riscv64-linux-gnu-gcc编译生成32位cpu二进制

• x86平台下64位设备，默认情况下编译的是elf64，比如简单的helloworld代码如下，使用readelf查看elf头部信息，格式为ELF64。

?  cat helloworld.c
#include <stdio.h>

int main()
{
    printf("hello!!!\n");

    return 0;
}

?  gcc -Wall helloworld.c -g -o helloworld
?  readelf -h helloworld
ELF 头：
  Magic：  7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  类别:                              ELF64
  数据:                              2 补码，小端序 (little endian)
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI 版本:                          0
  类型:                              DYN (Position-Independent Executable file)
  系统架构:                          Advanced Micro Devices X86-64
  版本:                              0x1
  入口点地址：              0x1040
  程序头起点：              64 (bytes into file)
  Start of section headers:          14080 (bytes into file)
  标志：             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         13
  Size of section headers:           64 (bytes)
  Number of section headers:         36
  Section header string table index: 35

• 要生成32位代码时只需要指定 -m 32即可。

?  gcc -Wall -m32 helloworld.c -g -o helloworld
?  readelf -h helloworld
ELF 头：
  Magic：  7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  类别:                              ELF32
  数据:                              2 补码，小端序 (little endian)
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI 版本:                          0
  类型:                              DYN (Position-Independent Executable file)
  系统架构:                          Intel 80386
  版本:                              0x1
  入口点地址：              0x1060
  程序头起点：              52 (bytes into file)
  Start of section headers:          13996 (bytes into file)
  标志：             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         12
  Size of section headers:           40 (bytes)
  Number of section headers:         36
  Section header string table index: 35

• 我的设备是MANJARO发行版本，通过包管理器安装了risc-v编译工具链riscv64-linux-gnu-gcc

?  uname -a
Linux mt-pc 5.10.202-1-MANJARO #1 SMP PREEMPT Tue Nov 28 19:17:16 UTC 2023 x86_64 GNU/Linux
?   pacman -Q riscv64-linux-gnu-gcc
riscv64-linux-gnu-gcc 12.2.0-1

• 对于riscv汇编程序，默认生成elf64二进制。

?  cat test.s
# Add
# Format:
#       ADD RD, RS1, RS2
# Description:
#       The contents of RS1 is added to the contents of RS2 and the result is
#       placed in RD.

        .text                   # Define beginning of text section
        .global _start          # Define entry _start

_start:
        li x6, 1                # x6 = 1
        li x7, 2                # x7 = 2
        add x5, x6, x7          # x5 = x6 + x7

stop:
        j stop                  # Infinite loop to stop execution

        .end                    # End of file

?  riscv64-linux-gnu-gcc -nostdlib -fno-builtin -g -Wall test.s -o test
?  riscv64-linux-gnu-readelf -h test
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              DYN (Position-Independent Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x29a
  Start of program headers:          64 (bytes into file)
  Start of section headers:          5536 (bytes into file)
  Flags:                             0x5, RVC, double-float ABI
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         8
  Size of section headers:           64 (bytes)
  Number of section headers:         19
  Section header string table index: 18

• 参照x86下的方式，使用 -m 32 发现没这个选项。查看github官方说明文档，发现–print-multi-lib可以打印出支持的架构，执行发现啥也没打印出来。

?  riscv64-linux-gnu-gcc --print-multi-lib
.;

• 继续看文档，发现有个 -march/-mabi 选项，继续搜查之。发现gnu官方文档-march gnu官方文档-mabi有关于这两个选项的说明。
• 查看默认情况下这两个选项的值为 ‘-march=rv64gc’ ‘-mabi=lp64d’。使用readelf查看header信息，生成的是elf64。

?  riscv64-linux-gnu-gcc -v -nostdlib -fno-builtin -g -Wall test.s -o test
...
COLLECT_GCC_OPTIONS='-v' '-nostdlib' '-fno-builtin' '-g' '-Wall' '-o' 'test' '-march=rv64gc' '-mabi=lp64d' '-misa-spec=20191213' '-march=rv64imafdc_zicsr_zifencei' '-dumpdir' 'test.'
...

?  tmp riscv64-linux-gnu-readelf -h test
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              DYN (Position-Independent Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x29a
  Start of program headers:          64 (bytes into file)
  Start of section headers:          5536 (bytes into file)
  Flags:                             0x5, RVC, double-float ABI
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         8
  Size of section headers:           64 (bytes)
  Number of section headers:         19
  Section header string table index: 18

• 基于以上信息，生成32二进制选项可设置为 “-march=rv32ima -mabi=ilp32”。

?  riscv64-linux-gnu-gcc -nostdlib -fno-builtin -march=rv32ima -mabi=ilp32 -g -Wall test.s -o test
?  riscv64-linux-gnu-readelf -h test
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:                              DYN (Position-Independent Executable file)
  Machine:                           RISC-V
  Version:                           0x1
  Entry point address:               0x1b8
  Start of program headers:          52 (bytes into file)
  Start of section headers:          5220 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         8
  Size of section headers:           40 (bytes)
  Number of section headers:         19
  Section header string table index: 18