Use a pooled array to create hashes from.

src/Workspaces/Core/Portable/Workspace/Solution/Checksum_Factory.cs

@@ -14,31 +14,53 @@ namespace Microsoft.CodeAnalysis
     // all these are just helper methods
     internal partial class Checksum
     {
-        private static readonly Stack<SHA1> s_sha1Stack = new Stack<SHA1>();
+        private static readonly Stack<IncrementalHash> s_hashStack = new Stack<IncrementalHash>();
         private static readonly object s_gate = new object();
 
         public static Checksum Create(Stream stream)
         {
-            SHA1 sha1;
+            IncrementalHash hash;
             lock (s_gate)
             {
-                sha1 = s_sha1Stack.Count > 0
-                    ? s_sha1Stack.Pop()
-                    : SHA1.Create();
+                hash = s_hashStack.Count > 0
+                    ? s_hashStack.Pop()
+                    : IncrementalHash.CreateHash(HashAlgorithmName.SHA1);
             }
 
-            stream.Seek(0, SeekOrigin.Begin);
-
-            var checksum = new Checksum(sha1.ComputeHash(stream));
+            var checksum = ComputeChecksum(stream, hash);
 
             lock (s_gate)
             {
-                s_sha1Stack.Push(sha1);
+                s_hashStack.Push(hash);
             }
 
             return checksum;
         }
 
+        private static Checksum ComputeChecksum(Stream stream, IncrementalHash hash)
+        {
+            using (var pooledBuffer = SharedPools.ByteArray.GetPooledObject())
+            {
+                stream.Seek(0, SeekOrigin.Begin);
+
+                var buffer = pooledBuffer.Object;
+                var bufferLength = buffer.Length;
+                int bytesRead;
+                do
+                {
+                    bytesRead = stream.Read(buffer, 0, bufferLength);
+                    if (bytesRead > 0)
+                    {
+                        hash.AppendData(buffer, 0, bytesRead);
+                    }
+                }
+                while (bytesRead > 0);
+
+                var bytes = hash.GetHashAndReset();
+                return new Checksum(bytes);
+            }
+        }
+
         public static Checksum Create(string kind, IObjectWritable @object)
         {
             using (var stream = SerializableBytes.CreateWritableStream())