Seek perf extra

See #541

Author: benaadams

src/Microsoft.AspNet.Server.Kestrel/Infrastructure/MemoryPoolIterator2.cs

@@ -177,9 +177,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector)
                 return -1;
             }
 
-            var following = _block.End - _index;
             var block = _block;
             var index = _index;
+            var following = block.End - index;
             byte[] array;
             var byte0 = byte0Vector[0];
 
@@ -224,9 +224,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector)
 #if !DEBUG // Need unit tests to test Vector path
                     }
 #endif
-                    fixed (byte* ptr = block.Array)
+                    fixed (byte* ptr = &block.Array[index])
                     {
-                        var pCurrent = ptr + index;
+                        var pCurrent = ptr;
                         var pEnd = pCurrent + following;
                         do
                         {
@@ -254,9 +254,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector, ref Vector<byte> byte1Vecto
                 return -1;
             }
 
-            var following = _block.End - _index;
             var block = _block;
             var index = _index;
+            var following = block.End - index;
             byte[] array;
             int byte0Index = int.MaxValue;
             int byte1Index = int.MaxValue;
@@ -323,9 +323,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector, ref Vector<byte> byte1Vecto
 #if !DEBUG // Need unit tests to test Vector path
                     }
 #endif
-                    fixed (byte* ptr = block.Array)
+                    fixed (byte* ptr = &block.Array[index])
                     {
-                        var pCurrent = ptr + index;
+                        var pCurrent = ptr;
                         var pEnd = pCurrent + following;
                         do
                         {
@@ -359,9 +359,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector, ref Vector<byte> byte1Vecto
                 return -1;
             }
 
-            var following = _block.End - _index;
             var block = _block;
             var index = _index;
+            var following = block.End - index;
             byte[] array;
             int byte0Index = int.MaxValue;
             int byte1Index = int.MaxValue;
@@ -456,9 +456,9 @@ public unsafe int Seek(ref Vector<byte> byte0Vector, ref Vector<byte> byte1Vecto
 #if !DEBUG // Need unit tests to test Vector path
                     }
 #endif
-                    fixed (byte* ptr = block.Array)
+                    fixed (byte* ptr = &block.Array[index])
                     {
-                        var pCurrent = ptr + index;
+                        var pCurrent = ptr;
                         var pEnd = pCurrent + following;
                         do
                         {