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

Willy Tarreau
2022-03-22T13:34:24Z

On Tue, Mar 22, 2022 at 08:26:37PM +0700, Ammar Faizi wrote:
> On 3/22/22 7:13 PM, Willy Tarreau wrote:
> > 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 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.
>
> I turned out GCC refuses to use "rm" if we compile without -fomit-frame-pointer
> (e.g. without optimization / -O0). So I will still use "m" here.

OK that's fine. then you can probably simplify it like this:

long _arg6 = (long)(arg6); /* Might be in memory */ \
\
asm volatile ( \
"pushl %%ebp\n\t" \
"movl %[_arg6], %%ebp\n\t" \
"int $0x80\n\t" \
"popl %%ebp\n\t" \
: "=a"(_ret) \
: "r"(_num), "r"(_arg1), "r"(_arg2), "r"(_arg3), \
"r"(_arg4),"r"(_arg5), [_arg6]"m"(_arg6) \
: "memory", "cc" \
); \

See ? no more push, no more addl, direct load from memory.

Willy