c# - IF 是否比 IF-ELSE 表现更好？

Question

这些代码块中哪一个执行得更好，哪一个更具可读性？我猜 yield 可以忽略不计，尤其是在第二个街区。我只是好奇。

区 block #1

string height;
string width;
if (myFlag == 1)
{
    height = "60%";
    width = "60%";
}
else
{
    height = "80%";
    width = "80%";
}

block #2

string height = "80%";
string width = "80%";
if (myFlag == 1)
{
    height = "60%";
    width = "60%";
}

---

已更新

我测试上面代码的结果是两个 block 执行相同

区 block #1

myFlag = 1:   3 Milliseconds
myFlag = 0:   3 Milliseconds

block #2

myFlag = 1:   3 Milliseconds
myFlag = 0:   3 Milliseconds

但我在这里注意到的一件重要事情(感谢 Matthew Steeples answer here )是因为我测试的代码块没有使用变量高度和宽度，除了 if-else 和 if 代码块 block 中的赋值-1 和 2，编译器通过完全删除有问题的 if 和 if-else block 优化了 IL 代码，从而在此处显示我们测试的无效结果。

我已经更新了两个代码块以将高度和宽度的值写入文件，从而再次使用它们并强制编译器运行我们的测试 block (我希望如此)，但您可以从代码中观察到实际的文件写入部分不会影响我们的测试结果

这是更新后的结果，C# 和 IL 代码

结果

区 block #1

myFlag = 1:   1688 Milliseconds
myFlag = 0:   1664 Milliseconds

block #2

myFlag = 1:   1700 Milliseconds
myFlag = 0:   1677 Milliseconds

C#.net代码

区 block #1

    public long WithIfAndElse(int myFlag)
    {
        Stopwatch myTimer = new Stopwatch();
        string someString = "";
        myTimer.Start();
        for (int i = 0; i < 1000000; i++)
        {
            string height;
            string width;
            if (myFlag == 1)
            {
                height = "60%";
                width = "60%";
            }
            else
            {
                height = "80%";
                width = "80%";
            }
            someString = "Height: " + height + Environment.NewLine + "Width: " + width;
        }
        myTimer.Stop();
        File.WriteAllText("testifelse.txt", someString);
        return myTimer.ElapsedMilliseconds;
    }

block #2

    public long WithOnlyIf(int myFlag)
    {
         Stopwatch myTimer = new Stopwatch();
        string someString = "";
        myTimer.Start();
        for (int i = 0; i < 1000000; i++)
        {
            string height = "80%";
            string width = "80%";
            if (myFlag == 1)
            {
                height = "60%";
                width = "60%";
            }
            someString = "Height: " + height + Environment.NewLine + "Width: " + width;
        }
        myTimer.Stop();
        File.WriteAllText("testif.txt", someString);
        return myTimer.ElapsedMilliseconds;
    }

