用GCC扩展ASM将Intel ASM转换为AT&T ASM

Question

在过去的两天里，我一直在学习AT&T内联装配，但是我在转换这个组件时遇到了一些问题：

static char vendername[50] = {0};
_asm {
    mov     eax, 0
    cpuid
    mov     dword ptr [vendername], ebx
    mov     dword ptr [vendername+4], edx
    mov     dword ptr [vendername+8], ecx
}

以下是我的尝试：

static char vendername[50] = {0};
__asm__(
    "movl $0,%%eax \n"
    "cpuid \n"
    "movl %%ebx, %[vendername] \n"
    "movl %%edx, %[vendername+$4] \n"
    "movl %%ecx, %[vendername+$8] \n"
    :"=r"(vendername) //user vendername as output
    :[vendername]"m"(vendername) //use vendername as input
    :"eax","ebx","edx","ecx" // clobbers those regs
);

这个代码不起作用，gcc给了我以下错误：

error: undefined named operand ‘vendername+4’

和

error: undefined named operand ‘vendername+8’

如何将代码从Intel程序集转换为AT&T程序集？

Answer 1

你可以这样做：

#include <stdio.h>

int main(int argc, char **argv) {
  static char vendername[50] = {0};

  __asm__ __volatile__ (
    "movl $0, %%eax\n"
    "cpuid\n"
    "movl %%ebx, %0\n"
    "movl %%edx, %0 + 4\n"
    "movl %%ecx, %0 + 8\n"
    :"=m"(vendername)
    :
    :"eax", "ebx", "edx", "ecx"
  );

  printf("%s\n", vendername);

  return 0;
}

Answer 2

经验法则是，如果您曾经用内联asm编写mov，您可能做错了:)编译器可以自己为您加载/存储值。

int dummy;
union {
    char text[12];
    struct {
        int ebx;
        int edx;
        int ecx;
    };
} vendorname;
__asm__(
    "cpuid \n"
    : "=b" (vendorname.ebx), "=d" (vendorname.edx), "=c" (vendorname.ecx), "=a" (dummy)
    : "a" (0)
);

请注意，由于必须将3个dword解释为一个字符串，情况变得复杂了。