x86 CMP 指令差异
问题
以下两条 x86 指令之间有什么（重要的）区别？
39 /r CMP r/m32,r32 Compare r32 with r/m32
3B /r CMP r32,r/m32 Compare r/m32 with r32
背景
我正在构建一个 Java 汇编器，我的编译器的中间语言将使用它来生成 Windows-32 可执行文件。
目前我有以下代码：
final ModelBase mb = new ModelBase(); // create new memory model
mb.addCode(new Compare(Register.ECX, Register.EAX)); // add code
mb.addCode(new Compare(Register.EAX, Register.ECX)); // add code
final FileOutputStream fos = new FileOutputStream(new File("test.exe"));
mb.writeToFile(fos);
fos.close();
输出一个有效的可执行文件，其中在文本部分包含两个 CMP 指令。输出到“text.exe”的可执行文件不会做任何有趣的事情，但这不是重点。 Compare 类是 CMP 指令的包装器。
上面的代码产生（用 OllyDbg 检查）：
Address Hex dump Command
0040101F |. 3BC8 CMP ECX,EAX
00401021 |. 3BC1 CMP EAX,ECX
区别很微妙：如果我使用 39 字节操作码：
Address Hex dump Command
0040101F |. 39C1 CMP ECX,EAX
00401021 |. 39C8 CMP EAX,ECX
这让我想知道它们的同义词以及为什么它存在。
Question
What is the (non-trivial) difference between the following two x86 instructions?
39 /r CMP r/m32,r32 Compare r32 with r/m32
3B /r CMP r32,r/m32 Compare r/m32 with r32
Background
I'm building a Java assembler, which will be used by my compiler's intermediate language to produce Windows-32 executables.
Currently I have following code:
final ModelBase mb = new ModelBase(); // create new memory model
mb.addCode(new Compare(Register.ECX, Register.EAX)); // add code
mb.addCode(new Compare(Register.EAX, Register.ECX)); // add code
final FileOutputStream fos = new FileOutputStream(new File("test.exe"));
mb.writeToFile(fos);
fos.close();
To output a valid executable file, which contains two CMP instruction in a TEXT-section. The executable outputted to "text.exe" will do nothing interesting, but that's not the point. The class Compare is a wrapper around the CMP instruction.
The above code produces (inspecting with OllyDbg):
Address Hex dump Command
0040101F |. 3BC8 CMP ECX,EAX
00401021 |. 3BC1 CMP EAX,ECX
The difference is subtle: if I use the 39 byte-opcode:
Address Hex dump Command
0040101F |. 39C1 CMP ECX,EAX
00401021 |. 39C8 CMP EAX,ECX
Which makes me wonder about their synonymity and why this even exists.
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如果比较两个寄存器，使用哪个操作码并不重要。唯一的区别是在将寄存器与内存操作数进行比较时，因为所使用的操作码决定了将从哪个操作数中减去哪个操作数。
至于为什么存在：x86 指令格式使用 ModR/M 字节来表示内存地址或寄存器。每条指令只能有一个ModR/M值，即只能访问一个内存地址（不包括MOVSB等特殊指令）。所以这意味着不能有通用的
cmp r/m32, r/m32指令，我们需要两个不同的操作码：cmp r/m32, r32和<代码>cmp r32，r/m32。作为副作用，这会在比较两个寄存器时产生一些冗余。It doesn't matter which opcode you use if you compare two registers. The only difference is when comparing a register with a memory operand, as the opcode used determines which will be subtracted from which.
As for why this exists: The x86 instruction format uses the ModR/M byte to denote either a memory address or a register. Each instruction can only have one ModR/M value, which means it can only access one memory address (not including special instructions like MOVSB). So this means that there can't be a general
cmp r/m32, r/m32instruction, and we need two different opcodes:cmp r/m32, r32andcmp r32, r/m32. As a side effect, this creates some redundancy when comparing two registers.这是x86 的冗余。类似这样的案例还有很多。编译器/汇编器可以自由使用任何有效的操作码。
某些汇编器允许您选择要发出的操作码。例如，在 GAS 上，您可以附加“.s”以使用其他指令编码
It's redundancy of x86. There are much more many cases like this. A compiler/assembler is free to use any of the valid opcodes
Some assembler allows you to choose which opcode to emit. For example on GAS you can attach ".s" to use the other instruction encoding