c# - 为什么这里没有发生尾调用优化？

Question

我们正在使用递归来查找因子并收到 StackOverflow 异常。我们读过 the C# compiler on x64 computers performs tail call optimizations :

JIT definitely does tailcals when running optimized code and not debugging.

在我们的程序中运行 dotnet --configuration release 可以做到这一点:

...                      
7214 is a factor of 1234567890
7606 is a factor of 1234567890
10821 is a factor of 1234567890
11409 is a factor of 1234567890                

Process is terminated due to StackOverflowException.

为什么没有发生尾调用优化？

class Program
{
    static void Main(string[] args)
    {
        const long firstCandidate = 1;
        WriteAllFactors(1234567890, firstCandidate);
    }

    private static void WriteAllFactors(long number, long candidate)
    {
        if (number % candidate == 0)
        {
            System.Console.WriteLine($"{candidate} is a factor of {number}");
        }

        candidate = candidate + 1;
        if(candidate > number / 2)
        {
            return;
        }

        WriteAllFactors(number, candidate);
    }
}

Answer 1

VSadov 在他的回复中给出了明确的理由:

Generally JIT emits tail calls when it finds that profitable.

此外，他接着说:

This is a part that is not expressible in C#. Unlike inlining, which can be forced via attributes, tailcalling cannot be currently forced. If one needs to write the code like emitted by EmitMethodCall, he cannot use C#.

所以答案是虽然尾调用肯定可用和使用，但无法预测它们何时会被使用或强制它们在 C# 中使用。