【发布时间】：2012-01-26 14:55:38
【问题描述】：
编辑：我在最后添加了 ASM。
我相信学习如何在平台上编写优秀代码的最佳方式是对平台进行试验，从而了解它。因此，这个问题是为了更好地理解 CLR，而不是尝试纳米优化。
尽管如此，我还是想到融合设置和评估变量这两个操作会更快。事实证明，确实如此。在下面的代码中，第二个循环的执行时间大约是第一个循环的 60%：
private sealed class Temp
{
public int val;
}
private void button13_Click(object sender, EventArgs e)
{
Temp t = new Temp();
Temp t1;
int T1 = Environment.TickCount;
for (int i = 0; i < 1000000000; i++)
{
t1 = t;
if (t1.val++ == 1000)
{
t1.val = 0;
}
}
int T2 = Environment.TickCount;
for (int i = 0; i < 1000000000; i++)
{
if ((t1 = t).val++ == 1000)
{
t1.val = 0;
}
}
int T3 = Environment.TickCount;
MessageBox.Show((T2 - T1).ToString() + Environment.NewLine +
(T3 - T2).ToString() + Environment.NewLine +
t.val.ToString());
}
在大多数情况下，CIL 编译器会在堆栈上创建设置值的副本，这意味着不需要通常需要的存储和获取。这将解释明显显着的速度增加。
但是，这段特定代码的反编译 C# 和 IL 并没有这样做，而是增加了 开销。然而它的速度几乎是原来的两倍。
EDIT2：我在物理上切换了循环，发现第二个循环总是快两倍。为什么？所以我添加了一个“热身”循环，这导致第一个循环的速度大约是原来的两倍。它基本上是相同的代码（ASM 方面）。幕后发生了什么？
{
Temp t1;
Temp t = new Temp();
int T1 = Environment.TickCount;
for (int i = 0; i < 0x3b9aca00; i++)
{
t1 = t;
if (t1.val++ == 0x3e8)
{
t1.val = 0;
}
}
int T2 = Environment.TickCount;
for (int i = 0; i < 0x3b9aca00; i++)
{
Temp temp1 = t1 = t;
if (temp1.val++ == 0x3e8)
{
t1.val = 0;
}
}
int T3 = Environment.TickCount;
string[] CS$0$0002 = new string[] { (T2 - T1).ToString(), Environment.NewLine, (T3 - T2).ToString(), Environment.NewLine, t.val.ToString() };
MessageBox.Show(string.Concat(CS$0$0002));
}
编辑：在 64 位 .Net 4 发布模式下编译
L_0000: newobj instance void DIRECT_UI.Form1/Temp::.ctor()
L_0005: stloc.0
L_0006: call int32 [mscorlib]System.Environment::get_TickCount()
L_000b: stloc.2
L_000c: ldc.i4.0
L_000d: stloc.3
L_000e: br.s L_0037
L_0010: ldloc.0
L_0011: stloc.1
L_0012: ldloc.1
L_0013: dup
L_0014: ldfld int32 DIRECT_UI.Form1/Temp::val
L_0019: dup
L_001a: stloc.s CS$0$0000
L_001c: ldc.i4.1
L_001d: add
L_001e: stfld int32 DIRECT_UI.Form1/Temp::val
L_0023: ldloc.s CS$0$0000
L_0025: ldc.i4 0x3e8
L_002a: bne.un.s L_0033
L_002c: ldloc.1
L_002d: ldc.i4.0
L_002e: stfld int32 DIRECT_UI.Form1/Temp::val
L_0033: ldloc.3
L_0034: ldc.i4.1
L_0035: add
L_0036: stloc.3
L_0037: ldloc.3
L_0038: ldc.i4 0x3b9aca00
L_003d: blt.s L_0010
L_003f: call int32 [mscorlib]System.Environment::get_TickCount()
L_0044: stloc.s T2
L_0046: ldc.i4.0
L_0047: stloc.s V_5
L_0049: br.s L_0074
L_004b: ldloc.0
L_004c: dup
L_004d: stloc.1
L_004e: dup
L_004f: ldfld int32 DIRECT_UI.Form1/Temp::val
L_0054: dup
L_0055: stloc.s CS$0$0001
L_0057: ldc.i4.1
L_0058: add
L_0059: stfld int32 DIRECT_UI.Form1/Temp::val
L_005e: ldloc.s CS$0$0001
L_0060: ldc.i4 0x3e8
L_0065: bne.un.s L_006e
L_0067: ldloc.1
L_0068: ldc.i4.0
L_0069: stfld int32 DIRECT_UI.Form1/Temp::val
