Adding more LZC benchmarks

Author: LeeCampbell

HdrHistogram.Benchmarking/LeadingZeroCount/BBarry32BitIfShiftLookupWith64BitShiftBranch.cs

@@ -46,7 +46,6 @@ private static int Log2(int i)
     public static class BBarry32BitIfShiftLookupWith64BitShiftBranch_2
     {
         private static readonly int[] Lookup;
-        private const long IntOverflow = int.MaxValue + 1L;
         static BBarry32BitIfShiftLookupWith64BitShiftBranch_2()
         {
             Lookup = new int[256];
@@ -57,7 +56,7 @@ static BBarry32BitIfShiftLookupWith64BitShiftBranch_2()
         }
         public static int GetLeadingZeroCount(long value)
         {
-            if (value < IntOverflow)
+            if (value <= int.MaxValue)
                 return 63 - Log2((uint)value);
             return 32 - Log2((uint)(value >> 31));
         }
@@ -70,4 +69,41 @@ private static int Log2(uint i)
             return Lookup[i];
         }
     }
+
+    /// <summary>
+    /// Contributed from @BBarry at https://github.com/HdrHistogram/HdrHistogram.NET/issues/42
+    /// This variation perfoms very well. Similar profile to the "Current Impl".
+    /// Faster on LegacyJIT/CLR, but much slower on RyuJIT (CLR & Core)
+    /// This just compares only lessThan operator instead of lessThanEqualTo. #completeness
+    /// </summary>
+    public static class BBarry32BitIfShiftLookupWith64BitShiftBranch_3
+    {
+        private static readonly int[] Lookup;
+        private const long IntOverflow = int.MaxValue + 1L;
+        static BBarry32BitIfShiftLookupWith64BitShiftBranch_3()
+        {
+            Lookup = new int[256];
+            for (int i = 1; i < 256; ++i)
+            {
+                Lookup[i] = (int)(Math.Log(i) / Math.Log(2));
+            }
+        }
+        public static int GetLeadingZeroCount(long value)
+        {
+            if (value < IntOverflow)
+                return 63 - Log2((uint)value);
+            return 32 - Log2((uint)(value >> 31));
+        }
+
+        private const int Bit24Range = 0x1000000 - 1;
+        private const int Bit16Range = 0x10000 - 1;
+        private const int Bit8Range = 0x100 - 1;
+        private static int Log2(uint i)
+        {
+            if (i > Bit24Range) { return Lookup[i >> 24] + 24; }
+            if (i > Bit16Range) { return Lookup[i >> 16] + 16; }
+            if (i > Bit8Range) { return Lookup[i >> 8] + 8; }
+            return Lookup[i];
+        }
+    }
 }

HdrHistogram.Benchmarking/LeadingZeroCount/LeadingZeroCountBenchmarkBase.cs

@@ -56,6 +56,7 @@ public void OneOffValidationOfImplementations()
                 {"DeBruijn128Bits", LeadingZeroCount.DeBruijn128Bits.GetLeadingZeroCount},
                 {"BBarry32BitIfShiftLookupWith64BitShiftBranch", LeadingZeroCount.BBarry32BitIfShiftLookupWith64BitShiftBranch.GetLeadingZeroCount},
                 {"BBarry32BitIfShiftLookupWith64BitShiftBranch_2", LeadingZeroCount.BBarry32BitIfShiftLookupWith64BitShiftBranch_2.GetLeadingZeroCount},
+                {"BBarry32BitIfShiftLookupWith64BitShiftBranch_3", LeadingZeroCount.BBarry32BitIfShiftLookupWith64BitShiftBranch_3.GetLeadingZeroCount},
                 {"BBarryIfShiftLookup", LeadingZeroCount.BBarryIfShiftLookup.GetLeadingZeroCount},
             };
             ValidateImplementations(expectedData, functions);
@@ -190,6 +191,16 @@ public int BBarry_imp1()
             return sum;
         }
         [Benchmark]
+        public int BBarry_imp2()
+        {
+            var sum = 0;
+            for (int i = 0; i < _testValues.Length; i++)
+            {
+                sum += LeadingZeroCount.BBarryIfShiftLookup.GetLeadingZeroCount(_testValues[i]);
+            }
+            return sum;
+        }
+        [Benchmark]
         public int BBarry_imp3()
         {
             var sum = 0;
@@ -200,16 +211,17 @@ public int BBarry_imp3()
             return sum;
         }
         [Benchmark]
-        public int BBarry_imp2()
+        public int BBarry_imp4()
         {
             var sum = 0;
             for (int i = 0; i < _testValues.Length; i++)
             {
-                sum += LeadingZeroCount.BBarryIfShiftLookup.GetLeadingZeroCount(_testValues[i]);
+                sum += LeadingZeroCount.BBarry32BitIfShiftLookupWith64BitShiftBranch_3.GetLeadingZeroCount(_testValues[i]);
             }
             return sum;
         }
 
+
         private class CalculationExpectation
         {
             public long Value { get; }