title: Why are operators so much slower than method calls?  (structs are slower only on older JITs)
body: <p><strong>Intro:</strong>  I write high-performance code in C#.  Yes, I know C++ would give me better optimization, but I still choose to use C#.  I do not wish to debate that choice.  Rather, I'd like to hear from those who, like me, are trying to write high-performance code on the .NET Framework.</p>
  
  <p><strong>Questions:</strong></p>
  
  <ul>
  <li>Why is the operator in the code below slower than the equivalent
  method call??</li>
  <li>Why is the method passing two doubles in the code below
  faster than the equivalent method passing a struct that has two
  doubles inside? (A: older JITs optimize structs poorly)</li>
  <li>Is there a way to get the .NET JIT Compiler to treat
  simple structs as efficiently as the members of the struct? (A: get newer JIT)</li>
  </ul>
  
  <p><strong>What I think I know:</strong>
  The original .NET JIT Compiler would not inline anything that involved a struct.  Bizarre given structs should only be used where you need small value types that should be optimized like built-ins, but true.  Fortunately, in .NET 3.5SP1 and .NET 2.0SP2, they made some improvements to the JIT Optimizer, including improvements to inlining, particularly for structs.  (I am guessing they did that because otherwise the new Complex struct that they were introducing would have performed horribly... so the Complex team was probably pounding on the JIT Optimizer team.)  So, any documentation prior to .NET 3.5 SP1 is probably not too relevant to this issue.</p>
  
  <p><strong>What my testing shows:</strong>
  I have verified that I do have the newer JIT Optimizer by checking that C:\Windows\Microsoft.NET\Framework\v2.0.50727\mscorwks.dll file does have version >= 3053 and so should have those improvements to the JIT optimizer.  However, even with that, what my timings and looks at the disassembly both show are:</p>
  
  <p>The JIT-produced code for passing a struct with two doubles is far less efficient than code that directly passes the two doubles.</p>
  
  <p>The JIT-produced code for a struct method passes in 'this' far more efficiently than if you passed a struct as an argument.</p>
  
  <p>The JIT still inlines better if you pass two doubles rather than passing a struct with two doubles, even with the multiplier due to being clearly in a loop.</p>
  
  <p><strong>The Timings:</strong>
  Actually, looking at the disassembly I realize that most of the time in the loops is just accessing the test data out of the List.  The difference between the four ways of making the same calls is dramatically different if you factor out the overhead code of the loop and the accessing of the data.  I get anywhere from 5x to 20x speedups for doing PlusEqual(double, double) instead of PlusEqual(Element).  And 10x to 40x for doing PlusEqual(double, double) instead of operator +=.  Wow.  Sad.</p>
  
  <p>Here's one set of timings:</p>
  
  <pre><code>Populating List&lt;Element&gt; took 320ms.
  The PlusEqual() method took 105ms.
  The 'same' += operator took 131ms.
  The 'same' -= operator took 139ms.
  The PlusEqual(double, double) method took 68ms.
  The do nothing loop took 66ms.
  The ratio of operator with constructor to method is 124%.
  The ratio of operator without constructor to method is 132%.
  The ratio of PlusEqual(double,double) to PlusEqual(Element) is 64%.
  If we remove the overhead time for the loop accessing the elements from the List...
  The ratio of operator with constructor to method is 166%.
  The ratio of operator without constructor to method is 187%.
  The ratio of PlusEqual(double,double) to PlusEqual(Element) is 5%.
  </code></pre>
  
  <p><strong>The Code:</strong></p>
  
  <pre><code>namespace OperatorVsMethod
  {
    public struct Element
    {
      public double Left;
      public double Right;
  
      public Element(double left, double right)
      {
        this.Left = left;
        this.Right = right;
      }
  
      public static Element operator +(Element x, Element y)
      {
        return new Element(x.Left + y.Left, x.Right + y.Right);
      }
  
      public static Element operator -(Element x, Element y)
      {
        x.Left += y.Left;
        x.Right += y.Right;
        return x;
      }    
  
      /// &lt;summary&gt;
      /// Like the += operator; but faster.
      /// &lt;/summary&gt;
      public void PlusEqual(Element that)
      {
        this.Left += that.Left;
        this.Right += that.Right;
      }    
  
