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

Ammar Faizi
2022-03-22T12:03:07Z

On 3/22/22 6:39 PM, David Laight wrote:
>> + __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"(eax)
>> + : "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),
>
> Does having "=a" for an output constraint and "a" for an input
> constraint actually DTRT?
> There is a special syntax for tying input and output to
> the same register.
> Or you could use "+a"(nr_rval) and 'return nr_rval'.

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?

--
Ammar Faizi