Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

On 3/22/22 7:02 PM, Ammar Faizi wrote:
> Well, I agree with your previous email. Now since we no longer use a #pragma
> optimize with -fomit-frame-pointer, the function is not needed. I propose the
> following macro (this is not so much different with other my_syscall macro),
> expect the 6th argument can be in reg or mem.
> 
> The "rm" constraint here gives the opportunity for the compiler to use %ebp
> instead of memory if -fomit-frame-pointer is turned on.
> 
> #define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) \
> ({                                                         \
>      long _ret;                                             \
>      register long _num asm("eax") = (num);                 \
>      register long _arg1 asm("ebx") = (long)(arg1);         \
>      register long _arg2 asm("ecx") = (long)(arg2);         \
>      register long _arg3 asm("edx") = (long)(arg3);         \
>      register long _arg4 asm("esi") = (long)(arg4);         \
>      register long _arg5 asm("edi") = (long)(arg5);         \
>      long _arg6 = (long)(arg6); /* Might be in memory */    \
>                                                             \
>      asm volatile (                                         \
>          "pushl  %[_arg6]\n\t"                              \
>          "pushl  %%ebp\n\t"                                 \
>          "movl   4(%%esp), %%ebp\n\t"                       \
>          "int    $0x80\n\t"                                 \
>          "popl   %%ebp\n\t"                                 \
>          "addl   $4,%%esp\n\t"                              \
>          : "=a"(_ret)                                       \
>          : "r"(_num), "r"(_arg1), "r"(_arg2), "r"(_arg3),   \
>            "r"(_arg4),"r"(_arg5), [_arg6]"rm"(_arg6)        \
>          : "memory", "cc"                                   \
>      );                                                     \
>      _ret;                                                  \
> })
> 
> What do you think?
> 

For the following code:

   int main()
   {
     mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
     return 0;
   }

GCC generates this:

   00001000 <main>:
     1000: push   %ebp
     1001: mov    $0xc0,%eax
     1006: mov    $0x1000,%ecx
     100b: mov    $0x3,%edx
     1010: push   %edi
     1011: xor    %ebp,%ebp
     1013: mov    $0xffffffff,%edi
     1018: push   %esi
     1019: mov    $0x22,%esi
     101e: push   %ebx
     101f: xor    %ebx,%ebx
     1021: push   %ebp        <--- arg6 here
     1022: push   %ebp
     1023: mov    0x4(%esp),%ebp
     1027: int    $0x80
     1029: pop    %ebp
     102a: add    $0x4,%esp
     102d: xor    %eax,%eax
     102f: pop    %ebx
     1030: pop    %esi
     1031: pop    %edi
     1032: pop    %ebp
     1033: ret

-- 
Ammar Faizi