Make Env smaller

Commit 20866a7031ca823055a221653b77986faa167329 added a ‘withAttrs’
field to Env, which is annoying because it makes every Env structure
bigger and we allocate millions of them.  E.g. NixOS evaluation took
18 MiB more.  So this commit squeezes ‘withAttrs’ into values[0].
Probably should use a union...

2 files changed, 23 insertions, 21 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 904a95d160..f891496a75 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -307,25 +307,26 @@ void mkPath(Value & v, const char * s)
 inline Value * EvalState::lookupVar(Env * env, const VarRef & var, bool noEval)
 {
     for (unsigned int l = var.level; l; --l, env = env->up) ;
-    
-    if (var.fromWith) {
-        while (1) {
-            if (!env->values[0]) {
-                if (noEval) return 0;
-                env->values[0] = allocValue();
-                evalAttrs(*env->up, env->withAttrs, *env->values[0]);
-            }
-            Bindings::iterator j = env->values[0]->attrs->find(var.name);
-            if (j != env->values[0]->attrs->end()) {
-                if (countCalls && j->pos) attrSelects[*j->pos]++;
-                return j->value;
-            }
-            if (env->prevWith == 0)
-                throwEvalError("undefined variable `%1%'", var.name);
-            for (unsigned int l = env->prevWith; l; --l, env = env->up) ;
+
+    if (!var.fromWith) return env->values[var.displ];
+
+    while (1) {
+        if (!env->haveWithAttrs) {
+            if (noEval) return 0;
+            Expr * attrs = (Expr *) env->values[0];
+            env->values[0] = allocValue();
+            evalAttrs(*env->up, attrs, *env->values[0]);
+            env->haveWithAttrs = true;
         }
-    } else
-        return env->values[var.displ];
+        Bindings::iterator j = env->values[0]->attrs->find(var.name);
+        if (j != env->values[0]->attrs->end()) {
+            if (countCalls && j->pos) attrSelects[*j->pos]++;
+            return j->value;
+        }
+        if (!env->prevWith)
+            throwEvalError("undefined variable `%1%'", var.name);
+        for (unsigned int l = env->prevWith; l; --l, env = env->up) ;
+    }
 }
 
 
@@ -829,7 +830,8 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
     Env & env2(state.allocEnv(1));
     env2.up = &env;
     env2.prevWith = prevWith;
-    env2.withAttrs = attrs;
+    env2.haveWithAttrs = false;
+    env2.values[0] = (Value *) attrs;
 
     body->eval(state, env2, v);
 }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index c820b28f49..6f6b2a9009 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -52,8 +52,8 @@ struct PrimOp
 struct Env
 {
     Env * up;
-    unsigned int prevWith; // nr of levels up to next `with' environment
-    Expr * withAttrs;
+    unsigned short prevWith; // nr of levels up to next `with' environment
+    bool haveWithAttrs;
     Value * values[0];
 };