I'm trying to build a code sample显示编译器在乘以 2 的幂时对代码的优化。然而，当我在 IL 上打开优化代码时，主要保持不变。知道我在这里做错了什么吗？
代码:
int nr;
int result;
var stopwatch = new Stopwatch();
nr = 5;
stopwatch.Start();
result = nr * 4;
stopwatch.Stop();
Console.WriteLine(result);
Console.WriteLine(stopwatch.Elapsed.ToString() + "ms ellapsed");
stopwatch.Reset();
stopwatch.Start();
result = nr << 2;
stopwatch.Stop();
Console.WriteLine(result);
Console.WriteLine(stopwatch.Elapsed.ToString() + "ms ellapsed");
非优化 IL:
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 130 (0x82)
.maxstack 2
.locals init ([0] int32 nr,
[1] int32 result,
[2] class [System]System.Diagnostics.Stopwatch stopwatch,
[3] valuetype [mscorlib]System.TimeSpan CS$0$0000,
[4] valuetype [mscorlib]System.TimeSpan CS$0$0001)
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.2
IL_0006: ldc.i4.5
IL_0007: stloc.0
IL_0008: ldloc.2
IL_0009: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_000e: ldloc.0
IL_000f: ldc.i4.4
IL_0010: mul
IL_0011: stloc.1
IL_0012: ldloc.2
IL_0013: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_0018: ldloc.1
IL_0019: call void [mscorlib]System.Console::WriteLine(int32)
IL_001e: ldloc.2
IL_001f: callvirt instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
IL_0024: stloc.3
IL_0025: ldloca.s CS$0$0000
IL_0027: constrained. [mscorlib]System.TimeSpan
IL_002d: callvirt instance string [mscorlib]System.Object::ToString()
IL_0032: ldstr "ms ellapsed"
IL_0037: call string [mscorlib]System.String::Concat(string,
string)
IL_003c: call void [mscorlib]System.Console::WriteLine(string)
IL_0041: ldloc.2
IL_0042: callvirt instance void [System]System.Diagnostics.Stopwatch::Reset()
IL_0047: ldloc.2
IL_0048: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_004d: ldloc.0
IL_004e: ldc.i4.2
IL_004f: shl
IL_0050: stloc.1
IL_0051: ldloc.2
IL_0052: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_0057: ldloc.1
IL_0058: call void [mscorlib]System.Console::WriteLine(int32)
IL_005d: ldloc.2
IL_005e: callvirt instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
IL_0063: stloc.s CS$0$0001
IL_0065: ldloca.s CS$0$0001
IL_0067: constrained. [mscorlib]System.TimeSpan
IL_006d: callvirt instance string [mscorlib]System.Object::ToString()
IL_0072: ldstr "ms ellapsed"
IL_0077: call string [mscorlib]System.String::Concat(string,
string)
IL_007c: call void [mscorlib]System.Console::WriteLine(string)
IL_0081: ret
} // end of method Program::Main
优化的 IL:
.method private hidebysig static void Main(string[] args) cil managed
{
.entrypoint
// Code size 130 (0x82)
.maxstack 2
.locals init ([0] int32 nr,
[1] int32 result,
[2] class [System]System.Diagnostics.Stopwatch stopwatch,
[3] valuetype [mscorlib]System.TimeSpan CS$0$0000,
[4] valuetype [mscorlib]System.TimeSpan CS$0$0001)
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.2
IL_0006: ldc.i4.5
IL_0007: stloc.0
IL_0008: ldloc.2
IL_0009: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_000e: ldloc.0
IL_000f: ldc.i4.4
IL_0010: mul
IL_0011: stloc.1
IL_0012: ldloc.2
IL_0013: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_0018: ldloc.1
IL_0019: call void [mscorlib]System.Console::WriteLine(int32)
IL_001e: ldloc.2
IL_001f: callvirt instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
IL_0024: stloc.3
IL_0025: ldloca.s CS$0$0000
IL_0027: constrained. [mscorlib]System.TimeSpan
IL_002d: callvirt instance string [mscorlib]System.Object::ToString()
IL_0032: ldstr "ms ellapsed"
IL_0037: call string [mscorlib]System.String::Concat(string,
string)
IL_003c: call void [mscorlib]System.Console::WriteLine(string)
IL_0041: ldloc.2
IL_0042: callvirt instance void [System]System.Diagnostics.Stopwatch::Reset()
IL_0047: ldloc.2
IL_0048: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_004d: ldloc.0
IL_004e: ldc.i4.2
IL_004f: shl
IL_0050: stloc.1
IL_0051: ldloc.2
IL_0052: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_0057: ldloc.1
IL_0058: call void [mscorlib]System.Console::WriteLine(int32)
IL_005d: ldloc.2
IL_005e: callvirt instance valuetype [mscorlib]System.TimeSpan [System]System.Diagnostics.Stopwatch::get_Elapsed()
IL_0063: stloc.s CS$0$0001
IL_0065: ldloca.s CS$0$0001
IL_0067: constrained. [mscorlib]System.TimeSpan
IL_006d: callvirt instance string [mscorlib]System.Object::ToString()
IL_0072: ldstr "ms ellapsed"
IL_0077: call string [mscorlib]System.String::Concat(string,
string)
IL_007c: call void [mscorlib]System.Console::WriteLine(string)
IL_0081: ret
} // end of method Program::Main
我以为编译器会将 mul 语句优化为 shl 语句？
我对 IL 的了解非常有限(如果不是不存在的话)。
最佳答案
这是发布版本中抖动产生的代码:
0000003e mov ecx,14h
当优化器知道操作数值时，它太聪明了，无法为乘法生成代码。如果替换 nr = 5;使用 nr = int.Parse("5") 以便抖动无法知道操作数值，然后它会为乘法生成此代码:
0000005c lea ebx,[rdi*4+00000000h]
它利用了 cpu 上地址生成逻辑中内置的乘法器，允许指令与使用 ALU 的另一条指令重叠。这使得乘法基本上是免费的。这是 64 位抖动的输出，32 位抖动生成这个:
0000004d shl edi,2
这就是您所希望的。我在 this post 中记录了由抖动执行的优化类型。 .
关于c# - 编译器什么时候优化我的代码，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7076479/