unroll

Author: Alex Peck

BitFaster.Caching/BitFaster.Caching.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;netcoreapp3.1;net6.0</TargetFrameworks>
-    <LangVersion>10.0</LangVersion>
+    <LangVersion>11.0</LangVersion>
     <Authors>Alex Peck</Authors>
     <Company />
     <Product>BitFaster.Caching</Product>

BitFaster.Caching/Lfu/CmSketchCore.cs

@@ -186,24 +186,31 @@ private unsafe int EstimateFrequencyStd(T value)
 
         private unsafe void IncrementStd(T value)
         {
-            var index = stackalloc int[8];
             int blockHash = Spread(comparer.GetHashCode(value));
             int counterHash = Rehash(blockHash);
             int block = (blockHash & blockMask) << 3;
 
-            for (int i = 0; i < 4; i++)
-            {
-                int h = (int)((uint)counterHash >> (i << 3));
-                index[i] = (h >> 1) & 15;
-                int offset = h & 1;
-                index[i + 4] = block + offset + (i << 1);
-            }
+            // Loop unrolling improves throughput by 10m ops/s
+            int h0 = counterHash;
+            int h1 = counterHash >>> 8;
+            int h2 = counterHash >>> 16;
+            int h3 = counterHash >>> 24;
+
+            int index0 = (h0 >>> 1) & 15;
+            int index1 = (h1 >>> 1) & 15;
+            int index2 = (h2 >>> 1) & 15;
+            int index3 = (h3 >>> 1) & 15;
+
+            int slot0 = block + (h0 & 1);
+            int slot1 = block + (h1 & 1) + 2;
+            int slot2 = block + (h2 & 1) + 4;
+            int slot3 = block + (h3 & 1) + 6;
 
             bool added =
-                  IncrementAt(index[4], index[0])
-                | IncrementAt(index[5], index[1])
-                | IncrementAt(index[6], index[2])
-                | IncrementAt(index[7], index[3]);
+                  IncrementAt(slot0, index0)
+                | IncrementAt(slot1, index1)
+                | IncrementAt(slot2, index2)
+                | IncrementAt(slot3, index3);
 
             if (added && (++size == sampleSize))
             {