From 17daec0b832205df781f6443292667c9aa617047 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 4 Jan 2022 19:23:11 +0100
Subject: [PATCH] Move empty attrset optimisation

---
 src/libexpr/attr-set.cc | 8 +++-----
 src/libexpr/eval.cc     | 3 +--
 src/libexpr/eval.hh     | 2 +-
 3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/src/libexpr/attr-set.cc b/src/libexpr/attr-set.cc
index d74243f45..fc971957d 100644
--- a/src/libexpr/attr-set.cc
+++ b/src/libexpr/attr-set.cc
@@ -12,6 +12,8 @@ namespace nix {
    structure. */
 Bindings * EvalState::allocBindings(size_t capacity)
 {
+    if (capacity == 0)
+        return &emptyBindings;
     if (capacity > std::numeric_limits<Bindings::size_t>::max())
         throw Error("attribute set of size %d is too big", capacity);
     return new (allocBytes(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings((Bindings::size_t) capacity);
@@ -20,10 +22,6 @@ Bindings * EvalState::allocBindings(size_t capacity)
 
 void EvalState::mkAttrs(Value & v, size_t capacity)
 {
-    if (capacity == 0) {
-        v = vEmptySet;
-        return;
-    }
     v.mkAttrs(allocBindings(capacity));
     nrAttrsets++;
     nrAttrsInAttrsets += capacity;
@@ -63,7 +61,7 @@ Value & BindingsBuilder::alloc(std::string_view name, ptr<Pos> pos)
 
 void Bindings::sort()
 {
-    std::sort(begin(), end());
+    if (size_) std::sort(begin(), end());
 }
 
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index cd7d19747..112690189 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -413,6 +413,7 @@ EvalState::EvalState(
     , sSelf(symbols.create("self"))
     , sEpsilon(symbols.create(""))
     , repair(NoRepair)
+    , emptyBindings(0)
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , regexCache(makeRegexCache())
@@ -454,8 +455,6 @@ EvalState::EvalState(
         }
     }
 
-    vEmptySet.mkAttrs(allocBindings(0));
-
     createBaseEnv();
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index c7f74d7b7..c0d88201a 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -91,7 +91,7 @@ public:
        mode. */
     std::optional<PathSet> allowedPaths;
 
-    Value vEmptySet;
+    Bindings emptyBindings;
 
     /* Store used to materialise .drv files. */
     const ref<Store> store;