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Date:   Wed, 23 Mar 2022 13:29:39 +0700
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From:   Ammar Faizi <ammarfaizi2@gnuweeb.org>
To:     David Laight <David.Laight@ACULAB.COM>, 'Willy Tarreau' <w@1wt.eu>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
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Subject: Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6
 arguments
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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):
```
   <source>: In function '__sys_mmap':
   <source>:35:1: error: unable to find a register to spill
    }
    ^
   <source>: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])) <source>:33 86 {*movsi_internal}
        (expr_list:REG_DEAD (reg:SI 16 argp)
           (nil)))
   <source>: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