ildasm.exe 生成的 IL 代码

区 block #1

.method public hidebysig instance int64  WithIfAndElse(int32 myFlag) cil managed
{
  // Code size       144 (0x90)
  .maxstack  3
  .locals init ([0] class [System]System.Diagnostics.Stopwatch myTimer,
           [1] string someString,
           [2] int32 i,
           [3] string height,
           [4] string width,
           [5] string[] CS$0$0000)
  IL_0000:  newobj     instance void [System]System.Diagnostics.Stopwatch::.ctor()
  IL_0005:  stloc.0
  IL_0006:  ldstr      ""
  IL_000b:  stloc.1
  IL_000c:  ldloc.0
  IL_000d:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_0012:  ldc.i4.0
  IL_0013:  stloc.2
  IL_0014:  br.s       IL_0070
  IL_0016:  ldarg.1
  IL_0017:  ldc.i4.1
  IL_0018:  bne.un.s   IL_0029
  IL_001a:  ldstr      "60%"
  IL_001f:  stloc.3
  IL_0020:  ldstr      "60%"
  IL_0025:  stloc.s    width
  IL_0027:  br.s       IL_0036
  IL_0029:  ldstr      "80%"
  IL_002e:  stloc.3
  IL_002f:  ldstr      "80%"
  IL_0034:  stloc.s    width
  IL_0036:  ldc.i4.5
  IL_0037:  newarr     [mscorlib]System.String
  IL_003c:  stloc.s    CS$0$0000
  IL_003e:  ldloc.s    CS$0$0000
  IL_0040:  ldc.i4.0
  IL_0041:  ldstr      "Height: "
  IL_0046:  stelem.ref
  IL_0047:  ldloc.s    CS$0$0000
  IL_0049:  ldc.i4.1
  IL_004a:  ldloc.3
  IL_004b:  stelem.ref
  IL_004c:  ldloc.s    CS$0$0000
  IL_004e:  ldc.i4.2
  IL_004f:  call       string [mscorlib]System.Environment::get_NewLine()
  IL_0054:  stelem.ref
  IL_0055:  ldloc.s    CS$0$0000
  IL_0057:  ldc.i4.3
  IL_0058:  ldstr      "Width: "
  IL_005d:  stelem.ref
  IL_005e:  ldloc.s    CS$0$0000
  IL_0060:  ldc.i4.4
  IL_0061:  ldloc.s    width
  IL_0063:  stelem.ref
  IL_0064:  ldloc.s    CS$0$0000
  IL_0066:  call       string [mscorlib]System.String::Concat(string[])
  IL_006b:  stloc.1
  IL_006c:  ldloc.2
  IL_006d:  ldc.i4.1
  IL_006e:  add
  IL_006f:  stloc.2
  IL_0070:  ldloc.2
  IL_0071:  ldc.i4     0xf4240
  IL_0076:  blt.s      IL_0016
  IL_0078:  ldloc.0
  IL_0079:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_007e:  ldstr      "testifelse.txt"
  IL_0083:  ldloc.1
  IL_0084:  call       void [mscorlib]System.IO.File::WriteAllText(string,
                                                                   string)
  IL_0089:  ldloc.0
  IL_008a:  callvirt   instance int64 [System]System.Diagnostics.Stopwatch::get_ElapsedMilliseconds()
  IL_008f:  ret
} // end of method frmResearch::WithIfAndElse

block #2

.method public hidebysig instance int64  WithOnlyIf(int32 myFlag) cil managed
{
  // Code size       142 (0x8e)
  .maxstack  3
  .locals init ([0] class [System]System.Diagnostics.Stopwatch myTimer,
           [1] string someString,
           [2] int32 i,
           [3] string height,
           [4] string width,
           [5] string[] CS$0$0000)
  IL_0000:  newobj     instance void [System]System.Diagnostics.Stopwatch::.ctor()
  IL_0005:  stloc.0
  IL_0006:  ldstr      ""
  IL_000b:  stloc.1
  IL_000c:  ldloc.0
  IL_000d:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Start()
  IL_0012:  ldc.i4.0
  IL_0013:  stloc.2
  IL_0014:  br.s       IL_006e
  IL_0016:  ldstr      "80%"
  IL_001b:  stloc.3
  IL_001c:  ldstr      "80%"
  IL_0021:  stloc.s    width
  IL_0023:  ldarg.1
  IL_0024:  ldc.i4.1
  IL_0025:  bne.un.s   IL_0034
  IL_0027:  ldstr      "60%"
  IL_002c:  stloc.3
  IL_002d:  ldstr      "60%"
  IL_0032:  stloc.s    width
  IL_0034:  ldc.i4.5
  IL_0035:  newarr     [mscorlib]System.String
  IL_003a:  stloc.s    CS$0$0000
  IL_003c:  ldloc.s    CS$0$0000
  IL_003e:  ldc.i4.0
  IL_003f:  ldstr      "Height: "
  IL_0044:  stelem.ref
  IL_0045:  ldloc.s    CS$0$0000
  IL_0047:  ldc.i4.1
  IL_0048:  ldloc.3
  IL_0049:  stelem.ref
  IL_004a:  ldloc.s    CS$0$0000
  IL_004c:  ldc.i4.2
  IL_004d:  call       string [mscorlib]System.Environment::get_NewLine()
  IL_0052:  stelem.ref
  IL_0053:  ldloc.s    CS$0$0000
  IL_0055:  ldc.i4.3
  IL_0056:  ldstr      "Width: "
  IL_005b:  stelem.ref
  IL_005c:  ldloc.s    CS$0$0000
  IL_005e:  ldc.i4.4
  IL_005f:  ldloc.s    width
  IL_0061:  stelem.ref
  IL_0062:  ldloc.s    CS$0$0000
  IL_0064:  call       string [mscorlib]System.String::Concat(string[])
  IL_0069:  stloc.1
  IL_006a:  ldloc.2
  IL_006b:  ldc.i4.1
  IL_006c:  add
  IL_006d:  stloc.2
  IL_006e:  ldloc.2
  IL_006f:  ldc.i4     0xf4240
  IL_0074:  blt.s      IL_0016
  IL_0076:  ldloc.0
  IL_0077:  callvirt   instance void [System]System.Diagnostics.Stopwatch::Stop()
  IL_007c:  ldstr      "testif.txt"
  IL_0081:  ldloc.1
  IL_0082:  call       void [mscorlib]System.IO.File::WriteAllText(string,
                                                                   string)
  IL_0087:  ldloc.0
  IL_0088:  callvirt   instance int64 [System]System.Diagnostics.Stopwatch::get_ElapsedMilliseconds()
  IL_008d:  ret
} // end of method frmResearch::WithOnlyIf

