Mitigate torn writes for current time on 32-bit platforms (#592)

* torn

* x86 bench

* jit

* rem bench

---------

Author: bitfaster

BitFaster.Caching/Lru/Time.cs

@@ -1,15 +1,53 @@
-namespace BitFaster.Caching.Lru
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading;
+
+namespace BitFaster.Caching.Lru
 {
     /// <summary>
     /// During reads, the policy evaluates ShouldDiscard and Touch. To avoid Getting the current time twice
     /// introduce a simple time class that holds the last time. This is class with a mutable field, because the 
     /// policy structs are readonly.
     /// </summary>
+    /// <remarks>
+    /// This class mitigates torn writes when running on 32-bit systems using Interlocked read and write.
+    /// </remarks>
     internal class Time
     {
+        private static readonly bool Is64Bit = Environment.Is64BitProcess;
+
+        private long time;
+
         /// <summary>
         /// Gets or sets the last time.
         /// </summary>
-        internal long Last { get; set; }
+        internal long Last 
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            { 
+                if (Is64Bit)
+                {
+                    return time;
+                }
+                else
+                {
+                    return Interlocked.Read(ref time);
+                }
+            }
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            set
+            {
+                if (Is64Bit)
+                {
+                    time = value;
+                }
+                else
+                {
+                    Interlocked.CompareExchange(ref time, value, time);
+                }
+            }
+        }
     }
 }