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

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Date:	Tue, 20 Oct 2009 09:21:32 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86-64: adjust ia32entry branches for better static	
 prediction

Jan Beulich a écrit :
> Swap branch pairs so that the static branch prediction logic would
> consider the syscall-number-in-range case the taken path.
> 
> Signed-off-by: Jan Beulich <jbeulich@...ell.com>
> 
> ---
>  arch/x86/ia32/ia32entry.S |   18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> --- 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...

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