有一个运行时的好处是用const的局部变量? [英] Is there a runtime benefit to using const local variables?
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问题描述
的确保它们不能被改变(到一个编译器错误的曲调)之外,没有了JIT作出任何的优化为常量当地人
例如
公共静态INT主要(字串[] args)
{
const int的timesToLoop = 50;
的for(int i = 0; I< timesToLoop;我++)
{
// ...
}
}
解决方案
生成的IL是不同的(使用拍摄模式):
使用当地不变采用本地正常
--------------------- ------------------------------------------------
.entrypoint .entrypoint
.maxstack 2 .maxstack 2
.locals的init(.locals的init(
[0] INT32我)[0] INT32 timesToLoop,
L_0000: ldc.i4.0 [1] INT32 I)
L_0001:stloc.0 L_0000:ldc.i4.s 50
L_0002:br.s L_0008 L_0002:stloc.0
L_0004:ldloc 0.0 L_0003:ldc.i4.0
L_0005:ldc.i4.1 L_0004:stloc.1
L_0006:添加L_0005:br.s L_000b
L_0007:stloc.0 L_0007:ldloc .1
L_0008:ldloc.0 L_0008:ldc.i4.1
L_0009:ldc.i4.s 50 L_0009:加
L_000b:blt.s L_0004 L_000a:stloc.1
L_000d:RET L_000b:ldloc.1
L_000c:ldloc.0
L_000d:blt.s L_0007
L_000f:RET
正如你所看到的编译器,这导致在一个较小的堆栈中的常量的值替换所有变量的用法。
Outside of the ensuring that they cannot be changed (to the tune of a compiler error), does the JIT make any optimisations for const locals?
Eg.
public static int Main(string[] args)
{
const int timesToLoop = 50;
for (int i=0; i<timesToLoop; i++)
{
// ...
}
}
解决方案
The generated IL is different (using Release mode):
using constant local using normal local
---------------------------------------------------------------------
.entrypoint .entrypoint
.maxstack 2 .maxstack 2
.locals init ( .locals init (
[0] int32 i) [0] int32 timesToLoop,
L_0000: ldc.i4.0 [1] int32 i)
L_0001: stloc.0 L_0000: ldc.i4.s 50
L_0002: br.s L_0008 L_0002: stloc.0
L_0004: ldloc.0 L_0003: ldc.i4.0
L_0005: ldc.i4.1 L_0004: stloc.1
L_0006: add L_0005: br.s L_000b
L_0007: stloc.0 L_0007: ldloc.1
L_0008: ldloc.0 L_0008: ldc.i4.1
L_0009: ldc.i4.s 50 L_0009: add
L_000b: blt.s L_0004 L_000a: stloc.1
L_000d: ret L_000b: ldloc.1
L_000c: ldloc.0
L_000d: blt.s L_0007
L_000f: ret
As you can see the compiler replaces all variable usages by the value of the constant which results in a smaller stack.
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