Merge pull request #9681 from edolstra/eval-optimisations

Optimize empty list constants

(cherry picked from commit 315aade89d00c692715e5953c36a1b7d6528b703)
Change-Id: I0f28ef8a27ccedc45acf44243eec9dc35b733300

src/libexpr/eval.cc

@@ -527,6 +527,8 @@ EvalState::EvalState(
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 
+    vEmptyList.mkList(0);
+
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
         for (auto & i : _searchPath.elements)
@@ -1405,6 +1407,15 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
 }
 
 
+Value * ExprList::maybeThunk(EvalState & state, Env & env)
+{
+    if (elems.empty()) {
+        return &state.vEmptyList;
+    }
+    return Expr::maybeThunk(state, env);
+}
+
+
 void ExprVar::eval(EvalState & state, Env & env, Value & v)
 {
     Value * v2 = state.lookupVar(&env, *this, false);

src/libexpr/eval.hh

@@ -218,6 +218,11 @@ public:
 
     Bindings emptyBindings;
 
+    /**
+     * Empty list constant.
+     */
+    Value vEmptyList;
+
     const SourcePath derivationInternal;
 
     /**

src/libexpr/nixexpr.hh

@@ -298,6 +298,7 @@ struct ExprList : Expr
     std::vector<Expr *> elems;
     ExprList() { };
     COMMON_METHODS
+    Value * maybeThunk(EvalState & state, Env & env) override;
 
     PosIdx getPos() const override
     {