性能是否会提升"GOTO"声明存在的"真正"目的？

Question

今天我打开了一个微软的.Net正则表达式实现,它让我觉得这可能是gotoC#语言中存在语句的唯一原因(它主要强调'开发者'概念的清晰度,其中goto似乎完全没有位置)也可能是微软使用CLR库实现的唯一原因 - 性能提升.我还记得在System.Web.UI.dll中微软的ASP.NET页面/控件渲染中看到类似的优化.这是一个有效的(或记录在案的？)假设吗？你见过它使用微软下,除了提高代码执行时间任何其他情况？谢谢.

以下是微软自己的正则表达式实现中的摘录(不仅仅是在细节上进行检查),它们再次广泛使用goto来实现性能提升:

public override void Go() 
{
    int num4;
    int num5;
    string runtext = base.runtext;
    int runtextstart = base.runtextstart;
    int runtextbeg = base.runtextbeg;
    int runtextend = base.runtextend;
    int runtextpos = base.runtextpos;
    int[] runtrack = base.runtrack;
    int runtrackpos = base.runtrackpos;
    int[] runstack = base.runstack;
    int runstackpos = base.runstackpos;
    runtrack[--runtrackpos] = runtextpos;
    runtrack[--runtrackpos] = 0;
    runstack[--runstackpos] = runtextpos;
    runtrack[--runtrackpos] = 1;
    if ((((runtextpos != base.runtextstart) || 
       (4 > (runtextend - runtextpos))) || 
       ((runtext[runtextpos] != '<') || 
       (runtext[runtextpos + 1] != '%'))) || 
       ((runtext[runtextpos + 2] != '-') || 
       (runtext[runtextpos + 3] != '-')))
    {
        goto Label_02F8;
    }
    runtextpos += 4;
    runstack[--runstackpos] = -1;
    runtrack[--runtrackpos] = 1;
    goto Label_0213;
Label_0161:
    if (num5 > num4)
    {
        runtrack[--runtrackpos] = (num5 - num4) - 1;
        runtrack[--runtrackpos] = runtextpos - 1;
        runtrack[--runtrackpos] = 2;
    }
Label_0194:
    num4 = runstack[runstackpos++];
    this.Capture(2, num4, runtextpos);
    runtrack[--runtrackpos] = num4;
    runtrack[--runtrackpos] = 3;
    if (runtextpos >= runtextend)
    {
        goto Label_02F8;
    }
    runtextpos++;
    if (runtext[runtextpos] != '-')
    {
        goto Label_02F8;
    }
    num4 = runstack[runstackpos++];
    this.Capture(1, num4, runtextpos);
    runtrack[--runtrackpos] = num4;
    runtrack[--runtrackpos] = 3;
Label_0213:
    if (num4 != -1)
    {
        runtrack[--runtrackpos] = num4;
    }
    else
    {
        runtrack[--runtrackpos] = runtextpos;
    }
    if ((num4 = runstack[runstackpos++]) != runtextpos)
    {
        runtrack[--runtrackpos] = runtextpos;
        runtrack[--runtrackpos] = 4;
    }
    else
    {
        runstack[--runstackpos] = num4;
        runtrack[--runtrackpos] = 5;
    }
    if (((3 > (runtextend - runtextpos)) || 
    (runtext[runtextpos] != '-')) || 
    ((runtext[runtextpos + 1] != '%') || 
    (runtext[runtextpos + 2] != '>')))
    {
        goto Label_02F8;
    }
    runtextpos += 3;
    num4 = runstack[runstackpos++];
    this.Capture(0, num4, runtextpos);
    runtrack[--runtrackpos] = num4;
    runtrack[--runtrackpos] = 3;
Label_02EF:
    base.runtextpos = runtextpos;
    return;
Label_02F8:
    base.runtrackpos = runtrackpos;
    base.runstackpos = runstackpos;
    this.EnsureStorage();
    runtrackpos = base.runtrackpos;
    runstackpos = base.runstackpos;
    runtrack = base.runtrack;
    runstack = base.runstack;
    switch (runtrack[runtrackpos++])
    {
        case 1:
            runstackpos++;
            goto Label_02F8;

        case 2:
            runtextpos = runtrack[runtrackpos++];
            num4 = runtrack[runtrackpos++];
            if (num4 > 0)
            {
                runtrack[--runtrackpos] = num4 - 1;
                runtrack[--runtrackpos] = runtextpos - 1;
                runtrack[--runtrackpos] = 2;
            }
            goto Label_0194;

        case 3:
            runstack[--runstackpos] = runtrack[runtrackpos++];
            this.Uncapture();
            goto Label_02F8;

        case 4:
            runtextpos = runtrack[runtrackpos++];
            runstack[--runstackpos] = runtextpos;
            runtrack[--runtrackpos] = 5;
            if ((runtrackpos > 40) && (runstackpos > 30))
            {
                runstack[--runstackpos] = runtextpos;
                runtrack[--runtrackpos] = 1;
                runstack[--runstackpos] = runtextpos;
                runtrack[--runtrackpos] = 1;
                num4 = (num5 = runtextend - runtextpos) + 1;
                do
                {
                    if (--num4 <= 0)
                    {
                        goto Label_0161;
                    }
                    runtextpos++;
                }
                while (runtext[runtextpos] != '-');
                runtextpos--;
                goto Label_0161;
            }
            runtrack[--runtrackpos] = 6;
            goto Label_02F8;

        case 5:
            runstack[runstackpos] = runtrack[runtrackpos++];
            goto Label_02F8;
    }
    runtextpos = runtrack[runtrackpos++];
    goto Label_02EF;
}

Answer 1

我看到的一个用例是自动生成的代码.高级控制流构造对我们人类来说很容易,但不一定方便作为代码生成算法的输出.

你的代码是否被反编译？在这种情况下,它可能只是在不使用gotos的情况下以反编译器无法处理的方式编写.在IL级别上,所有控制流构造都被转换为gotos,反编译器试图猜测它们是什么.在复杂的情况下,它可能无法找到使用高级构造表示它的好方法,因此可以使用高级构造goto.

另一个用例是状态机.您有许多状态,在每个状态之后,它可以使用goto语句转换到新状态.解析器是状态机的常见应用.