c# - 为什么代码行为在发布和 Debug模式下不同？

Question

考虑以下代码:

private static void Main(string[] args)
{
    var ar = new double[]
    {
        100
    };

    FillTo(ref ar, 5);
    Console.WriteLine(string.Join(",", ar.Select(a => a.ToString()).ToArray()));
}

public static void FillTo(ref double[] dd, int N)
{
    if (dd.Length >= N)
        return;

    double[] Old = dd;
    double d = double.NaN;
    if (Old.Length > 0)
        d = Old[0];

    dd = new double[N];

    for (int i = 0; i < Old.Length; i++)
    {
        dd[N - Old.Length + i] = Old[i];
    }
    for (int i = 0; i < N - Old.Length; i++)
        dd[i] = d;
}

Debug模式下的结果是:100,100,100,100,100。但在 Release模式下它是:100,100,100,100,0。

发生了什么事？

它使用 .NET Framework 4.7.1 和 .NET Core 2.0.0 进行了测试。

Answer 1

这似乎是一个 JIT 错误；我测试过:

// ... existing code unchanged
for (int i = 0; i < N - Old.Length; i++)
{
    // Console.WriteLine(i); // <== comment/uncomment this line
    dd[i] = d;
}

并添加 Console.WriteLine(i)修复它。唯一的 IL 变化是:

// ...
L_0040: ldc.i4.0 
L_0041: stloc.3 
L_0042: br.s L_004d
L_0044: ldarg.0 
L_0045: ldind.ref 
L_0046: ldloc.3 
L_0047: ldloc.1 
L_0048: stelem.r8 
L_0049: ldloc.3 
L_004a: ldc.i4.1 
L_004b: add 
L_004c: stloc.3 
L_004d: ldloc.3 
L_004e: ldarg.1 
L_004f: ldloc.0 
L_0050: ldlen 
L_0051: conv.i4 
L_0052: sub 
L_0053: blt.s L_0044
L_0055: ret 

对比

// ...
L_0040: ldc.i4.0 
L_0041: stloc.3 
L_0042: br.s L_0053
L_0044: ldloc.3 
L_0045: call void [System.Console]System.Console::WriteLine(int32)
L_004a: ldarg.0 
L_004b: ldind.ref 
L_004c: ldloc.3 
L_004d: ldloc.1 
L_004e: stelem.r8 
L_004f: ldloc.3 
L_0050: ldc.i4.1 
L_0051: add 
L_0052: stloc.3 
L_0053: ldloc.3 
L_0054: ldarg.1 
L_0055: ldloc.0 
L_0056: ldlen 
L_0057: conv.i4 
L_0058: sub 
L_0059: blt.s L_0044
L_005b: ret 

看起来完全正确(唯一的区别是额外的 ldloc.3 和 call void [System.Console]System.Console::WriteLine(int32) ，以及 br.s 的不同但等效的目标)。

我怀疑它需要 JIT 修复。

环境:

• Environment.Version : 4.0.30319.42000
• <TargetFramework>netcoreapp2.0</TargetFramework>
• VS:15.5.0 预览版 5.0
• dotnet --version : 2.1.1