* Store lists as lists of pointers to values rather than as lists of

values.  This improves sharing and gives another speed up.
  Evaluation of the NixOS system attribute is now almost 7 times
  faster than the old evaluator.

src/libexpr/attr-path.cc

@@ -61,7 +61,7 @@ void findAlongAttrPath(EvalState & state, const string & attrPath,
             if (attrIndex >= v.list.length)
                 throw Error(format("list index %1% in selection path `%2%' is out of range") % attrIndex % curPath);
 
-            v = v.list.elems[attrIndex];
+            v = *v.list.elems[attrIndex];
         }
         
     }

src/libexpr/eval-test.cc

@@ -113,6 +113,7 @@ void run(Strings args)
     doTest(state, "with { x = 1; }; let inherit x; y = x; in y");
     doTest(state, "builtins.toXML 123");
     doTest(state, "builtins.toXML { a.b = \"x\" + \"y\"; c = [ 1 2 ] ++ [ 3 4 ]; }");
+    doTest(state, "builtins.attrNames { x = 1; y = 2; }");
 
     state.printStats();
 }

src/libexpr/eval.cc

@@ -50,7 +50,7 @@ std::ostream & operator << (std::ostream & str, Value & v)
     case tList:
         str << "[ ";
         for (unsigned int n = 0; n < v.list.length; ++n)
-            str << v.list.elems[n] << " ";
+            str << *v.list.elems[n] << " ";
         str << "]";
         break;
     case tThunk:
@@ -102,7 +102,7 @@ EvalState::EvalState()
     , baseEnvDispl(0)
     , staticBaseEnv(false, 0)
 {
-    nrEnvs = nrValuesInEnvs = nrValuesInLists = nrValues = 0;
+    nrEnvs = nrValuesInEnvs = nrValues = nrListElems = 0;
     nrEvaluated = recursionDepth = maxRecursionDepth = 0;
     deepestStack = (char *) -1;
 
@@ -251,6 +251,7 @@ Value * EvalState::lookupVar(Env * env, const VarRef & var)
 
 Value * EvalState::allocValues(unsigned int count)
 {
+    nrValues += count;
     return new Value[count]; // !!! check destructor
 }
 
@@ -268,8 +269,8 @@ void EvalState::mkList(Value & v, unsigned int length)
 {
     v.type = tList;
     v.list.length = length;
-    v.list.elems = allocValues(length);
+    v.list.elems = new Value *[length];
-    nrValuesInLists += length;
+    nrListElems += length;
 }
 
 
@@ -461,8 +462,11 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
 void ExprList::eval(EvalState & state, Env & env, Value & v)
 {
     state.mkList(v, elems.size());
-    for (unsigned int n = 0; n < v.list.length; ++n)
+    Value * vs = state.allocValues(v.list.length);
-        mkThunk(v.list.elems[n], env, elems[n]);
+    for (unsigned int n = 0; n < v.list.length; ++n) {
+        v.list.elems[n] = &vs[n];
+        mkThunk(vs[n], env, elems[n]);
+    }
 }
 
 
@@ -543,7 +547,6 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v)
             primOp->primOp.fun(*this, vArgs, v);
         } else {
             Value * v2 = allocValues(2);
-            nrValues += 2;
             v2[0] = fun;
             v2[1] = arg;
             v.type = tPrimOpApp;
@@ -734,8 +737,6 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
     Value v2; state.eval(env, e2, v2);
     state.forceList(v2);
     state.mkList(v, v1.list.length + v2.list.length);
-    /* !!! This loses sharing with the original lists.  We could use a
-       tCopy node, but that would use more memory. */
     for (unsigned int n = 0; n < v1.list.length; ++n)
         v.list.elems[n] = v1.list.elems[n];
     for (unsigned int n = 0; n < v2.list.length; ++n)
@@ -810,7 +811,7 @@ void EvalState::strictForceValue(Value & v)
     
     else if (v.type == tList) {
         for (unsigned int n = 0; n < v.list.length; ++n)
-            strictForceValue(v.list.elems[n]);
+            strictForceValue(*v.list.elems[n]);
     }
 }
 