      /// &lt;summary&gt;
      /// Like the += operator; but faster.
      /// &lt;/summary&gt;
      public void PlusEqual(double thatLeft, double thatRight)
      {
        this.Left += thatLeft;
        this.Right += thatRight;
      }    
    }    
  
    [TestClass]
    public class UnitTest1
    {
      [TestMethod]
      public void TestMethod1()
      {
        Stopwatch stopwatch = new Stopwatch();
  
        // Populate a List of Elements to multiply together
        int seedSize = 4;
        List&lt;double&gt; doubles = new List&lt;double&gt;(seedSize);
        doubles.Add(2.5d);
        doubles.Add(100000d);
        doubles.Add(-0.5d);
        doubles.Add(-100002d);
  
        int size = 2500000 * seedSize;
        List&lt;Element&gt; elts = new List&lt;Element&gt;(size);
  
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          int di = ii % seedSize;
          double d = doubles[di];
          elts.Add(new Element(d, d));
        }
        stopwatch.Stop();
        long populateMS = stopwatch.ElapsedMilliseconds;
  
        // Measure speed of += operator (calls ctor)
        Element operatorCtorResult = new Element(1d, 1d);
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          operatorCtorResult += elts[ii];
        }
        stopwatch.Stop();
        long operatorCtorMS = stopwatch.ElapsedMilliseconds;
  
        // Measure speed of -= operator (+= without ctor)
        Element operatorNoCtorResult = new Element(1d, 1d);
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          operatorNoCtorResult -= elts[ii];
        }
        stopwatch.Stop();
        long operatorNoCtorMS = stopwatch.ElapsedMilliseconds;
  
        // Measure speed of PlusEqual(Element) method
        Element plusEqualResult = new Element(1d, 1d);
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          plusEqualResult.PlusEqual(elts[ii]);
        }
        stopwatch.Stop();
        long plusEqualMS = stopwatch.ElapsedMilliseconds;
  
        // Measure speed of PlusEqual(double, double) method
        Element plusEqualDDResult = new Element(1d, 1d);
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          Element elt = elts[ii];
          plusEqualDDResult.PlusEqual(elt.Left, elt.Right);
        }
        stopwatch.Stop();
        long plusEqualDDMS = stopwatch.ElapsedMilliseconds;
  
        // Measure speed of doing nothing but accessing the Element
        Element doNothingResult = new Element(1d, 1d);
        stopwatch.Reset();
        stopwatch.Start();
        for (int ii = 0; ii &lt; size; ++ii)
        {
          Element elt = elts[ii];
          double left = elt.Left;
          double right = elt.Right;
        }
        stopwatch.Stop();
        long doNothingMS = stopwatch.ElapsedMilliseconds;
  
        // Report results
        Assert.AreEqual(1d, operatorCtorResult.Left, "The operator += did not compute the right result!");
        Assert.AreEqual(1d, operatorNoCtorResult.Left, "The operator += did not compute the right result!");
        Assert.AreEqual(1d, plusEqualResult.Left, "The operator += did not compute the right result!");
        Assert.AreEqual(1d, plusEqualDDResult.Left, "The operator += did not compute the right result!");
        Assert.AreEqual(1d, doNothingResult.Left, "The operator += did not compute the right result!");
  
        // Report speeds
        Console.WriteLine("Populating List&lt;Element&gt; took {0}ms.", populateMS);
        Console.WriteLine("The PlusEqual() method took {0}ms.", plusEqualMS);
        Console.WriteLine("The 'same' += operator took {0}ms.", operatorCtorMS);
        Console.WriteLine("The 'same' -= operator took {0}ms.", operatorNoCtorMS);
        Console.WriteLine("The PlusEqual(double, double) method took {0}ms.", plusEqualDDMS);
        Console.WriteLine("The do nothing loop took {0}ms.", doNothingMS);
  
