gpt4 book ai didi

performance - 为什么 i=i+1 比 i++ 快?

转载 作者:行者123 更新时间:2023-12-04 17:11:15 28 4
gpt4 key购买 nike

在 Flash 中测试此代码:

var i:int = 0;
for (var j:int = 0; j < 5000000; j++)
{
i=i+1;
}// use about 300ms.

i = 0;
for (var j:int = 0; j < 5000000; j++)
{
i++;
}// use about 400ms

i = 0;
for (var j:int = 0; j < 5000000; j++)
{
++i;
}// use about 400ms too

为什么是 i=i+1在 ActionScript 3 中更快,而在其他中则更慢?

对不起,我弄错了。上面的代码使用了相同的时间。
但是如果放到函数里,结果就不一样了。
var i:int;
var j:int;
var startTime:Number;

function func1():void
{
i = i + 1;
}

function func2():void
{
i++;
}

startTime = getTimer();
i = 0;
for (j = 0; j < 10000000; j++)
{
func1();
}
trace(getTimer() - startTime);//5 times:631,628,641,628,632

startTime = getTimer();
i = 0;
for (j = 0; j < 10000000; j++)
{
func2();
}
trace(getTimer() - startTime);//5 times:800,814,791,832,777

最佳答案

循环所在的位置会对性能产生很大影响。如果您的循环在函数内部,Flash 将使用本地寄存器执行计算。包含 i++ 的循环因此产生以下操作码:

000013 inclocal_i (REG_2)  ; increment i 
000015 inclocal_i (REG_3) ; increment j
000017 getlocal (REG_3) ; push j onto stack
000018 pushint 5000000 ; push 5000000 onto stack
000020 iflt -12 ; jump backward if less than

包含 i = i + 1 的循环产生以下内容:
000013 getlocal (REG_2)    ; push i onto stack
000014 pushbyte 1 ; push 1 onto stack
000016 add ; add the two
000017 convert_i ; coerce to integer
000018 setlocal (REG_2) ; save i back to register 2
000019 inclocal_i (REG_3)
000021 getlocal (REG_3)
000022 pushint 5000000
000024 iflt -16
i++i = i + 1 快这里因为 inclocal_i 直接修改寄存器,而不必将寄存器加载到堆栈上并将其保存回来。

当你把它放在一个框架脚本中时,循环的效率会大大降低。 Flash 将声明的变量存储为类变量。访问这些需要更多的工作。 i++循环结果如下:
000017 getlocal (REG_0, this)   ; push this onto stack
000018 dup ; duplicate it
000019 setlocal (REG_2) ; save this to register 2
000020 getproperty i ; get property "i"
000022 increment_i ; add one to it
000023 setlocal (REG_3) ; save result to register 3
000024 getlocal (REG_2) ; get this from register 2
000025 getlocal (REG_3) ; get value from register 3
000026 setproperty i ; set property "i"
000028 kill (REG_3) ; kill register 2
000030 kill (REG_2) ; kill register 3
000032 getlocal (REG_0, this) ; do the same thing with j...
000033 dup
000034 setlocal (REG_2)
000035 getproperty j
000037 increment_i
000038 setlocal (REG_3)
000039 getlocal (REG_2)
000040 getlocal (REG_3)
000041 setproperty j
000043 kill (REG_3)
000045 kill (REG_2)
000047 getlocal (REG_0, this)
000048 getproperty j
000050 pushint 5000000
000052 iflt -40
i = i + 1版本有点短:
000017 getlocal (REG_0, this)   ; push this onto stack
000018 getlocal (REG_0, this) ; push this onto stack
000019 getproperty i ; get property "i"
000021 pushbyte 1 ; push 1 onto stack
000023 add ; add the two
000024 initproperty i ; save result to property "i"
000026 getlocal (REG_0, this) ; increment j...
000027 dup
000028 setlocal (REG_2)
000029 getproperty j
000031 increment_i
000032 setlocal (REG_3)
000033 getlocal (REG_2)
000034 getlocal (REG_3)
000035 setproperty j
000037 kill (REG_3)
000039 kill (REG_2)
000041 getlocal (REG_0, this)
000042 getproperty j
000044 pushint 5000000
000046 iflt -34

关于performance - 为什么 i=i+1 比 i++ 快?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/12208421/

28 4 0
Copyright 2021 - 2024 cfsdn All Rights Reserved 蜀ICP备2022000587号
广告合作:1813099741@qq.com 6ren.com