Optimize HashCode.AddBytes for inputs larger than 16 bytes. (#70095)

* Optimize `HashCode.AddBytes` for inputs larger than 16 bytes.

* Address PR feedback, refactor control flow and inline UnsafeAddMany.

Co-authored-by: Tanner Gooding <tagoo@outlook.com>

* Add more asserts before memory reads and move one inside the main loop.

* Address PR feedback.

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Co-authored-by: Tanner Gooding <tagoo@outlook.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: 3 people

src/libraries/System.Private.CoreLib/src/System/HashCode.cs

@@ -42,6 +42,7 @@ A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
@@ -319,8 +320,62 @@ public void AddBytes(ReadOnlySpan<byte> value)
             ref byte pos = ref MemoryMarshal.GetReference(value);
             ref byte end = ref Unsafe.Add(ref pos, value.Length);
 
+            if (value.Length < (sizeof(int) * 4))
+            {
+                goto Small;
+            }
+
+            // Usually Add calls Initialize but if we haven't used HashCode before it won't have been called.
+            if (_length == 0)
+            {
+                Initialize(out _v1, out _v2, out _v3, out _v4);
+            }
+            else
+            {
+                // If we have at least 16 bytes to hash, we can add them in 16-byte batches,
+                // but we first have to add enough data to flush any queued values.
+                switch (_length % 4)
+                {
+                    case 1:
+                        Debug.Assert(Unsafe.ByteOffset(ref pos, ref end) >= sizeof(int));
+                        Add(Unsafe.ReadUnaligned<int>(ref pos));
+                        pos = ref Unsafe.Add(ref pos, sizeof(int));
+                        goto case 2;
+                    case 2:
+                        Debug.Assert(Unsafe.ByteOffset(ref pos, ref end) >= sizeof(int));
+                        Add(Unsafe.ReadUnaligned<int>(ref pos));
+                        pos = ref Unsafe.Add(ref pos, sizeof(int));
+                        goto case 3;
+                    case 3:
+                        Debug.Assert(Unsafe.ByteOffset(ref pos, ref end) >= sizeof(int));
+                        Add(Unsafe.ReadUnaligned<int>(ref pos));
+                        pos = ref Unsafe.Add(ref pos, sizeof(int));
+                        break;
+                }
+            }
+
+            // With the queue clear, we add sixteen bytes at a time until the input has fewer than sixteen bytes remaining.
+            // We first have to round the end pointer to the nearest 16-byte block from the offset. This makes the loop's condition simpler.
+            ref byte blockEnd = ref Unsafe.Subtract(ref end, Unsafe.ByteOffset(ref pos, ref end) % (sizeof(int) * 4));
+            while (Unsafe.IsAddressLessThan(ref pos, ref blockEnd))
+            {
+                Debug.Assert(Unsafe.ByteOffset(ref pos, ref blockEnd) >= (sizeof(int) * 4));
+                uint v1 = Unsafe.ReadUnaligned<uint>(ref pos);
+                _v1 = Round(_v1, v1);
+                uint v2 = Unsafe.ReadUnaligned<uint>(ref Unsafe.Add(ref pos, sizeof(int) * 1));
+                _v2 = Round(_v2, v2);
+                uint v3 = Unsafe.ReadUnaligned<uint>(ref Unsafe.Add(ref pos, sizeof(int) * 2));
+                _v3 = Round(_v3, v3);
+                uint v4 = Unsafe.ReadUnaligned<uint>(ref Unsafe.Add(ref pos, sizeof(int) * 3));
+                _v4 = Round(_v4, v4);
+
+                _length += 4;
+                pos = ref Unsafe.Add(ref pos, sizeof(int) * 4);
+            }
+
+        Small:
             // Add four bytes at a time until the input has fewer than four bytes remaining.
-            while ((nint)Unsafe.ByteOffset(ref pos, ref end) >= sizeof(int))
+            while (Unsafe.ByteOffset(ref pos, ref end) >= sizeof(int))
             {
                 Add(Unsafe.ReadUnaligned<int>(ref pos));
                 pos = ref Unsafe.Add(ref pos, sizeof(int));