Blog Post · For(;;) a While(true)

For(;;) a While(true) ^

Posted On Saturday 26 February 2011 by Freddie Nash (FredFace)

Incase someone is wondering, for(;;) and while(true) compile identically in CPP and C#. C# is my forte, so I'll demonstrate the intermediate code produced (the same) for each. Consider the following C# Code:

for (;;)
    if (threadBreaker)
while (true)
    if (threadBreaker)

Clearly, it represents two loops which will be broken out of when a variable (bool threadBreaker) is set to true (this is just give something to look at).

After moving each loop to a separate function and compiling (without optimizatins) the IL for the functions is identical, and thus:

  .maxstack  2
  .locals init ([0] bool CS$4$0000)
  IL_0000:  nop
  IL_0001:  br.s       IL_0013
  IL_0003:  nop
  IL_0004:  ldsfld     bool ConsoleTestGround.Program::threadBreaker
  IL_0009:  ldc.i4.0
  IL_000a:  ceq
  IL_000c:  stloc.0
  IL_000d:  ldloc.0
  IL_000e:  brtrue.s   IL_0012
  IL_0010:  br.s       IL_0017
  IL_0012:  nop
  IL_0013:  ldc.i4.1
  IL_0014:  stloc.0
  IL_0015:  br.s       IL_0003
  IL_0017:  ret

A quick skim will show up how very inefficiant this is - the need for a stored local variable alone is uncalled for, but it does show that the said variable is assigned the value true (IL_0013 onwards), but it is never compared to anything.

With oprtimizations, the code size is reduced dramatically, but oddly the maxstack is quadrupled in size. Both functions are again, identical:

  .maxstack  8
  IL_0000:  ldsfld     bool ConsoleTestGround.Program::threadBreaker
  IL_0005:  brfalse.s  IL_0000
  IL_0007:  ret

Well, nothing more to say...