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

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Message-ID: <878weyy2xm.fsf@basil.nowhere.org>
Date:	Mon, 26 Oct 2009 09:28:21 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jan Beulich <JBeulich@...ell.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86-64: adjust ia32entry branches for better static	 prediction

"H. Peter Anvin" <hpa@...or.com> writes:

> 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.

But it's also not necessarily faster.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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