因此我们可以说 IF-Else block ( block #1)比 if block ( block #2)运行得更快，正如本论坛中许多人指出的那样。

Answer 1

测试结果

block 1 的 10,000,000 次迭代

myFlag = 0:    23.8ns per iteration
myFlag = 1:    23.8ns per iteration

block 2 的 10,000,000 次迭代

myFlag = 0:    23.8ns per iteration
myFlag = 1:    46.8ns per iteration

Block 2 比 Block 1 慢 96%。这是有道理的，因为 Block 2 在悲观情况下做了两倍的工作。

i prefer either case, depending on the situation. If myFlag is rarely ever 1, then it want it to stand out as the edge case that we have to handle. If both are equally likely, i want the if-else syntax. But that's preference, not fact.

---

几十年前，如果执行条件跳转，intel 80286 双流水线就会停止，而不是继续执行下一条指令。到远去的奔腾时代； CPU 预取两个分支路径。但在我的脑海深处，每当我编写的代码在 else 子句中具有最常见的结果时，我仍然会感到恐惧。每次我都必须提醒自己，这已经不重要了。

---

Int32 reps = 10000000;

private void Block1(int myFlag)
{
    String width;
    String height;

    Stopwatch sw = new Stopwatch();
    sw.Start();
    for (int i = 0; i < reps; i++)
    {
        if (myFlag == 1)
        {
            width = String.Format("{0:g}%", 60);
            height = String.Format("{0:g}%", 60);
        }
        else
        {
            width = String.Format("{0:g}%", 80);
            height = String.Format("{0:g}%", 80);
        }
    }
    sw.Stop();
    Double time = (Double)sw.Elapsed.Ticks / Stopwatch.Frequency * 1000000000.0 / reps;
    MessageBox.Show(time.ToString() + " ns");
}

private void Block2(int myFlag)
{
    String width;
    String height;

    Stopwatch sw = new Stopwatch();
    sw.Start();
    for (int i = 0; i < reps; i++)
    {
        width = String.Format("{0:g}%", 80);
        height = String.Format("{0:g}%", 80);
        if (myFlag == 1)
        {
            width = String.Format("{0:g}%", 60);
            height = String.Format("{0:g}%", 60);
        }
    }
    sw.Stop();

    Double time = (Double)sw.Elapsed.Ticks / Stopwatch.Frequency * 1000000000.0 / reps;
    MessageBox.Show(time.ToString() + " ns");
}

• String.Format 使 IF 慢 96%
• GetPercentageString(0.60) 使 IF 慢 96%

---

const
   reps = 10000000;

procedure Block1(myflag: Integer);
var
   width, height: string;
   i: Integer;
   t1, t2: Int64;
   time: Extended;
   freq: Int64;
begin
   QueryPerformanceCounter(t1);
   for i := 1 to reps do
   begin
      if myFlag = 1 then
      begin
         width := '60%';
         height := '60%';
      end
      else
      begin
         width := '80%';
         height := '80%';
      end;
   end;
   QueryPerformanceCounter(t2);
   QueryPerformanceFrequency(freq);

   time := (t2-t1) / freq * 1000000000 / reps;
   ShowMessage(FloatToStr(time)+ 'ns');
end;

procedure Block2(myflag: Integer);
var
   width, height: string;
   i: Integer;
   t1, t2: Int64;
   time: Extended;
   freq: Int64;
begin
   QueryPerformanceCounter(t1);
   for i := 1 to reps do
   begin
      width := '80%';
      height := '80%';
      if myFlag = 1 then
      begin
         width := '60%';
         height := '60%';
      end;
   end;
   QueryPerformanceCounter(t2);
   QueryPerformanceFrequency(freq);

   time := (t2-t1) / freq * 1000000000 / reps;
   ShowMessage(FloatToStr(time)+ 'ns');
end;

---

做两倍的工作量大约需要两倍的时间。

答案:IF 的性能并不比 IF-ELSE 好。

---