From 6d6200f37afe10e8da3b08582a926245538af5d6 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Mon, 2 Aug 2010 16:31:05 +0000
Subject: [PATCH] * Optimisation in the // operator: if one of the sets is
 empty, return   the other set.

---
 src/libexpr/eval.cc | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index eb1d8d432..96bda43a3 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -750,13 +750,15 @@ void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
 
 void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 {
-    Value v2;
-    state.evalAttrs(env, e1, v2);
-        
-    state.cloneAttrs(v2, v);
-        
+    Value v1, v2;
+    state.evalAttrs(env, e1, v1);
     state.evalAttrs(env, e2, v2);
-    
+
+    if (v1.attrs->size() == 0) { v = v2; return; }
+    if (v2.attrs->size() == 0) { v = v1; return; }
+
+    state.cloneAttrs(v1, v);
+
     foreach (Bindings::iterator, i, *v2.attrs) {
         Attr & a = (*v.attrs)[i->first];
         mkCopy(a.value, i->second.value);