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Re: [PATCH 5/6] Optimize i386 syscall inlining
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 15 Oct 2015 05:22:27 -0700
- Subject: Re: [PATCH 5/6] Optimize i386 syscall inlining
- Authentication-results: sourceware.org; auth=none
- References: <20151012232028 dot GC8797 at intel dot com>
On Mon, Oct 12, 2015 at 4:20 PM, H.J. Lu <hongjiu.lu@intel.com> wrote:
> Since GCC 5 and above can properly spill %ebx when needed, we can inline
> syscalls with 6 arguments if GCC 5 or above is used to compile glibc.
> This patch rewrites INTERNAL_SYSCALL macros and skips __libc_do_syscall
> for GCC 5.
>
> For sysdeps/unix/sysv/linux/i386/brk.c, with -O2 -march=i686
> -mtune=generic, GCC 5.2 now generates:
>
> <__brk>:
> 0: push %ebx
> 1: mov $0x2d,%eax
> 6: mov 0x8(%esp),%ebx
> a: call b <__brk+0xb> b: R_386_PC32 __x86.get_pc_thunk.dx
> f: add $0x2,%edx 11: R_386_GOTPC _GLOBAL_OFFSET_TABLE_
> 15: call *%gs:0x10
> 1c: mov 0x0(%edx),%edx 1e: R_386_GOT32 __curbrk
> 22: cmp %eax,%ebx
> 24: mov %eax,(%edx)
> 26: ja 30 <__brk+0x30>
> 28: xor %eax,%eax
> 2a: pop %ebx
> 2b: ret
>
> instead of
>
> <__brk>:
> 0: push %ebx
> 1: mov 0x8(%esp),%ecx
> 5: call 6 <__brk+0x6> 6: R_386_PC32 __x86.get_pc_thunk.bx
> a: add $0x2,%ebx c: R_386_GOTPC _GLOBAL_OFFSET_TABLE_
> 10: xchg %ecx,%ebx
> 12: mov $0x2d,%eax
> 17: call *%gs:0x10
> 1e: xchg %ecx,%ebx
> 20: mov %eax,%edx
> 22: mov 0x0(%ebx),%eax 24: R_386_GOT32 __curbrk
> 28: mov %edx,(%eax)
> 2a: xor %eax,%eax
> 2c: cmp %edx,%ecx
> 2e: ja 38 <__brk+0x38>
> 30: pop %ebx
> 31: ret
>
> The new one is shorter by 2 instructions.
>
> * sysdeps/unix/sysv/linux/i386/libc-do-syscall.S
> (__libc_do_syscall): Defined only if !__GNUC_PREREQ (5,0).
> * sysdeps/unix/sysv/linux/i386/sysdep.h: Define assembler macros
> only if !__GNUC_PREREQ (5,0).
> (INTERNAL_SYSCALL_MAIN_6): Optimize for GCC 5.
> (INTERNAL_SYSCALL_MAIN_INLINE): Likewise.
> (INTERNAL_SYSCALL_NCS): Likewise.
> (LOADREGS_0): New macro for GCC 5.
> (ASMARGS_0): Likewise.
> (LOADREGS_1): Likewise.
> (ASMARGS_1): Likewise.
> (LOADREGS_2): Likewise.
> (ASMARGS_2): Likewise.
> (LOADREGS_3): Likewise.
> (ASMARGS_3): Likewise.
> (LOADREGS_4): Likewise.
> (ASMARGS_4): Likewise.
> (LOADREGS_5): Likewise.
> (ASMARGS_5): Likewise.
> (LOADREGS_6): Likewise.
> (ASMARGS_6): Likewise.
I am checking in this patch now.
--
H.J.