@@ -951,11 +952,11 @@ string EvalState::coerceToString(Value & v, PathSet & context,
         if (v.type == tList) {
             string result;
             for (unsigned int n = 0; n < v.list.length; ++n) {
-                result += coerceToString(v.list.elems[n],
+                result += coerceToString(*v.list.elems[n],
                     context, coerceMore, copyToStore);
                 if (n < v.list.length - 1
                     /* !!! not quite correct */
-                    && (v.list.elems[n].type != tList || v.list.elems[n].list.length != 0))
+                    && (v.list.elems[n]->type != tList || v.list.elems[n]->list.length != 0))
                     result += " ";
             }
             return result;
@@ -1009,14 +1010,14 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         case tList:
             if (v2.type != tList || v1.list.length != v2.list.length) return false;
             for (unsigned int n = 0; n < v1.list.length; ++n)
-                if (!eqValues(v1.list.elems[n], v2.list.elems[n])) return false;
+                if (!eqValues(*v1.list.elems[n], *v2.list.elems[n])) return false;
             return true;
 
         case tAttrs: {
             if (v2.type != tAttrs || v1.attrs->size() != v2.attrs->size()) return false;
             Bindings::iterator i, j;
             for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
-                if (!eqValues(i->second, j->second)) return false;
+                if (i->first != j->first || !eqValues(i->second, j->second)) return false;
             return true;
         }
 
@@ -1046,8 +1047,8 @@ void EvalState::printStats()
         % nrEnvs % (nrEnvs * sizeof(Env)));
     printMsg(v, format("  values allocated in environments: %1% (%2% bytes)")
         % nrValuesInEnvs % (nrValuesInEnvs * sizeof(Value)));
-    printMsg(v, format("  values allocated in lists: %1% (%2% bytes)")
+    printMsg(v, format("  list elements: %1% (%2% bytes)")
-        % nrValuesInLists % (nrValuesInLists * sizeof(Value)));
+        % nrListElems % (nrListElems * sizeof(Value *)));
     printMsg(v, format("  misc. values allocated: %1% (%2% bytes) ")
         % nrValues % (nrValues * sizeof(Value)));
     printMsg(v, format("  symbols in symbol table: %1%") % symbols.size());

src/libexpr/eval.hh

@@ -76,7 +76,7 @@ struct Value
         Bindings * attrs;
         struct {
             unsigned int length;
-            Value * elems;
+            Value * * elems;
         } list;
         struct {
             Env * env;
@@ -282,8 +282,8 @@ private:
     
     unsigned long nrEnvs;
     unsigned long nrValuesInEnvs;
-    unsigned long nrValuesInLists;
     unsigned long nrValues;
+    unsigned long nrListElems;
     unsigned long nrEvaluated;
     unsigned int recursionDepth;
     unsigned int maxRecursionDepth;

src/libexpr/get-drvs.cc

@@ -48,7 +48,7 @@ MetaInfo DrvInfo::queryMetaInfo(EvalState & state) const
         } else if (i->second.type == tList) {
             value.type = MetaValue::tpStrings;
             for (unsigned int j = 0; j < i->second.list.length; ++j)
-                value.stringValues.push_back(state.forceStringNoCtx(i->second.list.elems[j]));
+                value.stringValues.push_back(state.forceStringNoCtx(*i->second.list.elems[j]));
         } else continue;
         meta[i->first] = value;
     }
@@ -206,8 +206,8 @@ static void getDerivations(EvalState & state, Value & vIn,
             startNest(nest, lvlDebug,
                 format("evaluating list element"));
             string pathPrefix2 = addToPath(pathPrefix, (format("%1%") % n).str());
-            if (getDerivation(state, v.list.elems[n], pathPrefix2, drvs, done))
+            if (getDerivation(state, *v.list.elems[n], pathPrefix2, drvs, done))
-                getDerivations(state, v.list.elems[n], pathPrefix2, autoArgs, drvs, done);
+                getDerivations(state, *v.list.elems[n], pathPrefix2, autoArgs, drvs, done);
         }
     }
 

