Question

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Why doesn't .net/C# eliminate tail recursion?

Take the following C# code:

using System;

namespace TailTest
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            Counter(0);
        }

        static void Counter(int i)
        {
            Console.WriteLine(i);
            if (i < int.MaxValue) Counter(++i);
        }
    }
}

The C# compiler (mine anyway) will compile the Counter method into the following CIL:

.method private static hidebysig default void Counter (int32 i) cil managed 
{
.maxstack 8
IL_0000:  ldarg.0 
IL_0001:  call void class [mscorlib]System.Console::WriteLine(int32)
IL_0006:  ldarg.0 
IL_0007:  ldc.i4 2147483647
IL_000c:  bge IL_0019
IL_0011:  ldarg.0 
IL_0012:  ldc.i4.1 
IL_0013:  add 
IL_0014:  call void class TailTest.MainClass::Counter(int32)
IL_0019:  ret 
}

The problem with the above code is that it will cause a stack overflow (at about i=262000 on my hardware). To get around this problem, some languages do what is called tail-call elimination or tail-call optimization (TCO). Essentially, they turn the recursive call into a loop.

My understanding is the the 64-bit implementation of the .NET 4 JIT now performs TCO and avoids the overflow on code like the above CIL. However, the 32-bit JIT does not. Mono does not seem to either. It is interesting that the JIT (which is under time and resource pressure) does TCO while the C# compiler does not. My question is why isn’t the C# compiler itself more TCO aware?

There is a CIL instruction that tells the JIT to perform TCO. For example, the C# compiler could instead generate the following CIL:

.method private static hidebysig default void Counter (int32 i) cil managed 
{
.maxstack 8
IL_0000:  ldarg.0 
IL_0001:  call void class [mscorlib]System.Console::WriteLine(int32)
IL_0006:  ldarg.0 
IL_0007:  ldc.i4 2147483647
IL_000c:  bge IL_001c
IL_0011:  ldarg.0 
IL_0012:  ldc.i4.1 
IL_0013:  add 
IL_0014:  tail.
IL_0017:  call void class TailTest.MainClass::Counter(int32)
IL_001c:  ret 
}

Unlike the original, this code will not overflow and will run to completion even on the 32-bit JIT (both .NET and Mono). The magic is in the tail. CIL instruction. Compilers like F# will generate CIL that includes this instruction automatically.

So my question is, is there a technical reason that the C# compiler does not do this?

I get that it has historically maybe just not been worth it. Code like Counter() has not been common in idiomatic C# and/or the .NET framework. You could easily view TCO for C# as an unnecessary or premature optimization.

With the introduction of LINQ and other things, it seems like both C# and C# developers are moving in more functional directions. So, it would be nice if using recursion was not such an unsafe thing to do. But my question is really a more technical one.

Am missing something that makes TCO like this a bad idea (or a risky one) for C#. Or is there something that makes it particularly tricky to get right? That is really what I am hoping to understand. Any insight?

EDIT: Thanks for the great information. I just wanted to be clear that I am not criticizing the lack of or even demanding this feature. I am not super interested in the rational around prioritization. My curiosity is what obstacles might I not perceive or understand that make this a difficult, dangerous, or undesirable thing to do.

Perhaps a different context will help focus the conversation…

Let’s say I was going to implement my own C#-like language on the CLR. Why would I not (other than opportunity cost) include automatic and transparent emission of the ‘tail.’ instruction where appropriate? What challenges would I encounter or what constraints would I introduce in supporting this feature in a language very much like C#.

Thank you again (and in advance) for the responses.

Answer 1

check the following links

Why doesn't .NET/C# optimize for tail-call recursion?
/491463#491463
http://social.msdn.microsoft.com/Forums/en-US/netfxtoolsdev/thread/67b6d908-8811-430f-bc84-0081f4393336?StatusCode=1
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=166013&wa=wsignin1.0

The following statement is MS official (Luke Hoban Visual C# Compiler Program Manager) and copied from last link

Thanks for the suggestion. We’ve considered emiting tail call instructions at a number
of points in the development of the C# compiler. However, there are some subtle issues
which have pushed us to avoid this so far: 1) There is actually a non-trivial overhead
cost to using the .tail instruction in the CLR (it is not just a jump instruction as
tail calls ultimately become in many less strict environments such as functional
language runtime environments where tail calls are heavily optimized). 2) There are
few real C# methods where it would be legal to emit tail calls (other languages
encourage coding patterns which have more tail recursion, and many that rely heavily
on tail call optimization actually do global re-writing (such as Continuation Passing
transformations) to increase the amount of tail recursion). 3) Partly because of 2),
cases where C# methods stack overflow due to deep recursion that should have succeeded
are fairly rare.

All that said, we continue to look at this, and we may in a future release of the
compiler find some patterns where it makes sense to emit .tail instructions.