C 和 RISCV 汇编

IR (Intermediate Representation): 中间代码的作用: 相当于有一个中间缓冲, 可以看成编译器的前端, 通过增加这一个模块, 可以让编译器的前端和后端解耦, 方便支持多种语言和多种目标架构.

             frontend                              backend
           +----------+                        +------------+
      C -> |  Clang   | -+                 +-> |  llvm-x86  | -> x86
           +----------+  |                 |   +------------+
           +----------+  +-> +----------+ -+   +------------+
Fortran -> | llvm-gcc | ---> | llvm-opt | ---> |  llvm-arm  | -> ARM
           +----------+  +-> +----------+ -+   +------------+
           +----------+  |                 |   +------------+
Haskell -> |    GHC   | -+                 +-> | llvm-riscv | -> RISC-V
           +----------+  LLVM IR      LLRM IR  +------------+

volatile 关键字: 告诉编译器不要优化这个语句.

在 C 中插入汇编代码 [1]:

一般我们不会显式地指明用哪些寄存器或者内存地址, 让编译器来决定!

// 相当于 add c, a, b
// r 代表用寄存器来做对应 (m 表示用内存来做对应)
int foo (int a, int b)
{
    int c;
    asm volatile (
        "add %0, %1, %2\n" // 指令内容用双引号
        : "=r" (c) // 输出
        : "r" (a), "r" (b) // 输入
    );
    return c;
}

也可以用:

asm volatile (
    "add %[sum], %[add1], %[add2]\n"
    : [sum] "=r" (c) // 输出
    : [add1] "r" (a), [add2] "r" (b) // 输入
);

C 编译过程

Compiler, Assembler, Linker:
- Watch this video
gcc 编译选项:
- -E: 只预处理, 不编译.
- -S: 生成汇编代码 (.s 文件)
- -c: 只编译, 不链接. (生成 .o 文件)
- -o <file>: 指定输出文件名.
- -g: 在输出的文件中加入支持调试的信息.
- -v (verbose): 显示编译过程中的详细信息.

装好 RISCV 编译工具链, 创建一个简单的 C 语言程序 hello.c:

hello.c

#include <stdio.h>

int global_init = 0x11111111;
const int global_const = 0x22222222;

void main()
{
    static int static_var = 0x33333333;
    static int static_var_uninit;
    int auto_var = 0x44444444;
    printf("hello world!\n");
    return;
}

运行:

riscv32-unknown-elf-gcc -c hello.c -o hello.o # 只编译, 不链接

用 riscv32-unknown-elf-readelf 查看生成的 .o 文件:

riscv32-unknown-elf-readelf -h hello.o > header_info.log # 查看 Header 信息, 并输出到文件

得到:

header_info.log

ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              REL (Relocatable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          0 (bytes into file)
  Start of section headers:          916 (bytes into file)
  Flags:                             0x5, RVC, double-float ABI
  Size of this header:               52 (bytes)
  Size of program headers:           0 (bytes)
  Number of program headers:         0
  Size of section headers:           40 (bytes)
  Number of section headers:         14
  Section header string table index: 13

再查看 Section 信息:

riscv32-unknown-elf-readelf -S hello.o > section_info.log # 查看 Section 信息, 并输出到文件

得到:

section_info.log

There are 14 section headers, starting at offset 0x394:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00000000 000034 00002e 00  AX  0   0  2
  [ 2] .rela.text        RELA            00000000 0002d8 000048 0c   I 11   1  4
  [ 3] .data             PROGBITS        00000000 000062 000000 00  WA  0   0  1
  [ 4] .bss              NOBITS          00000000 000064 000004 00  WA  0   0  4
  [ 5] .sdata            PROGBITS        00000000 000064 000008 00  WA  0   0  4
  [ 6] .srodata          PROGBITS        00000000 00006c 000004 00   A  0   0  4
  [ 7] .rodata           PROGBITS        00000000 000070 00000d 00   A  0   0  4
  [ 8] .comment          PROGBITS        00000000 00007d 00001a 01  MS  0   0  1
  [ 9] .note.GNU-stack   PROGBITS        00000000 000097 000000 00      0   0  1
  [10] .riscv.attributes RISCV_ATTRIBUTE 00000000 000097 000066 00      0   0  1
  [11] .symtab           SYMTAB          00000000 000100 000130 10     12  15  4
  [12] .strtab           STRTAB          00000000 000230 0000a7 00      0   0  1
  [13] .shstrtab         STRTAB          00000000 000320 000074 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), p (processor specific)

对 .o 文件进行反汇编:

riscv32-unknown-elf-objdump -S hello.o > disassembly.s # 反汇编, 并输出到文件

得到:

disassembly.s


hello.o:     file format elf32-littleriscv


Disassembly of section .text:

00000000 <main>:
   0:   1101                    addi    sp,sp,-32
   2:   ce06                    sw  ra,28(sp)
   4:   cc22                    sw  s0,24(sp)
   6:   1000                    addi    s0,sp,32
   8:   444447b7            lui a5,0x44444
   c:   44478793            addi    a5,a5,1092 # 44444444 <static_var.0+0x44444440>
  10:   fef42623            sw  a5,-20(s0)
  14:   000007b7            lui a5,0x0
  18:   00078513            mv  a0,a5
  1c:   00000097            auipc   ra,0x0
  20:   000080e7            jalr    ra # 1c <main+0x1c>
  24:   0001                    nop
  26:   40f2                    lw  ra,28(sp)
  28:   4462                    lw  s0,24(sp)
  2a:   6105                    addi    sp,sp,32
  2c:   8082                    ret

RISCV 汇编语言

基本格式:
```
<label>: <operation> # <comment>
```
- <operation> 的类型 [1]:
  - instruction (指令)
  - pseudo-instruction (伪指令): li x6, 5, nop, RISCV 中有定义
  - directive (伪操作): .macro ... .endm, .end, .text, .global _start 告知汇编器的指示, 与 RISCV 规范无关.
  - macro (宏):
```
.macro do_nothing
    nop
.endm

_start: do_nothing # 调用宏, 汇编器会将它展开
```

标签其实表示地址:

_start: li x6, 5 # _start 标签其实就是这条指令的地址

[1]

Lazyparser, “Compilation and linker,” 2025, Available: https://www.bilibili.com/video/BV1Q5411w7z5?spm_id_from=333.788.videopod.episodes&vd_source=42579e22289b6144ba0b2bdcf99834e3&p=5