组装中的功能是什么? [英] What is function in assembly?

问题描述

我正在尝试研究汇编，编译器(LLVM)和提升器.

I am trying to study about assembly, compiler(LLVM) and lifter.

我可以用nasm编写汇编代码.(例如此)

I can write just assembly code by nasm.(like this)

下面是我的汇编代码.

section .data
hello_string db "Hello World!", 0x0d, 0x0a
hello_string_len equ $ - hello_string

section .text
global _start

_start:
    mov eax, 4 ; eax <- 4, syscall number (print) But, never execute.
    mov ebx, 1 ; ebx <- 1, syscall argument1 (stdout) But, never execute.
    mov ecx, hello_string ; ecx <- exit_string, syscall argument2 (string ptr) But, never execute.
    mov edx, hello_string_len ; edx <- exit_string_len, syscall argument3 (string len) But, never execute.
    int 0x80; ; syscall But, never execute.
    mov eax, 1 ; eax <- 1, syscall number (exit) But, never execute.
    mov ebx, 0 ; ebx <- 0, syscall argument1 (return value) But, never execute.
    int 0x80; syscall But, never execute.

;nasm -felf32 hello.x86.s -o hello.o
;ld -m elf_i386 hello.o -o hello.out

然后我检查二进制文件.

And I check binary file.

在这里，我找不到功能.并且我同意该呼叫和退回指令是一些指令的组合.

Here, I can't find Function. and i agree with that call and ret instructions are something combined some instructions.

$readelf -s hello.o
Symbol table '.symtab' contains 7 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 FILE    LOCAL  DEFAULT  ABS hello.x86.s
     2: 00000000     0 SECTION LOCAL  DEFAULT    1 
     3: 00000000     0 SECTION LOCAL  DEFAULT    2 
     4: 00000000     0 NOTYPE  LOCAL  DEFAULT    1 hello_string
     5: 0000000e     0 NOTYPE  LOCAL  DEFAULT  ABS hello_string_len
     6: 00000000     0 NOTYPE  GLOBAL DEFAULT    2 _start

但是.如果我编译C程序并通过readelf检查该二进制文件.然后我可以找到功能".

But. If i compile c program and check that binary file by readelf. then i can find "function".

PS

$readelf -s function.o | grep FUNC
     3: 0000000000000000    18 FUNC    GLOBAL DEFAULT    2 add
     4: 0000000000000020    43 FUNC    GLOBAL DEFAULT    2 main

在这里我可以看到什么是功能.

here i can see what is function.

功能不同的NOTYPE标签是什么?

what is function different NOTYPE label?

推荐答案

ELF符号元数据可以由某些汇编程序设置，例如在NASM中，使用 global main:function 将符号类型标记为FUNC.( https://nasm.us/doc/nasmdoc8.html#section-8.9.5 ).

ELF symbol metadata can be set by some assemblers, e.g. in NASM, global main:function to mark the symbol type as FUNC. (https://nasm.us/doc/nasmdoc8.html#section-8.9.5).

等效的GAS语法(C编译器发出)是 .type main，function .例如在 https://godbolt.org 上放置一些代码，并禁用过滤以在编译器输出中查看asm指令.

The GAS syntax equivalent (which C compilers emit) is .type main, function. e.g. put some code on https://godbolt.org and disable filtering to see asm directives in compiler output.

但是请注意，这只是供链接器和调试器使用的元数据.CPU在执行时看不到它.这就是为什么没人为NASM示例而烦恼的原因.

But note this is just metadata for linkers and debuggers to use; the CPU doesn't see that when executing. That's why nobody bothers with it for NASM examples.

汇编语言并没有真正的功能，只是实现该概念的工具，例如跳转并将返回地址存储在某个地方= call ，间接跳转到返回地址= ret .在x86上，返回地址被压入并弹出堆栈.

Assembly language doesn't truly have functions, just the tools to implement that concept, e.g. jump and store a return address somewhere = call, indirect jump to a return address = ret. On x86, return addresses are pushed and popped on the stack.

执行模型是纯粹顺序的和局部的，一次只执行一条指令(在大多数ISA上，但有些ISA是VLIW，例如一次执行3条，但在范围上仍然是局部的)，每条指令仅构成一个对架构状态进行明确定义的更改.CPU本身不知道或不在乎它是否在功能中".或其他有关嵌套的内容，但返回地址预测变量堆栈乐观地认为 ret 将实际使用由相应的 call 推送的返回地址.但这是性能上的优化；如果代码做得很奇怪(例如，上下文切换)，有时您会得到不匹配的调用/返回.

The model of execution is purely sequential and local, one instruction at a time (on most ISAs, but some ISAs are VLIW and execute 3 at a time for example, but still local in scope), with each instruction just making a well-defined change to the architectural state. The CPU itself doesn't know or care that it's "in a function" or anything about nesting, other than the return-address predictor stack which optimistically assumes that ret will actually use a return address pushed by a corresponding call. But that's a performance optimization; you do sometimes get mismatched call/ret if code is doing something weird (e.g. a context switch).

C编译器不会在函数之外放置任何指令.

A C compiler won't put any instructions outside of functions.

从技术上讲，间接调用 main 的 _start 入口点不是函数；它不能返回，必须进行 exit 系统调用，但这是用asm编写的，并且是libc的一部分.它不是由C编译器适当生成的，仅与C编译器的输出链接以创建工作程序.)请参见

Technically the _start entry point that indirectly calls main isn't a function; it can't return and has to make an exit system call, but that's written in asm and is part of libc. It's not generated by the C compiler proper, only linked with the C compiler's output to make a working program.) See Linux x86 Program Start Up or - How the heck do we get to main()? for example.

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