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

Ammar Faizi
2022-03-23T06:29:48Z

On 3/22/22 8:37 PM, David Laight wrote:
> dunno, 'asm' register variables are rather more horrid and
> should probably only be used (for asm statements) when there aren't
> suitable register constraints.
>
> (I'm sure there is a comment about that in the gcc docs.)

^ Hey David, yes you're right, that is very interesting...

I hit a GCC bug when playing with syscall6() implementation here.

Using register variables for all inputs for syscall6() causing GCC 11.2
stuck in an endless loop with 100% CPU usage. Reproducible with several
versions of GCC.

In GCC 6.3, the syscall6() implementation above yields ICE (Internal
Compiler Error):
```
: In function '__sys_mmap':
:35:1: error: unable to find a register to spill
}
^
:35:1: error: this is the insn:
(insn 14 13 30 2 (set (reg:SI 95 [92])
(mem/c:SI (plus:SI (reg/f:SI 16 argp)
(const_int 28 [0x1c])) [1 offset+0 S4 A32])) :33 86 {*movsi_internal}
(expr_list:REG_DEAD (reg:SI 16 argp)
(nil)))
:35: confused by earlier errors, bailing out
Compiler returned: 1
```
See the full show here: https://godbolt.org/z/dYeKaYWY3

Using the appropriate constraints, it compiles nicely, now it looks
like this:
```
#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) \
({ \
long _eax = (long)(num); \
long _arg6 = (long)(arg6); /* Always 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"(_eax) /* %eax */ \
: "b"(arg1), /* %ebx */ \
"c"(arg2), /* %ecx */ \
"d"(arg3), /* %edx */ \
"S"(arg4), /* %esi */ \
"D"(arg5), /* %edi */ \
[_arg6]"m"(_arg6) /* memory */ \
: "memory", "cc" \
); \
_eax; \
})
```
Link: https://godbolt.org/z/ozGbYWbPY

Will use that in the next patchset version.

--
Ammar Faizi