Re: [PATCH] x86-64: adjust ia32entry branches for better static prediction

[H. Peter Anvin]

On 10/20/2009 04:21 PM, Eric Dumazet wrote:
> > --- linux-2.6.32-rc5/arch/x86/ia32/ia32entry.S	2009-10-19 13:12:43.000000000 +0200
> > +++ 2.6.32-rc5-x86_64-ia32-syscall-trace-branch-dir/arch/x86/ia32/ia32entry.S	2009-10-19 09:36:39.000000000 +0200
> > @@ -249,9 +249,9 @@ sysenter_tracesys:
> >   	call	syscall_trace_enter
> >   	LOAD_ARGS32 ARGOFFSET  /* reload args from stack in case ptrace changed it */
> >   	RESTORE_REST
> > -	cmpl	$(IA32_NR_syscalls-1),%eax
> > -	ja	int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */
> > -	jmp	sysenter_do_call
> > +	cmpl	$IA32_NR_syscalls,%eax
> > +	jb	sysenter_do_call
> > +	jmp	int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */
> >   	CFI_ENDPROC
>
> on cmov enabled build, we could use cmov instruction to load eax with number
> that calls enosys_syscall(), to avoid loading -ENOSYS,RAX
> and these branches...

It's x86-64, cmov is guaranteed.

	-hpa
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/