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

[David Laight <David.Laight@ACULAB.COM>]

From: Willy Tarreau
> Sent: 22 March 2022 12:14
> 
> On Tue, Mar 22, 2022 at 07:02:53PM +0700, Ammar Faizi wrote:
> > 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?
> 
> Hmmm indeed that comes back to the existing constructs and is certainly
> more in line with the rest of the code (plus it will not be affected by
> -O0).

I'd add an 'always_inline' to the function.
That will force inline even with -O0.

> I seem to remember a register allocation issue which kept me away from
> implementing it this way on i386 back then, but given that my focus was
> not as much on i386 as it was on other platforms, it's likely that I have
> not insisted too much and not tried this one which looks like the way to
> go to me.

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.)

	David

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