        // Compare speeds
        long percentageRatio = 100L * operatorCtorMS / plusEqualMS;
        Console.WriteLine("The ratio of operator with constructor to method is {0}%.", percentageRatio);
        percentageRatio = 100L * operatorNoCtorMS / plusEqualMS;
        Console.WriteLine("The ratio of operator without constructor to method is {0}%.", percentageRatio);
        percentageRatio = 100L * plusEqualDDMS / plusEqualMS;
        Console.WriteLine("The ratio of PlusEqual(double,double) to PlusEqual(Element) is {0}%.", percentageRatio);
  
        operatorCtorMS -= doNothingMS;
        operatorNoCtorMS -= doNothingMS;
        plusEqualMS -= doNothingMS;
        plusEqualDDMS -= doNothingMS;
        Console.WriteLine("If we remove the overhead time for the loop accessing the elements from the List...");
        percentageRatio = 100L * operatorCtorMS / plusEqualMS;
        Console.WriteLine("The ratio of operator with constructor to method is {0}%.", percentageRatio);
        percentageRatio = 100L * operatorNoCtorMS / plusEqualMS;
        Console.WriteLine("The ratio of operator without constructor to method is {0}%.", percentageRatio);
        percentageRatio = 100L * plusEqualDDMS / plusEqualMS;
        Console.WriteLine("The ratio of PlusEqual(double,double) to PlusEqual(Element) is {0}%.", percentageRatio);
      }
    }
  }
  </code></pre>
  
  <p><strong>The IL:</strong> (aka. what some of the above gets compiled into)</p>
  
  <pre><code>public void PlusEqual(Element that)
      {
  00000000 push    ebp 
  00000001 mov     ebp,esp 
  00000003 push    edi 
  00000004 push    esi 
  00000005 push    ebx 
  00000006 sub     esp,30h 
  00000009 xor     eax,eax 
  0000000b mov     dword ptr [ebp-10h],eax 
  0000000e xor     eax,eax 
  00000010 mov     dword ptr [ebp-1Ch],eax 
  00000013 mov     dword ptr [ebp-3Ch],ecx 
  00000016 cmp     dword ptr ds:[04C87B7Ch],0 
  0000001d je     00000024 
  0000001f call    753081B1 
  00000024 nop       
        this.Left += that.Left;
  00000025 mov     eax,dword ptr [ebp-3Ch] 
  00000028 fld     qword ptr [ebp+8] 
  0000002b fadd    qword ptr [eax] 
  0000002d fstp    qword ptr [eax] 
        this.Right += that.Right;
  0000002f mov     eax,dword ptr [ebp-3Ch] 
  00000032 fld     qword ptr [ebp+10h] 
  00000035 fadd    qword ptr [eax+8] 
  00000038 fstp    qword ptr [eax+8] 
      }
  0000003b nop       
  0000003c lea     esp,[ebp-0Ch] 
  0000003f pop     ebx 
  00000040 pop     esi 
  00000041 pop     edi 
  00000042 pop     ebp 
  00000043 ret     10h 
   public void PlusEqual(double thatLeft, double thatRight)
      {
  00000000 push    ebp 
  00000001 mov     ebp,esp 
  00000003 push    edi 
  00000004 push    esi 
  00000005 push    ebx 
  00000006 sub     esp,30h 
  00000009 xor     eax,eax 
  0000000b mov     dword ptr [ebp-10h],eax 
  0000000e xor     eax,eax 
  00000010 mov     dword ptr [ebp-1Ch],eax 
  00000013 mov     dword ptr [ebp-3Ch],ecx 
  00000016 cmp     dword ptr ds:[04C87B7Ch],0 
  0000001d je     00000024 
  0000001f call    75308159 
  00000024 nop       
        this.Left += thatLeft;
  00000025 mov     eax,dword ptr [ebp-3Ch] 
  00000028 fld     qword ptr [ebp+10h] 
  0000002b fadd    qword ptr [eax] 
  0000002d fstp    qword ptr [eax] 
        this.Right += thatRight;
  0000002f mov     eax,dword ptr [ebp-3Ch] 
  00000032 fld     qword ptr [ebp+8] 
  00000035 fadd    qword ptr [eax+8] 
  00000038 fstp    qword ptr [eax+8] 
      }
  0000003b nop       
  0000003c lea     esp,[ebp-0Ch] 
  0000003f pop     ebx 
  00000040 pop     esi 
  00000041 pop     edi 
  00000042 pop     ebp 
  00000043 ret     10h 
  </code></pre>