From: Andi Kleen <andi@firstfloor.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
Jan Beulich <JBeulich@novell.com>,
mingo@elte.hu, tglx@linutronix.de, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86-64: adjust ia32entry branches for better static prediction
Date: Mon, 26 Oct 2009 09:28:21 +0100 [thread overview]
Message-ID: <878weyy2xm.fsf@basil.nowhere.org> (raw)
In-Reply-To: <4ADD65C5.5000206@zytor.com> (H. Peter Anvin's message of "Tue, 20 Oct 2009 16:24:53 +0900")
"H. Peter Anvin" <hpa@zytor.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@linux.intel.com -- Speaking for myself only.
prev parent reply other threads:[~2009-10-26 8:28 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-10-20 7:10 [PATCH] x86-64: adjust ia32entry branches for better static prediction Jan Beulich
2009-10-20 7:21 ` Eric Dumazet
2009-10-20 7:24 ` H. Peter Anvin
2009-10-26 8:28 ` Andi Kleen [this message]
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