From 5c9cfa46fc0cdf8ec67e022c0a7086a4dcb639b8 Mon Sep 17 00:00:00 2001
From: Philipp Otterbein <potterbein@blockstream.com>
Date: Sun, 15 Jun 2025 21:59:13 +0200
Subject: [PATCH] libexpr: don't allocate additional set in
 builtins.listToAttrs

---
 src/libexpr/primops.cc | 37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 60f44ca62..7f19871e4 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2961,25 +2961,42 @@ static void prim_listToAttrs(EvalState & state, const PosIdx pos, Value * * args
 {
     state.forceList(*args[0], pos, "while evaluating the argument passed to builtins.listToAttrs");
 
-    auto attrs = state.buildBindings(args[0]->listSize());
+    size_t listSize = args[0]->listSize();
+    auto & bindings = *state.allocBindings(listSize);
 
-    std::set<Symbol> seen;
-
-    for (auto v2 : args[0]->listItems()) {
+    for (const auto & [n, v2] : enumerate(args[0]->listItems())) {
         state.forceAttrs(*v2, pos, "while evaluating an element of the list passed to builtins.listToAttrs");
 
         auto j = state.getAttr(state.sName, v2->attrs(), "in a {name=...; value=...;} pair");
 
         auto name = state.forceStringNoCtx(*j->value, j->pos, "while evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs");
-
         auto sym = state.symbols.create(name);
-        if (seen.insert(sym).second) {
-            auto j2 = state.getAttr(state.sValue, v2->attrs(), "in a {name=...; value=...;} pair");
-            attrs.insert(sym, j2->value, j2->pos);
-        }
+
+        bindings[n] = Attr(sym, (Value *) &v2);
     }
 
-    v.mkAttrs(attrs);
+    std::sort(&bindings[0], &bindings[listSize], [](const Attr & a, const Attr & b) {
+        // a.value < b.value makes the sort stable because `value` points to an array item
+        return a < b || (!(a > b) && a.value < b.value);
+    });
+
+    Symbol prev;
+    for (size_t n = 0; n < listSize; n++) {
+        auto attr = bindings[n];
+        if (prev == attr.name) {
+            continue;
+        }
+        Value * v2 = *(Value * const *) attr.value;
+
+        auto j = state.getAttr(state.sValue, v2->attrs(), "in a {name=...; value=...;} pair");
+        prev = attr.name;
+        bindings.push_back({prev, j->value, j->pos});
+    }
+    // help GC and clear end of allocated array
+    for (size_t n = bindings.size(); n < listSize; n++) {
+        bindings[n] = Attr{};
+    }
+    v.mkAttrs(&bindings);
 }
 
 static RegisterPrimOp primop_listToAttrs({