Question

I’m trying to build a code sample to show the optimization of code by the compiler when multiplying with a power of 2 number. Yet when I turn Optimize code on the IL remains mainly the same. Any ideas what I’m doing wrong here?

The code:

int nr;
int result;
var stopwatch = new Stopwatch();

nr = 5;

stopwatch.Start();
    result = nr * 4;
stopwatch.Stop();

Console.WriteLine(result);
Console.WriteLine(stopwatch.Elapsed.ToString() + "ms ellapsed");
stopwatch.Reset();

stopwatch.Start();
result = nr << 2;
stopwatch.Stop();

Console.WriteLine(result);
Console.WriteLine(stopwatch.Elapsed.ToString() + "ms ellapsed");

Non Optimized IL:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       130 (0x82)
  .maxstack  2
  .locals init ([0] int32 nr,
           [1] int32 result,
           [2] class [System]System.Diagnostics.Stopwatch stopwatch,
           [3] valuetype [mscorlib]System.TimeSpan CS$0$0000,
           [4] valuetype [mscorlib]System.TimeSpan CS$0$0001)
  IL_0000:  newobj     instance void [System]System.Diagnostics.Stopwatch::.ctor()
  IL_0005:  stloc.2
  IL_0006:  ldc.i4.5
  IL_0007:  stloc.0
  IL_0008:  ldloc.2
  IL_0009:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_000e:  ldloc.0
  IL_000f:  ldc.i4.4
  IL_0010:  mul
  IL_0011:  stloc.1
  IL_0012:  ldloc.2
  IL_0013:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_0018:  ldloc.1
  IL_0019:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_001e:  ldloc.2
  IL_001f:  callvirt   instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
  IL_0024:  stloc.3
  IL_0025:  ldloca.s   CS$0$0000
  IL_0027:  constrained. [mscorlib]System.TimeSpan
  IL_002d:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0032:  ldstr      "ms ellapsed"
  IL_0037:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_003c:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0041:  ldloc.2
  IL_0042:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Reset()
  IL_0047:  ldloc.2
  IL_0048:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_004d:  ldloc.0
  IL_004e:  ldc.i4.2
  IL_004f:  shl
  IL_0050:  stloc.1
  IL_0051:  ldloc.2
  IL_0052:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_0057:  ldloc.1
  IL_0058:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_005d:  ldloc.2
  IL_005e:  callvirt   instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
  IL_0063:  stloc.s    CS$0$0001
  IL_0065:  ldloca.s   CS$0$0001
  IL_0067:  constrained. [mscorlib]System.TimeSpan
  IL_006d:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0072:  ldstr      "ms ellapsed"
  IL_0077:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_007c:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0081:  ret
} // end of method Program::Main

Optimized IL:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       130 (0x82)
  .maxstack  2
  .locals init ([0] int32 nr,
           [1] int32 result,
           [2] class [System]System.Diagnostics.Stopwatch stopwatch,
           [3] valuetype [mscorlib]System.TimeSpan CS$0$0000,
           [4] valuetype [mscorlib]System.TimeSpan CS$0$0001)
  IL_0000:  newobj     instance void [System]System.Diagnostics.Stopwatch::.ctor()
  IL_0005:  stloc.2
  IL_0006:  ldc.i4.5
  IL_0007:  stloc.0
  IL_0008:  ldloc.2
  IL_0009:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_000e:  ldloc.0
  IL_000f:  ldc.i4.4
  IL_0010:  mul
  IL_0011:  stloc.1
  IL_0012:  ldloc.2
  IL_0013:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_0018:  ldloc.1
  IL_0019:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_001e:  ldloc.2
  IL_001f:  callvirt   instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
  IL_0024:  stloc.3
  IL_0025:  ldloca.s   CS$0$0000
  IL_0027:  constrained. [mscorlib]System.TimeSpan
  IL_002d:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0032:  ldstr      "ms ellapsed"
  IL_0037:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_003c:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0041:  ldloc.2
  IL_0042:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Reset()
  IL_0047:  ldloc.2
  IL_0048:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_004d:  ldloc.0
  IL_004e:  ldc.i4.2
  IL_004f:  shl
  IL_0050:  stloc.1
  IL_0051:  ldloc.2
  IL_0052:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_0057:  ldloc.1
  IL_0058:  call       void [mscorlib]System.Console::WriteLine(int32)
  IL_005d:  ldloc.2
  IL_005e:  callvirt   instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
  IL_0063:  stloc.s    CS$0$0001
  IL_0065:  ldloca.s   CS$0$0001
  IL_0067:  constrained. [mscorlib]System.TimeSpan
  IL_006d:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0072:  ldstr      "ms ellapsed"
  IL_0077:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_007c:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0081:  ret
} // end of method Program::Main

I thought the compiler would optimize the mul statement to a shl statement?
My knowledge of IL is very limited (if not non-existing).

Answer 1

This is the code generated by the jitter in the Release build:

0000003e  mov         ecx,14h 

The optimizer is far too smart to generate code for a multiplication when it knows the operand values. If you replace nr = 5; with nr = int.Parse(“5”) so that the jitter cannot know the operand values then it generates this code for the multiplication:

0000005c  lea         ebx,[rdi*4+00000000h] 

Which takes advantage of the multiplier built into the address generation logic on the cpu, allowing the instruction to be overlapped by another instruction that uses the ALU. Which makes the multiplication essentially free. That’s output for the 64-bit jitter, the 32-bit jitter generates this:

0000004d  shl         edi,2 

Which is what you were hoping for. I documented the kind of optimizations performed by the jitter in this post.