MSIL指令及其参数详解(ldc指令诠释篇)

首先来看一下一个有代表性的: ldc.i4 num 这个指令.在这个指令中,num被放到了堆栈上面:

Note that there are special short (and hence more efficient) encodings for the integers -128 through 127, and especially short encodings for -1 through 8. All short encodings push 4 byte integers on the stack. Longer encodings are used for 8 byte integers and 4 and 8 byte floating-point numbers, as well as 4-byte values that do not fit in the short forms. There are three ways to push an 8 byte integer constant onto the stack.

参见:
http://msdn2.microsoft.com/en-us/library/system.reflection.emit.opcodes.ldc_i4(vs.71).aspx

ldc指令的语法如下:
ldc.type value
ldc.i4.number
ldc.i4.s number

在第一条语法中,ldc指令加载一个指定类型的常量到stack.
在第二条语法的指令当中,ldc指令更加有效.它传输一个整型值-1以及0到8之间的整数给计算堆栈.当我们传输-1到计算堆栈的时候,这条ldc的语法指令又进一步演变成为ldc.i4.ml.

这里,截取ECMA的C#标准里面关于这个指令的完整参考:

Ref:
引用部分来自ECMA关于CLI部分的说明.如果安装了.Net Framework SDK1.1的话,可以在这里找到:
C:/Program Files/Microsoft Visual Studio .NET 2003/SDK/v1.1/Tool Developers Guide/docs/Partition III CIL.doc 
 

Format	Assembly Format	Description
20 <int32>	ldc.i4 num	Push num of type int32 onto the stack as int32.
21 <int64>	ldc.i8 num	Push num of type int64 onto the stack as int64.
22 <float32>	ldc.r4 num	Push num of type float32 onto the stack as F.
23 <float64>	ldc.r8 num	Push num of type float64 onto the stack as F.
16	ldc.i4.0	Push 0 onto the stack as int32.
17	ldc.i4.1	Push 1 onto the stack as int32.
1E	ldc.i4.8	Push 8 onto the stack as int32.
15	ldc.i4.m1	Push -1 onto the stack as int32.
15	ldc.i4.M1	Push -1 of type int32 onto the stack as int32 (alias).
1F <int8>	ldc.i4.s num	Push num onto the stack as int32, short form.

Stack里面的传输方向:
… à …, num 

Description:
The ldc num instruction pushes number num or some constant onto the stack. There are special short encodings for the integers –128 through 127 (with especially short encodings for –1 through 8). All short encodings push 4-byte integers on the stack. Longer encodings are used for 8-byte integers and 4- and 8-byte floating-point numbers, as well as 4-byte values that do not fit in the short forms.

There are three ways to push an 8-byte integer constant onto the stack
1. For constants that shall be expressed in more than 32 bits, use the ldc.i8 instruction.
2. For constants that require 9–32 bits, use the ldc.i4 instruction followed by a conv.i8.
3. For constants that can be expressed in 8 or fewer bits, use a short form instruction followed by a conv.i8.

There is no way to express a floating-point constant that has a larger range or greater precision than a 64-bit IEC 60559:1989 number, since these representations are not portable across architectures.

Ref:引用结束

从这里,我们可以看到ldc指令极其参数的完整使用说明.

这里,只是举一个参数比较复杂的例子来说明IL instruction的使用方法,大家可以触类旁通,将类似的知识使用到其它的计算堆栈和托管堆指令中去.
另外更多MSIL指令的详细用法可以参考ECMA关于CLI的Reference的第三章.