Compiled C# Lambda Expressions Performance

Question

Consider the following simple manipulation over a collection:

static List<int> x = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; var result = x.Where(i => i % 2 == 0).Where(i => i > 5); 

Now let's use Expressions. The following code is roughly equivalent:

static void UsingLambda() {     Func<IEnumerable<int>, IEnumerable<int>> lambda = l => l.Where(i => i % 2 == 0).Where(i => i > 5);     var t0 = DateTime.Now.Ticks;     for (int j = 1; j < MAX; j++)          var sss = lambda(x).ToList();      var tn = DateTime.Now.Ticks;     Console.WriteLine("Using lambda: {0}", tn - t0); } 

But I want to build the expression on-the-fly, so here's a new test:

static void UsingCompiledExpression() {     var f1 = (Expression<Func<IEnumerable<int>, IEnumerable<int>>>)(l => l.Where(i => i % 2 == 0));     var f2 = (Expression<Func<IEnumerable<int>, IEnumerable<int>>>)(l => l.Where(i => i > 5));     var argX = Expression.Parameter(typeof(IEnumerable<int>), "x");     var f3 = Expression.Invoke(f2, Expression.Invoke(f1, argX));     var f = Expression.Lambda<Func<IEnumerable<int>, IEnumerable<int>>>(f3, argX);      var c3 = f.Compile();      var t0 = DateTime.Now.Ticks;     for (int j = 1; j < MAX; j++)          var sss = c3(x).ToList();      var tn = DateTime.Now.Ticks;     Console.WriteLine("Using lambda compiled: {0}", tn - t0); } 

Of course it isn't exactly like the above, so to be fair, I modify the first one slightly:

static void UsingLambdaCombined() {     Func<IEnumerable<int>, IEnumerable<int>> f1 = l => l.Where(i => i % 2 == 0);     Func<IEnumerable<int>, IEnumerable<int>> f2 = l => l.Where(i => i > 5);     Func<IEnumerable<int>, IEnumerable<int>> lambdaCombined = l => f2(f1(l));     var t0 = DateTime.Now.Ticks;     for (int j = 1; j < MAX; j++)          var sss = lambdaCombined(x).ToList();      var tn = DateTime.Now.Ticks;     Console.WriteLine("Using lambda combined: {0}", tn - t0); } 

Now comes the results for MAX = 100000, VS2008, debugging ON:

Using lambda compiled: 23437500 Using lambda:           1250000 Using lambda combined:  1406250 

And with debugging OFF:

Using lambda compiled: 21718750 Using lambda:            937500 Using lambda combined:  1093750 

Surprise. The compiled expression is roughly 17x slower than the other alternatives. Now here comes the questions:

1. Am I comparing non-equivalent expressions?
2. Is there a mechanism to make .NET "optimize" the compiled expression?
3. How do I express the same chain call l.Where(i => i % 2 == 0).Where(i => i > 5); programatically?

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Some more statistics. Visual Studio 2010, debugging ON, optimizations OFF:

Using lambda:           1093974 Using lambda compiled: 15315636 Using lambda combined:   781410 

Debugging ON, optimizations ON:

Using lambda:            781305 Using lambda compiled: 15469839 Using lambda combined:   468783 

Debugging OFF, optimizations ON:

Using lambda:            625020 Using lambda compiled: 14687970 Using lambda combined:   468765 

New Surprise. Switching from VS2008 (C#3) to VS2010 (C#4), makes the UsingLambdaCombined faster than the native lambda.

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Ok, I've found a way to improve the lambda compiled performance by more than an order of magnitude. Here's a tip; after running the profiler, 92% of the time is spent on:

System.Reflection.Emit.DynamicMethod.CreateDelegate(class System.Type, object) 

Hmmmm... Why is it creating a new delegate in every iteration? I'm not sure, but the solution follows in a separate post.

Answer 1

Could it be that the inner lambdas are not being compiled?!? Here's a proof of concept:

static void UsingCompiledExpressionWithMethodCall() {         var where = typeof(Enumerable).GetMember("Where").First() as System.Reflection.MethodInfo;         where = where.MakeGenericMethod(typeof(int));         var l = Expression.Parameter(typeof(IEnumerable<int>), "l");         var arg0 = Expression.Parameter(typeof(int), "i");         var lambda0 = Expression.Lambda<Func<int, bool>>(             Expression.Equal(Expression.Modulo(arg0, Expression.Constant(2)),                              Expression.Constant(0)), arg0).Compile();         var c1 = Expression.Call(where, l, Expression.Constant(lambda0));         var arg1 = Expression.Parameter(typeof(int), "i");         var lambda1 = Expression.Lambda<Func<int, bool>>(Expression.GreaterThan(arg1, Expression.Constant(5)), arg1).Compile();         var c2 = Expression.Call(where, c1, Expression.Constant(lambda1));          var f = Expression.Lambda<Func<IEnumerable<int>, IEnumerable<int>>>(c2, l);          var c3 = f.Compile();          var t0 = DateTime.Now.Ticks;         for (int j = 1; j < MAX; j++)         {             var sss = c3(x).ToList();         }          var tn = DateTime.Now.Ticks;         Console.WriteLine("Using lambda compiled with MethodCall: {0}", tn - t0);     } 

And now the timings are:

Using lambda:                            625020 Using lambda compiled:                 14687970 Using lambda combined:                   468765 Using lambda compiled with MethodCall:   468765 

Woot! Not only it is fast, it is faster than the native lambda. (Scratch head).

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Of course the above code is simply too painful to write. Let's do some simple magic:

static void UsingCompiledConstantExpressions() {     var f1 = (Func<IEnumerable<int>, IEnumerable<int>>)(l => l.Where(i => i % 2 == 0));     var f2 = (Func<IEnumerable<int>, IEnumerable<int>>)(l => l.Where(i => i > 5));     var argX = Expression.Parameter(typeof(IEnumerable<int>), "x");     var f3 = Expression.Invoke(Expression.Constant(f2), Expression.Invoke(Expression.Constant(f1), argX));     var f = Expression.Lambda<Func<IEnumerable<int>, IEnumerable<int>>>(f3, argX);      var c3 = f.Compile();      var t0 = DateTime.Now.Ticks;     for (int j = 1; j < MAX; j++) {         var sss = c3(x).ToList();     }      var tn = DateTime.Now.Ticks;     Console.WriteLine("Using lambda compiled constant: {0}", tn - t0); } 

And some timings, VS2010, Optimizations ON, Debugging OFF:

Using lambda:                            781260 Using lambda compiled:                 14687970 Using lambda combined:                   468756 Using lambda compiled with MethodCall:   468756 Using lambda compiled constant:          468756 

Now you could argue that I'm not generating the whole expression dynamically; just the chaining invocations. But in the above example I generate the whole expression. And the timings match. This is just a shortcut to write less code.

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From my understanding, what is going on is that the .Compile() method does not propagate the compilations to inner lambdas, and thus the constant invocation of CreateDelegate. But to truly understand this, I would love to have a .NET guru comment a little about the internal stuff going on.

And why, oh why is this now faster than a native lambda!?