Optimized HashTable.index

(`ops/s`, smaller is better)

`Before (9c5d3f8)`:
```scala
[info] # Run complete. Total time: 00:08:15
[info]
[info] Benchmark                                     (size)  Mode  Cnt      Score      Error  Units
[info] s.c.immutable.VectorMapBenchmark.groupBy          10  avgt   20    645.594 ±    9.435  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy         100  avgt   20   2084.216 ±   37.814  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy        1000  avgt   20  19878.481 ±  262.404  ns/op
[info] s.c.mutable.HashMapBenchmark.get                  10  avgt   20    689.941 ±    5.850  ns/op
[info] s.c.mutable.HashMapBenchmark.get                 100  avgt   20   7357.330 ±   45.956  ns/op
[info] s.c.mutable.HashMapBenchmark.get                1000  avgt   20  95767.200 ± 1550.771  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate      10  avgt   20    509.181 ±    2.683  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate     100  avgt   20   5563.301 ±   32.335  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate    1000  avgt   20  71965.365 ± 1809.738  ns/op
[info] s.c.mutable.HashMapBenchmark.put                  10  avgt   20    247.270 ±    3.972  ns/op
[info] s.c.mutable.HashMapBenchmark.put                 100  avgt   20   5646.185 ±  106.172  ns/op
[info] s.c.mutable.HashMapBenchmark.put                1000  avgt   20  81303.663 ±  954.938  ns/op
```

`Changed modulo to bitwise and in hash calculation (4c729fe)`:
```scala
[info] Benchmark                                     (size)  Mode  Cnt      Score      Error  Units
[info] s.c.immutable.VectorMapBenchmark.groupBy          10  avgt   20    631.291 ±    9.269  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy         100  avgt   20   2077.885 ±   59.737  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy        1000  avgt   20  15458.278 ±  317.347  ns/op
[info] s.c.mutable.HashMapBenchmark.get                  10  avgt   20    678.013 ±    4.453  ns/op
[info] s.c.mutable.HashMapBenchmark.get                 100  avgt   20   7258.522 ±   76.088  ns/op
[info] s.c.mutable.HashMapBenchmark.get                1000  avgt   20  94748.845 ± 1226.120  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate      10  avgt   20    498.042 ±    5.006  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate     100  avgt   20   5243.154 ±  110.372  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate    1000  avgt   20  68194.752 ±  655.436  ns/op
[info] s.c.mutable.HashMapBenchmark.put                  10  avgt   20    257.275 ±    1.411  ns/op
[info] s.c.mutable.HashMapBenchmark.put                 100  avgt   20   5318.532 ±  152.923  ns/op
[info] s.c.mutable.HashMapBenchmark.put                1000  avgt   20  79607.160 ±  651.779  ns/op

```

`Optimized HashTable.index (6cc1504)`:
```scala
[info] Benchmark                                     (size)  Mode  Cnt      Score      Error  Units
[info] s.c.immutable.VectorMapBenchmark.groupBy          10  avgt   20    616.164 ±    4.712  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy         100  avgt   20   2034.447 ±   14.495  ns/op
[info] s.c.immutable.VectorMapBenchmark.groupBy        1000  avgt   20  14712.164 ±  119.983  ns/op
[info] s.c.mutable.HashMapBenchmark.get                  10  avgt   20    679.046 ±    6.872  ns/op
[info] s.c.mutable.HashMapBenchmark.get                 100  avgt   20   7242.097 ±   41.244  ns/op
[info] s.c.mutable.HashMapBenchmark.get                1000  avgt   20  95342.919 ± 1521.328  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate      10  avgt   20    488.034 ±    4.554  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate     100  avgt   20   4883.123 ±   59.268  ns/op
[info] s.c.mutable.HashMapBenchmark.getOrElseUpdate    1000  avgt   20  65174.034 ±  496.759  ns/op
[info] s.c.mutable.HashMapBenchmark.put                  10  avgt   20    267.983 ±    1.797  ns/op
[info] s.c.mutable.HashMapBenchmark.put                 100  avgt   20   5097.351 ±  104.538  ns/op
[info] s.c.mutable.HashMapBenchmark.put                1000  avgt   20  78772.540 ±  543.935  ns/op
```

Summary, i.e. the effect of this PR, according to the benchmarks:
* `groupBy` has a `~35%` speedup
* `get` didn't change
* `getOrElseUpdate` has a `~10%` speedup
* `put` has a `~3%` speedup

Note: caching the `exponent` to a local private field (`Byte` or `Int`) didn't have any performance advantage (only a minor slowdown was measured, possibly because it's accessed via an interface now)
(cherry picked from commit a5014447861a5678c8b595e235019bb8fec098a7)

1 files changed, 7 insertions, 7 deletions

diff --git a/src/library/scala/collection/mutable/HashTable.scala b/src/library/scala/collection/mutable/HashTable.scala
index bb15788bdf..81e42f86b3 100644
--- a/src/library/scala/collection/mutable/HashTable.scala
+++ b/src/library/scala/collection/mutable/HashTable.scala
@@ -360,14 +360,14 @@ trait HashTable[A, Entry >: Null <: HashEntry[A, Entry]] extends HashTable.HashU
 
   protected def elemEquals(key1: A, key2: A): Boolean = (key1 == key2)
 
-  // Note:
-  // we take the most significant bits of the hashcode, not the lower ones
-  // this is of crucial importance when populating the table in parallel
-  protected final def index(hcode: Int) = {
+  /**
+    * Note: we take the most significant bits of the hashcode, not the lower ones
+    * this is of crucial importance when populating the table in parallel
+    */
+  protected final def index(hcode: Int): Int = {
     val ones = table.length - 1
-    val improved = improve(hcode, seedvalue)
-    val shifted = (improved >> (32 - java.lang.Integer.bitCount(ones))) & ones
-    shifted
+    val exponent = Integer.numberOfLeadingZeros(ones)
+    (improve(hcode, seedvalue) >>> exponent) & ones
   }
 
   protected def initWithContents(c: HashTable.Contents[A, Entry]) = {