大会我们需要结局吗?

问题描述

在汇编 (att) 中，以下内容是合法的:

In assembly (att) the following is legal:

mov %rax, %rbx

等于:

movq %rax, %rbx

其中 q 表示 第一个参数 是 64 位，我的问题是:

Where q means the first parameter is 64 bits, my question is:

这个 q(或其他结尾)是否仅用于人类阅读的简单性，或者在某些情况下，不写 q 会给出错误答案或与预期不同的结果，甚至导致代码崩溃(非法命令)，请如果可能的话，给我一个例子.

Is this q (or other endings) used only for simplicity of human reading or there could be some cases where no writing q would give wrong answer or different result than the expected one or even crash the code (illegal command), please give me an example if possible.

推荐答案

您在询问操作数大小后缀.有两种情况:

You're asking about the operand size suffix. There are two cases:

1. 对于许多指令，操作数的大小可以从操作数本身推断出来，通常是因为它们是特定大小的寄存器.这就像您的示例: mov %rax, %rbx 必须具有 64 位操作数大小，因为 %rax, %rbx 是 64 位寄存器.在这种情况下，后缀是可选的，无论你写 mov %rax, %rbx 还是 movq % 都会生成相同的机器码(48 89 c3)rax，%rbx.无论您是否选择包含它，这纯粹是一种风格问题(尽管人们对哪种方式更好有自己的看法).

1. For many instructions, the operand size can be inferred from the operands themselves, typically because they are registers of a particular size. This is like your example: mov %rax, %rbx must have a 64-bit operand size, because %rax, %rbx are 64-bit registers. In this case, the suffix is optional, and the same machine code (48 89 c3) is generated whether you write mov %rax, %rbx or movq %rax, %rbx. It's purely a matter of style whether you choose to include it or not (though certainly people have opinions about which way is better).

如果您提供的后缀与操作数不一致，例如movl %rax, %rbx，汇编器会给出警告或错误.

If you provide a suffix that is inconsistent with the operands, e.g. movl %rax, %rbx, the assembler will give a warning or error.

在其他情况下，无法推断操作数大小.最常见的情况是当一个操作数是立即数而另一个是内存引用时.如果您从注释中为 ecm 的示例编写 mov $1, (%rbx) ，这是模棱两可的:汇编程序是否应该发出一条指令来存储一个字节(机器码 c6 03 01)，或者一个字(两个字节，66 c7 03 01 00)，或者一个 long(四个字节，c7 03 01 00 00 00)，或者一个四元组(八个字节，48 c7 03 01 00 00 00)?所以在这种情况下需要一个后缀:你必须写 movb $1, (%rbx) 或 movw $1, (%rbx) 等等.

In others, the operand size cannot be inferred. The most common case is when one operand is an immediate and the other is a memory reference. If you write mov $1, (%rbx) for ecm's example from comments, it is ambiguous: should the assembler emit an instruction to store a byte (machine code c6 03 01), or a word (two bytes, 66 c7 03 01 00), or a long (four bytes, c7 03 01 00 00 00), or a quad (eight bytes, 48 c7 03 01 00 00 00)? So a suffix is required in this case: you must write movb $1, (%rbx) or movw $1, (%rbx) and so on.

如果你在这种情况下省略后缀，最近的汇编版本至少应该警告你.有些会因为错误而中止；其他人可能会猜测操作数的大小或使用一些内置的默认值，正如彼得在下面评论的那样.一些较旧的汇编器版本实际上会在没有警告的情况下恢复为默认值.

If you omit the suffix in such a case, recent assembler versions should at least warn you. Some will then abort with an error; others may guess at the operand size or use some built-in default, as Peter comments below. Some older assembler versions would actually revert to a default without warning.

所以原则上，是的，省略后缀可能会导致错误"代码，在某些情况下和一些汇编程序.但是，广泛使用的 AT&T 语法汇编器的当前版本至少会警告您.

So in principle, yes, omitting the suffix could lead to "wrong" code, in some cases and with some assemblers. However, current versions of the widely used AT&T-syntax assemblers would at least warn you.

然而，还有另一种可能发生这种情况:假设您想将 5 添加到 32 位寄存器 eax (addl $5, %eax)，但你打错了，省略了 e.如果你有使用后缀的习惯，你会写 addl $5, %ax 并得到一个汇编错误，提醒你你的错误.如果您的风格是省略它们，您可以编写 add $5, %ax 并且代码可以完美构建但会错误".

There is however one other way that this can sort of happen: suppose you want to add 5 to the 32-bit register eax (addl $5, %eax), but you make a typo and leave off the e. If you are in the habit of using the suffixes, you would write addl $5, %ax and get an assembly error, alerting you to your mistake. If your style is to omit them, you would write add $5, %ax and the code would build perfectly but would be "wrong".

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