[PATCH 5/6] Optimize i386 syscall inlining

[H.J. Lu]

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.