Messages in this thread | | | Date | Wed, 23 Mar 2022 13:29:39 +0700 | From | Ammar Faizi <> | 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
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