Don't allocate closure in List.hashCode.

Provided specialized version of hashing algorithm
for List that does not allocate the closure.

This saves us 124k closures allocated in a compiler
when compiling `Vector.scala` from std library.
That's aroudn 5% of all closures allocated for that
compiler run.

1 files changed, 19 insertions, 1 deletions

diff --git a/src/library/scala/util/hashing/MurmurHash3.scala b/src/library/scala/util/hashing/MurmurHash3.scala
index 3efd5b5e72..1cfb8276fe 100644
--- a/src/library/scala/util/hashing/MurmurHash3.scala
+++ b/src/library/scala/util/hashing/MurmurHash3.scala
@@ -157,6 +157,20 @@ private[hashing] class MurmurHash3 {
     // Finalization
     finalizeHash(h, data.length)
   }
+
+  final def listHash(xs: collection.immutable.List[_], seed: Int): Int = {
+    var n = 0
+    var h = seed
+    var elems = xs
+    while (!elems.isEmpty) {
+      val head = elems.head
+      val tail = elems.tail
+      h = mix(h, head.##)
+      n += 1
+      elems = tail
+    }
+    finalizeHash(h, n)
+  }
 }
 
 /**
@@ -199,7 +213,11 @@ object MurmurHash3 extends MurmurHash3 {
 
   /** To offer some potential for optimization.
    */
-  def seqHash(xs: collection.Seq[_]): Int    = orderedHash(xs, seqSeed)
+  def seqHash(xs: collection.Seq[_]): Int    = xs match {
+    case xs: List[_] => listHash(xs, seqSeed)
+    case xs => orderedHash(xs, seqSeed)
+  }
+
   def mapHash(xs: collection.Map[_, _]): Int = unorderedHash(xs, mapSeed)
   def setHash(xs: collection.Set[_]): Int    = unorderedHash(xs, setSeed)