feat(3p/nix): Statically pass bindings capacity where possible

To aid in making the decision of where to (currently just statically)
use a vector or btree as the backing implementation, add an extra
constructor argument to Bindings::NewGC for a capacity, and use
a (currently hardcoded at 32, for no good reason other than it felt like
a reasonable number) pivot to switch between our possible backing
implementations. Then, update all the call sites where it feels
reasonable that we know the capacity statically to *pass* that capacity
to the constructor.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I1858c161301a1cd0e83aeeb9a58839378869e71d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1124
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/attr-set.cc

@@ -11,6 +11,8 @@
 
 namespace nix {
 
+constexpr size_t ATTRS_CAPACITY_PIVOT = 32;
+
 BindingsIterator& BindingsIterator::operator++() {
   std::visit(util::overloaded{
                  [](AttributeMap::iterator& iter) { ++iter; },
@@ -132,27 +134,13 @@ void BTreeBindings::merge(Bindings& other) {
   }
 }
 
-void EvalState::mkAttrs(Value& v, size_t capacity) {
-  clearValue(v);
-  v.type = tAttrs;
-  v.attrs = Bindings::NewGC();
-  assert(v.attrs->begin() == v.attrs->begin());
-  assert(v.attrs->end() == v.attrs->end());
-  nrAttrsets++;
-  nrAttrsInAttrsets += capacity;
-}
-
-/* Create a new attribute named 'name' on an existing attribute set stored
-   in 'vAttrs' and return the newly allocated Value which is associated with
-   this attribute. */
-Value* EvalState::allocAttr(Value& vAttrs, const Symbol& name) {
-  Value* v = allocValue();
-  vAttrs.attrs->push_back(Attr(name, v));
-  return v;
-}
-
 class VectorBindings : public Bindings {
  public:
+  VectorBindings() {};
+  VectorBindings(size_t capacity) : attributes_() {
+    attributes_.reserve(capacity);
+  };
+
   size_t size() override;
   bool empty() override;
   void push_back(const Attr& attr) override;
@@ -246,8 +234,32 @@ Bindings::iterator VectorBindings::end() {
   return BindingsIterator{attributes_.end()};
 }
 
-// TODO pick what to do based on size
+Bindings* Bindings::NewGC(size_t capacity) {
-Bindings* Bindings::NewGC() { return new (GC) BTreeBindings; }
+  if (capacity > ATTRS_CAPACITY_PIVOT) {
-// Bindings* Bindings::NewGC() { return new (GC) VectorBindings; }
+    return new (GC) BTreeBindings;
+  } else {
+    return new (GC) VectorBindings(capacity);
+  }
+}
+
+void EvalState::mkAttrs(Value& v, size_t capacity) {
+  clearValue(v);
+  v.type = tAttrs;
+  v.attrs = Bindings::NewGC(capacity);
+  assert(v.attrs->begin() == v.attrs->begin());
+  assert(v.attrs->end() == v.attrs->end());
+  nrAttrsets++;
+  nrAttrsInAttrsets += capacity;
+}
+
+/* Create a new attribute named 'name' on an existing attribute set stored
+   in 'vAttrs' and return the newly allocated Value which is associated with
+   this attribute. */
+Value* EvalState::allocAttr(Value& vAttrs, const Symbol& name) {
+  Value* v = allocValue();
+  vAttrs.attrs->push_back(Attr(name, v));
+  return v;
+}
+
 
 }  // namespace nix

third_party/nix/src/libexpr/attr-set.hh

@@ -67,13 +67,9 @@ class Bindings {
  public:
   typedef BindingsIterator iterator;
 
-  // Allocate a new attribute set that is visible to the garbage
-  // collector.
-  static Bindings* NewGC();
-
   // Allocate a new attribute set with a static capacity that is visible to the
   // garbage collector.
-  // static Bindings* NewGC(size_t capacity);
+  static Bindings* NewGC(size_t capacity = 0);
 
   // Return the number of contained elements.
   virtual size_t size() = 0;

third_party/nix/src/libexpr/common-eval-args.cc

@@ -33,7 +33,7 @@ MixEvalArgs::MixEvalArgs() {
 }
 
 Bindings* MixEvalArgs::getAutoArgs(EvalState& state) {
-  Bindings* res = Bindings::NewGC();
+  Bindings* res = Bindings::NewGC(autoArgs.size());
   for (auto& i : autoArgs) {
     Value* v = state.allocValue();
     if (i.second[0] == 'E') {

third_party/nix/src/libexpr/eval.cc

@@ -1240,9 +1240,9 @@ void ExprOpUpdate::eval(EvalState& state, Env& env, Value& dest) {
 
   state.nrOpUpdates++;
 
-  state.mkAttrs(dest, /* capacity = */ 0);
+  state.mkAttrs(dest, v1.attrs->size() + v2.attrs->size());
 
-  /* Merge the sets, preferring values from the second set. */
+  // Merge the sets, preferring values from the second set.
   dest.attrs->merge(*v1.attrs);
   dest.attrs->merge(*v2.attrs);
 }

third_party/nix/src/libexpr/get-drvs.cc

@@ -294,7 +294,7 @@ bool DrvInfo::queryMetaBool(const std::string& name, bool def) {
 void DrvInfo::setMeta(const std::string& name, Value* v) {
   getMeta();
   Bindings* old = meta;
-  meta = Bindings::NewGC();
+  meta = Bindings::NewGC(old->size());
   Symbol sym = state->symbols.Create(name);
   if (old != nullptr) {
     for (auto i : *old) {