我只是想知道" if OR "和" if AND " 内部会发生什么.我有一种感觉,它只是使用语法糖&&,||并且在内部所有情况都构建为单个if语句.
紧凑形式||:
if(a || b || c)
{
DoSomething();
}
潜在的内部形式:
if(a)
{
DoSomething();
}
else if(b)
{
DoSomething();
}
else if(c)
{
DoSomething();
}
紧凑形式&&:
if(a && b && c)
{
DoSomething();
}
潜在的内部形式:
if(a)
{
if(b)
{
if(c)
{
DoSomething();
}
}
}
这两个例子的表现有什么不同吗？
*编辑:将其他内容添加到|| 案件
Tho*_*oub 33
首先,||并且&&是短路的.这意味着:
if(a || b || c)
DoSomething();
如果a是真实的,b而c不会进行评估.
其次,你的实现||是假的:
if(a)
DoSomething();
if(b)
DoSomething();
if(c)
DoSomething();
DoSomething()最多会被召唤3次.
它应该是:
if(a)
DoSomething();
else if(b)
DoSomething();
else if(c)
DoSomething();
要完成,如果你想表现先喜欢短呼叫您的条件:
if(aShortFunctionToExecute() || aVeryVeryLongFunctionToExecute())
DoSomething();
会比快
if(aVeryVeryLongFunctionToExecute() || aShortFunctionToExecute())
DoSomething();
因为懒惰的评价
如果您反汇编代码:
private static void Main()
{
if (a() && b() && c())
{
Console.WriteLine("DoSomething");
}
}
bool a(){
return true;
}
bool b(){
return 3 % 2 == 1;
}
bool c(){
return (3 % 2) / 1 == 1;
}
你会得到:
if (a() && b() && c())
00000022 call FFFFFFFFFFEE8D90
00000027 mov byte ptr [rbp+20h],al
0000002a movzx eax,byte ptr [rbp+20h]
0000002e test eax,eax
00000030 je 000000000000005A
00000032 call FFFFFFFFFFEE8D98
00000037 mov byte ptr [rbp+21h],al
0000003a movzx eax,byte ptr [rbp+21h]
0000003e test eax,eax
00000040 je 000000000000005A
00000042 call FFFFFFFFFFEE8DA0
00000047 mov byte ptr [rbp+22h],al
0000004a movzx ecx,byte ptr [rbp+22h]
0000004e xor eax,eax
00000050 test ecx,ecx
00000052 sete al
00000055 mov dword ptr [rbp+24h],eax
00000058 jmp 0000000000000062
0000005a nop
0000005b mov dword ptr [rbp+24h],1
00000062 nop
00000063 movzx eax,byte ptr [rbp+24h]
00000067 mov byte ptr [rbp+2Fh],al
0000006a movzx eax,byte ptr [rbp+2Fh]
0000006e test eax,eax
00000070 jne 0000000000000087
{
00000072 nop
Console.WriteLine("DoSomething");
00000073 mov rcx,12603398h
0000007d mov rcx,qword ptr [rcx]
00000080 call 00000000577A82A0
00000085 nop
}
并为代码:
private static void Main()
{
if (a())
if(b())
if(c())
Console.WriteLine("DoSomething");
}
static bool a(){
return true;
}
static bool b(){
return 3 % 2 == 1;
}
static bool c(){
return (3 % 2) / 1 == 1;
}
你会得到:
if (a())
00000022 call FFFFFFFFFFEE8D90
00000027 mov byte ptr [rbp+20h],al
0000002a movzx ecx,byte ptr [rbp+20h]
0000002e xor eax,eax
00000030 test ecx,ecx
00000032 sete al
00000035 mov dword ptr [rbp+24h],eax
00000038 movzx eax,byte ptr [rbp+24h]
0000003c mov byte ptr [rbp+3Fh],al
0000003f movzx eax,byte ptr [rbp+3Fh]
00000043 test eax,eax
00000045 jne 00000000000000A4
if(b())
00000047 call FFFFFFFFFFEE8D98
0000004c mov byte ptr [rbp+28h],al
0000004f movzx ecx,byte ptr [rbp+28h]
00000053 xor eax,eax
00000055 test ecx,ecx
00000057 sete al
0000005a mov dword ptr [rbp+2Ch],eax
0000005d movzx eax,byte ptr [rbp+2Ch]
00000061 mov byte ptr [rbp+3Fh],al
00000064 movzx eax,byte ptr [rbp+3Fh]
00000068 test eax,eax
0000006a jne 00000000000000A4
if(c())
0000006c call FFFFFFFFFFEE8DA0
00000071 mov byte ptr [rbp+30h],al
00000074 movzx ecx,byte ptr [rbp+30h]
00000078 xor eax,eax
0000007a test ecx,ecx
0000007c sete al
0000007f mov dword ptr [rbp+34h],eax
00000082 movzx eax,byte ptr [rbp+34h]
00000086 mov byte ptr [rbp+3Fh],al
00000089 movzx eax,byte ptr [rbp+3Fh]
0000008d test eax,eax
0000008f jne 00000000000000A4
Console.WriteLine("DoSomething");
00000091 mov rcx,125D3398h
0000009b mov rcx,qword ptr [rcx]
0000009e call 00000000577B82A0
000000a3 nop
这有点长:它需要40条指令而不是31条指令.
