringNoCopy("derivation");
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.s.type, &vDerivation);
     builder.insert(state.s.drvPath, &vError);
 
@@ -553,12 +553,12 @@ TEST_F(ValuePrintingTests, ansiColorsBlackhole)
 
 TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("a"), &vEmpty);
     builder.insert(state.symbols.create("b"), &vEmpty);
 
@@ -570,7 +570,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 
 TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
@@ -586,7 +586,7 @@ TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 
 TEST_F(ValuePrintingTests, listRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    BindingsBuilder emptyBuilder = state.buildBindings(1);
 
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
@@ -609,7 +609,7 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
     Value vTwo;
     vTwo.mkInt(2);
 
-    BindingsBuilder builder(state, state.allocBindings(10));
+    BindingsBuilder builder = state.buildBindings(10);
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
 
@@ -635,8 +635,6 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
 
 TEST_F(ValuePrintingTests, ansiColorsListElided)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
-
     Value vOne;
     vOne.mkInt(1);
 

src/libexpr/attr-set.cc

@@ -16,19 +16,19 @@ Bindings * EvalState::allocBindings(size_t capacity)
         throw Error("attribute set of size %d is too big", capacity);
     nrAttrsets++;
     nrAttrsInAttrsets += capacity;
-    return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings((Bindings::size_t) capacity);
+    return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings();
 }
 
 Value & BindingsBuilder::alloc(Symbol name, PosIdx pos)
 {
-    auto value = state.allocValue();
+    auto value = state.get().allocValue();
     bindings->push_back(Attr(name, value, pos));
     return *value;
 }
 
 Value & BindingsBuilder::alloc(std::string_view name, PosIdx pos)
 {
-    return alloc(state.symbols.create(name), pos);
+    return alloc(state.get().symbols.create(name), pos);
 }
 
 void Bindings::sort()

src/libexpr/eval.cc

@@ -205,7 +205,7 @@ EvalState::EvalState(
     , settings{settings}
     , symbols(StaticEvalSymbols::staticSymbolTable())
     , repair(NoRepair)
-    , emptyBindings(0)
+    , emptyBindings(Bindings())
     , storeFS(makeMountedSourceAccessor({
           {CanonPath::root, makeEmptySourceAccessor()},
           /* In the pure eval case, we can simply require
@@ -1218,7 +1218,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
                 *vOverrides,
                 [&]() { return vOverrides->determinePos(noPos); },
                 "while evaluating the `__overrides` attribute");
-            bindings.grow(state.allocBindings(bindings.capacity() + vOverrides->attrs()->size()));
+            bindings.grow(state.buildBindings(bindings.capacity() + vOverrides->attrs()->size()));
             for (auto & i : *vOverrides->attrs()) {
                 AttrDefs::iterator j = attrs.find(i.name);
                 if (j != attrs.end()) {

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

@@ -5,6 +5,7 @@
 #include "nix/expr/symbol-table.hh"
 
 #include <algorithm>
+#include <functional>
 
 namespace nix {
 
@@ -54,16 +55,14 @@ public:
     PosIdx pos;
 
 private:
-    size_t size_, capacity_;
+    size_t size_ = 0;
     Attr attrs[0];
 
-    Bindings(size_t capacity)
+    Bindings() = default;
-        : size_(0)
+    Bindings(const Bindings &) = delete;
-        , capacity_(capacity)
+    Bindings(Bindings &&) = delete;
-    {
+    Bindings & operator=(const Bindings &) = delete;
-    }
+    Bindings & operator=(Bindings &&) = delete;
-
-    Bindings(const Bindings & bindings) = delete;
 
 public:
     size_t size() const
@@ -82,7 +81,6 @@ public:
 
     void push_back(const Attr & attr)
     {
-        assert(size_ < capacity_);
         attrs[size_++] = attr;
     }
 
@@ -136,11 +134,6 @@ public:
 
     void sort();
 
-    size_t capacity() const
-    {
-        return capacity_;
-    }
-
     /**
      * Returns the attributes in lexicographically sorted order.
      */
@@ -165,22 +158,29 @@ public:
  * order at the end. The only way to consume a BindingsBuilder is to
  * call finish(), which sorts the bindings.
  */
-class BindingsBuilder
+class BindingsBuilder final
 {
-    Bindings * bindings;
-
 public:
     // needed by std::back_inserter
     using value_type = Attr;
+    using size_type = Bindings::size_t;
 
-    EvalState & state;
+private:
+    Bindings * bindings;
+    Bindings::size_t capacity_;
 
-    BindingsBuilder(EvalState & state, Bindings * bindings)
+    friend class EvalState;
+
+    BindingsBuilder(EvalState & state, Bindings * bindings, size_type capacity)
         : bindings(bindings)
+        , capacity_(capacity)
         , state(state)
     {
     }
 
+public:
+    std::reference_wrapper<EvalState> state;
+
     void insert(Symbol name, Value * value, PosIdx pos = noPos)
     {
         insert(Attr(name, value, pos));
@@ -193,6 +193,7 @@ public:
 
     void push_back(const Attr & attr)
     {
+        assert(bindings->size() < capacity_);
         bindings->push_back(attr);
     }
 
@@ -211,16 +212,16 @@ public:
         return bindings;
     }
 
-    size_t capacity()
+    size_t capacity() const noexcept
     {
-        return bindings->capacity();
+        return capacity_;
     }
 
-    void grow(Bindings * newBindings)
+    void grow(BindingsBuilder newBindings)
     {
         for (auto & i : *bindings)
-            newBindings->push_back(i);
+            newBindings.push_back(i);
-        bindings = newBindings;
+        std::swap(*this, newBindings);
     }
 
     friend struct ExprAttrs;

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

@@ -879,7 +879,7 @@ public:
 
     BindingsBuilder buildBindings(size_t capacity)
     {
-        return BindingsBuilder(*this, allocBindings(capacity));
+        return BindingsBuilder(*this, allocBindings(capacity), capacity);
     }
 
     ListBuilder buildList(size_t size)