C# Compiler Optimizations

Question

I\'m wondering if someone can explain to me what exactly the compiler might be doing for me to observe such extreme differences in performance for a simple method.

Answer 1

To look at what the C# compiler does for you, you need to look at the IL. If you want to see how that affects the JITted code, you'll need to look at the native code as described by Scott Chamberlain. Be aware that the JITted code will vary based on processor architecture, CLR version, how the process was launched, and possibly other things.

I would usually start with the IL, and then potentially look at the JITted code.

Comparing the IL using ildasm can be slightly tricky, as it includes a label for each instruction. Here are two versions of your method compiled with and without optimization (using the C# 5 compiler), with extraneous labels (and nop instructions) removed to make them as easy to compare as possible:

Optimized

  .method public hidebysig static uint32 
          CalculateCheckSum(string str) cil managed
  {
    // Code size       46 (0x2e)
    .maxstack  2
    .locals init (char[] V_0,
             uint32 V_1,
             char V_2,
             char[] V_3,
             int32 V_4)
    ldarg.0
    callvirt   instance char[] [mscorlib]System.String::ToCharArray()
    stloc.0
    ldc.i4.0
    stloc.1
    ldloc.0
    stloc.3
    ldc.i4.0
    stloc.s    V_4
    br.s       loopcheck
  loopstart:
    ldloc.3
    ldloc.s    V_4
    ldelem.u2
    stloc.2
    ldloc.1
    ldloc.2
    add
    stloc.1
    ldloc.s    V_4
    ldc.i4.1
    add
    stloc.s    V_4
  loopcheck:
    ldloc.s    V_4
    ldloc.3
    ldlen
    conv.i4
    blt.s      loopstart
    ldloc.1
    ldc.i4     0x100
    rem.un
    ret
  } // end of method Program::CalculateCheckSum

Unoptimized

  .method public hidebysig static uint32 
          CalculateCheckSum(string str) cil managed
  {
    // Code size       63 (0x3f)
    .maxstack  2
    .locals init (char[] V_0,
             uint32 V_1,
             char V_2,
             uint32 V_3,
             char[] V_4,
             int32 V_5,
             bool V_6)
    ldarg.0
    callvirt   instance char[] [mscorlib]System.String::ToCharArray()
    stloc.0
    ldc.i4.0
    stloc.1
    ldloc.0
    stloc.s    V_4
    ldc.i4.0
    stloc.s    V_5
    br.s       loopcheck

  loopstart:
    ldloc.s    V_4
    ldloc.s    V_5
    ldelem.u2
    stloc.2
    ldloc.1
    ldloc.2
    add
    stloc.1
    ldloc.s    V_5
    ldc.i4.1
    add
    stloc.s    V_5
  loopcheck:
    ldloc.s    V_5
    ldloc.s    V_4
    ldlen
    conv.i4
    clt
    stloc.s    V_6
    ldloc.s    V_6
    brtrue.s   loopstart

    ldloc.1
    ldc.i4     0x100
    rem.un
    stloc.3
    br.s       methodend

  methodend:
    ldloc.3
    ret
  }

Points to note:

• The optimized version uses fewer locals. This may allow the JIT to use registers more effectively.
• The optimized version uses blt.s rather than clt followed by brtrue.s when checking whether or not to go round the loop again (this is the reason for one of the extra locals).
• The unoptimized version uses an additional local to store the return value before returning, presumably to make debugging easier.
• The unoptimized version has an unconditional branch just before it returns.
• The optimized version is shorter, but I doubt that it's short enough to be inlined, so I suspect that's irrelevant.