c# – Help me fix the error

Question:

There is an object that inherits from IEnumerable . In MoveNext method of this class, a StackOverflowException occurs on the line with Regex . The regular expression itself looks for text matches in files. Remarkably, everything works on Windows 10 x64 with .Net 4.6, the error occurs on Windows 7 x64 with .Net 4.0.

I have 2 guesses why this is happening:

  1. First: perhaps due to differences in the implementation of the Regex method in .Net 4.0 and .Net 4.6, Regex takes more memory on the stack and therefore crashes with an exception.

  2. Second: maybe the stack size in Win10 is different from that in Win7.

How to check current stack size? How to check available stack size? And what else could be the reason for this error?

The number of files in the IEnumerable to be searched is 2760 ( _objEnumerator.Count ), each file is loaded in advance and stored as a string in this very IEnumerable . Below is an example code:

private class MyEnumerator : IEnumerable
{
    public bool MoveNext()
    {
        if (_objEnumerator == null)
        {
            _objEnumerator = _objects.GetEnumerator();
        }

        Match m;

        if (_current == null)
        {
            if (!_objEnumerator.MoveNext())
                return false;

            m = _regex.Match((_objEnumerator.Current).Text); // (_objEnumerator.Current).Text хранит  текст файла, ошибка падает в этой строчке
        }           

        if (m.Success)
        {
           // код выдающий результат
           return true;
        }
        else
        {
           _current = null;
           return MoveNext();
        }
    }
}

Answer:

The problem, I think, is precisely because of the recursion (depth 2700, this is a lot). In .NET, starting with, it seems, 4.5, they switched to a new JIT compiler that can detect tail recursion, and make a tailcall call instead of a normal recursive call. As a result, the stack is not clogged.

Since the presence of tail recursion optimization is not guaranteed by the language (and does not seem to work on 32-bit targets so far), the recursive implementation is a bug in the code. Rewrite your function iteratively.

Clarification: I tried code similar to yours and it doesn't generate tail call in IL code in VS 2015/.NET 4.5/x64/Release. So that may not be your problem. I'll try to investigate the cause further.

Further investigation: In the current version of .NET (4.5), tail recursion does not require the tail IL prefix. Example: here is the code

class Program
{
    [MethodImpl(MethodImplOptions.NoOptimization)]
    static void Main(string[] args)
    {
        var t = new Program();
        t.f(1);
        Console.ReadKey();        // здесь можно приаттачить отладчик
        t.f(100000000);
    }

    int f(int iterNo)
    {
        new DateTime(2017, 2, 3); // увеличим размер кода функции, чтобы сделать
                                  // хвостовую оптимизацию привлекательной для JIT
        if (iterNo == 0)
            return 0;
        else
            return f(iterNo - 1);
    }
}

generates the following IL, without the .tail prefix:

  .method private hidebysig instance int32 
          f(int32 iterNo) cil managed
  {
    // Code size       28 (0x1c)
    .maxstack  8
//000021: 
//000022:      int f(int iterNo)
//000023:      {
//000024:          new DateTime(2017, 2, 3); // увеличим размер кода функции, чтобы сделать
    IL_0000:  ldc.i4     0x7e1
    IL_0005:  ldc.i4.2
    IL_0006:  ldc.i4.3
    IL_0007:  newobj     instance void [mscorlib]System.DateTime::.ctor(int32,
                                                                        int32,
                                                                        int32)
    IL_000c:  pop
//000025:                                    // хвостовую оптимизацию привлекательной для JIT
//000026:          if (iterNo == 0)
    IL_000d:  ldarg.1
    IL_000e:  brtrue.s   IL_0012

//000027:              return 0;
    IL_0010:  ldc.i4.0
    IL_0011:  ret

//000028:          else
//000029:              return f(iterNo - 1);
    IL_0012:  ldarg.0
    IL_0013:  ldarg.1
    IL_0014:  ldc.i4.1
    IL_0015:  sub
    IL_0016:  call       instance int32 Tailcall.Program::f(int32)
    IL_001b:  ret
  } // end of method Program::f

However, the native code for f looks like this:

   24:             new DateTime(2017, 2, 3); // увеличим размер кода функции, чтобы сделать
push        rdi  
push        rsi  
sub         rsp,28h  
mov         rdi,rcx  
mov         esi,edx  
mov         ecx,7E1h  
mov         edx,2  
mov         r8d,3  
call        000007FEF1361730  
   25:                                       // хвостовую оптимизацию привлекательной для JIT
   26:             if (iterNo == 0)
test        esi,esi  
jne         000007FE932A053D  
   27:                 return 0;
xor         eax,eax  
add         rsp,28h  
pop         rsi  
pop         rdi  
ret  
lea         edx,[rsi-1]                  // вычли 1
mov         rcx,rdi
mov         rax,7FE932A0080h  
add         rsp,28h                      // очистили фрейм
pop         rsi  
pop         rdi  
jmp         rax                          // переход вместо возврата

This means that tail recursion can be applied at the JIT level , without the involvement of an IL compiler. So the fact that .NET 4.5 has a new JIT compiler might well help.

In any case, the correct solution to the problem is to rewrite the code in an iterative way.

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