ripper234 ripper234 - 2 months ago 8
C# Question

Why doesn't .NET/C# optimize for tail-call recursion?

I found this question about which languages optimize tail recursion. Why C# doesn't optimize tail recursion, whenever possible?

For a concrete case, why isn't this method optimized into a loop (Visual Studio 2008 32-bit, if that matters)?:

private static void Foo(int i)
{
if (i == 1000000)
return;

if (i % 100 == 0)
Console.WriteLine(i);

Foo(i+1);
}

Answer

JIT compilation is a tricky balancing act between not spending too much time doing the compilation phase (thus slowing down short lived applications considerably) vs. not doing enough analysis to keep the application competitive in the long term with a standard ahead-of-time compilation.

Interestingly the NGen compilation steps are not targeted to being more aggressive in their optimizations. I suspect this is because they simply don't want to have bugs where the behaviour is dependent on whether the JIT or NGen was responsible for the machine code.

The CLR itself does support tail call optimization, but the language-specific compiler must know how to generate the relevant opcode and the JIT must be willing to respect it. F#'s fsc will generate the relevant opcodes (though for a simple recursion it may just convert the whole thing into a while loop directly). C#'s csc does not.

See this blog post for some details (quite possibly now out of date given recent JIT changes). Note that the CLR changes for 4.0 the x86, x64 and ia64 will respect it.