src/libexpr/primops.cc

@@ -311,7 +311,7 @@ static void prim_derivationStrict(EvalState & state, Value * * args, Value & v)
             if (key == "args") {
                 state.forceList(i->second);
                 for (unsigned int n = 0; n < i->second.list.length; ++n) {
-                    string s = state.coerceToString(i->second.list.elems[n], context, true);
+                    string s = state.coerceToString(*i->second.list.elems[n], context, true);
                     drv.args.push_back(s);
                 }
             }
@@ -651,14 +651,17 @@ static void prim_attrNames(EvalState & state, Value * * args, Value & v)
     state.forceAttrs(*args[0]);
 
     state.mkList(v, args[0]->attrs->size());
+    Value * vs = state.allocValues(v.list.length);
 
     StringSet names;
     foreach (Bindings::iterator, i, *args[0]->attrs)
         names.insert(i->first);
 
     unsigned int n = 0;
-    foreach (StringSet::iterator, i, names)
+    foreach (StringSet::iterator, i, names) {
-        mkString(v.list.elems[n++], *i);
+        v.list.elems[n] = &vs[n];
+        mkString(vs[n++], *i);
+    }
 }
 
 
@@ -701,8 +704,8 @@ static void prim_removeAttrs(EvalState & state, Value * * args, Value & v)
     state.cloneAttrs(*args[0], v);
 
     for (unsigned int i = 0; i < args[1]->list.length; ++i) {
-        state.forceStringNoCtx(args[1]->list.elems[i]);
+        state.forceStringNoCtx(*args[1]->list.elems[i]);
-        v.attrs->erase(state.symbols.create(args[1]->list.elems[i].string.s));
+        v.attrs->erase(state.symbols.create(args[1]->list.elems[i]->string.s));
     }
 }
 
@@ -718,7 +721,7 @@ static void prim_listToAttrs(EvalState & state, Value * * args, Value & v)
     state.mkAttrs(v);
 
     for (unsigned int i = 0; i < args[0]->list.length; ++i) {
-        Value & v2(args[0]->list.elems[i]);
+        Value & v2(*args[0]->list.elems[i]);
         state.forceAttrs(v2);
         
         Bindings::iterator j = v2.attrs->find(state.sName);
@@ -815,8 +818,8 @@ static void prim_head(EvalState & state, Value * * args, Value & v)
     state.forceList(*args[0]);
     if (args[0]->list.length == 0)
         throw Error("`head' called on an empty list");
-    state.forceValue(args[0]->list.elems[0]);
+    state.forceValue(*args[0]->list.elems[0]);
-    v = args[0]->list.elems[0];
+    v = *args[0]->list.elems[0];
 }
 
 
@@ -840,11 +843,13 @@ static void prim_map(EvalState & state, Value * * args, Value & v)
     state.forceList(*args[1]);
 
     state.mkList(v, args[1]->list.length);
+    Value * vs = state.allocValues(v.list.length);
 
     for (unsigned int n = 0; n < v.list.length; ++n) {
-        v.list.elems[n].type = tApp;
+        v.list.elems[n] = &vs[n];
-        v.list.elems[n].app.left = args[0];
+        vs[n].type = tApp;
-        v.list.elems[n].app.right = &args[1]->list.elems[n];
+        vs[n].app.left = args[0];
+        vs[n].app.right = args[1]->list.elems[n];
     }
 }
 

src/libexpr/value-to-xml.cc

@@ -97,7 +97,7 @@ static void printValueAsXML(EvalState & state, bool strict, Value & v,
         case tList: {
             XMLOpenElement _(doc, "list");
             for (unsigned int n = 0; n < v.list.length; ++n)
-                printValueAsXML(state, strict, v.list.elems[n], doc, context, drvsSeen);
+                printValueAsXML(state, strict, *v.list.elems[n], doc, context, drvsSeen);
             break;
         }