L_006e: ldloc.s V_5
L_0070: ldc.i4.1
L_0071: add
L_0072: stloc.s V_5
L_0074: ldloc.s V_5
L_0076: ldc.i4 0x3b9aca00
L_007b: blt.s L_004b
L_007d: call int32 [mscorlib]System.Environment::get_TickCount()
L_0082: stloc.s T3
L_0084: ldc.i4.5
L_0085: newarr string
L_008a: stloc.s CS$0$0002
L_008c: ldloc.s CS$0$0002
L_008e: ldc.i4.0
L_008f: ldloc.s T2
L_0091: ldloc.2
L_0092: sub
L_0093: stloc.s CS$0$0003
L_0095: ldloca.s CS$0$0003
L_0097: call instance string [mscorlib]System.Int32::ToString()
L_009c: stelem.ref
L_009d: ldloc.s CS$0$0002
L_009f: ldc.i4.1
L_00a0: call string [mscorlib]System.Environment::get_NewLine()
L_00a5: stelem.ref
L_00a6: ldloc.s CS$0$0002
L_00a8: ldc.i4.2
L_00a9: ldloc.s T3
L_00ab: ldloc.s T2
L_00ad: sub
L_00ae: stloc.s CS$0$0004
L_00b0: ldloca.s CS$0$0004
L_00b2: call instance string [mscorlib]System.Int32::ToString()
L_00b7: stelem.ref
L_00b8: ldloc.s CS$0$0002
L_00ba: ldc.i4.3
L_00bb: call string [mscorlib]System.Environment::get_NewLine()
L_00c0: stelem.ref
L_00c1: ldloc.s CS$0$0002
L_00c3: ldc.i4.4
L_00c4: ldloc.0
L_00c5: ldflda int32 DIRECT_UI.Form1/Temp::val
L_00ca: call instance string [mscorlib]System.Int32::ToString()
L_00cf: stelem.ref
L_00d0: ldloc.s CS$0$0002
L_00d2: call string [mscorlib]System.String::Concat(string[])
L_00d7: call valuetype [System.Windows.Forms]System.Windows.Forms.DialogResult [System.Windows.Forms]System.Windows.Forms.MessageBox::Show(string)
L_00dc: pop
L_00dd: ret
这对我来说没有意义。它看起来像反向优化，但运行速度更快。任何人都可以对此有所了解吗？
ASM：
t1 = t;
000000ac mov rax,qword ptr [rsp+20h]
000000b1 mov qword ptr [rsp+28h],rax
if (t1.val++ == 1000)
000000b6 mov rax,qword ptr [rsp+28h]
000000bb mov eax,dword ptr [rax+8]
000000be mov dword ptr [rsp+74h],eax
000000c2 mov eax,dword ptr [rsp+74h]
000000c6 mov dword ptr [rsp+44h],eax
000000ca mov ecx,dword ptr [rsp+74h]
000000ce inc ecx
000000d0 mov rax,qword ptr [rsp+28h]
000000d5 mov dword ptr [rax+8],ecx
000000d8 cmp dword ptr [rsp+44h],3E8h
000000e0 jne 00000000000000EE
if ((t1 = t).val++ == 1000)
0000011d mov rax,qword ptr [rsp+20h]
00000122 mov qword ptr [rsp+28h],rax
00000127 mov rax,qword ptr [rsp+20h]
0000012c mov eax,dword ptr [rax+8]
0000012f mov dword ptr [rsp+7Ch],eax
00000133 mov eax,dword ptr [rsp+7Ch]
00000137 mov dword ptr [rsp+48h],eax
0000013b mov ecx,dword ptr [rsp+7Ch]
0000013f inc ecx
00000141 mov rax,qword ptr [rsp+20h]
00000146 mov dword ptr [rax+8],ecx
00000149 cmp dword ptr [rsp+48h],3E8h
00000151 jne 000000000000015F
【问题讨论】：
-
请指定平台（32/64位）和.NET版本。它们之间的优化差异很大。
-
另外，查看 IL，它似乎是在调试模式下编译的。也请注明，如果 NGEN 也这样做了！
-
查看 IL 只会让您了解性能的一部分。要了解详细信息，您确实需要查看特定平台的 JIT 编译代码。
-
我编辑了这个问题。它是 64 位 .Net 4.0 发布模式。
标签： .net optimization compiler-construction cil micro-optimization