正如thanosqr所指出的,性能还取决于您的条件成立的概率.举个例子:
如果a99%的时间都失败并且需要1秒才能运行,如果b
99%的时间成功并且需要10秒才能运行,超过100次尝试你将更快地b放在第一位:
if(b || a) => 10s 99% ==> 100 runs will take 99*10+11 = 1001s
if(b || a) => 11s 1%
if(a || b) => 11s 99% ==> 100 runs will take 99*11+1 = 1090s
if(a || b) => 1s 1%
另外,我建议你阅读" 为什么处理排序数组比未排序数组更快？ "这非常有趣!
使用紧凑形式,C#编译器发出的IL将不那么冗长,导致在运行时处理的指令更少.发出的IL语句及其逻辑实际上是相同的,因此没有花哨的内置支持来处理该情况或一些特殊指令(请记住,您可以将任何带有布尔结果的表达式放入其中if).
对于使用||运算符的紧凑形式(调试版本):
.method private hidebysig static void One() cil managed
{
// Code size 38 (0x26)
.maxstack 2
.locals init ([0] bool CS$4$0000)
IL_0000: nop
IL_0001: ldsfld bool ConsoleApplication4.Program::a
IL_0006: brtrue.s IL_0019
IL_0008: ldsfld bool ConsoleApplication4.Program::b
IL_000d: brtrue.s IL_0019
IL_000f: ldsfld bool ConsoleApplication4.Program::c
IL_0014: ldc.i4.0
IL_0015: ceq
IL_0017: br.s IL_001a
IL_0019: ldc.i4.0
IL_001a: nop
IL_001b: stloc.0
IL_001c: ldloc.0
IL_001d: brtrue.s IL_0025
IL_001f: call void ConsoleApplication4.Program::DoSomething()
IL_0024: nop
IL_0025: ret
} // end of method Program::One
使用您的内部表单(考虑您使用else if而不是if):
.method private hidebysig static void Two() cil managed
{
// Code size 60 (0x3c)
.maxstack 2
.locals init ([0] bool CS$4$0000)
IL_0000: nop
IL_0001: ldsfld bool ConsoleApplication4.Program::a
IL_0006: ldc.i4.0
IL_0007: ceq
IL_0009: stloc.0
IL_000a: ldloc.0
IL_000b: brtrue.s IL_0015
IL_000d: call void ConsoleApplication4.Program::DoSomething()
IL_0012: nop
IL_0013: br.s IL_003b
IL_0015: ldsfld bool ConsoleApplication4.Program::b
IL_001a: ldc.i4.0
IL_001b: ceq
IL_001d: stloc.0
IL_001e: ldloc.0
IL_001f: brtrue.s IL_0029
IL_0021: call void ConsoleApplication4.Program::DoSomething()
IL_0026: nop
IL_0027: br.s IL_003b
IL_0029: ldsfld bool ConsoleApplication4.Program::c
IL_002e: ldc.i4.0
IL_002f: ceq
IL_0031: stloc.0
IL_0032: ldloc.0
IL_0033: brtrue.s IL_003b
IL_0035: call void ConsoleApplication4.Program::DoSomething()
IL_003a: nop
IL_003b: ret
} // end of method Program::Two
因此,有更多的指令来处理附加if语句所需的所有跳转.因此,第一种形式更有效(实际上更具可读性:)).
在性能方面(每个方法使用10.000.000迭代测量10次并删除最高和最低值,发布版本):
紧凑形式:平均55ms
详细形式:平均56ms
所以没有太大的区别.