[PATCH] Optimize i386 syscall inlining for GCC 5

[H.J. Lu]

On Wed, Aug 12, 2015 at 4:37 PM, Zack Weinberg <zackw@panix.com> wrote:
> If I'm reading that right, it's still not quite optimal; there's an
> unnecessary register shuffle after the system call... better would be
>
>         push   %ebx
>         mov    $0x2d,%eax
>         mov    0x8(%esp),%ebx
>         call   __x86.get_pc_thunk.cx
>         add    $_GLOBAL_OFFSET_TABLE_,%ecx
>         call   *%gs:0x10
>         mov    __curbrk(%ecx),%edx
>         mov    %eax,(%edx)
>         cmp    %eax,%ebx
>         ja     1f
>         xor    %eax,%eax
>         pop    %ebx
>         ret
> 1:
>         ; set errno and return -1
>
> I think the compiler might've done it the way it did because then it
> could hoist the xor %eax,%eax above the comparison, but the branch is
> so unlikely that that's not a good choice.
>
> But also, in context, I think it would be more helpful if you could
> show the generated assembly for a *six*-argument syscall, ne?

I am looking into overhauling i386 syscall implementation.  I
think I can do much better for libc.so.

-- 
H.J.