From: "H. Peter Anvin" <hpa@zytor.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: 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: Tue, 20 Oct 2009 16:24:53 +0900 [thread overview]
Message-ID: <4ADD65C5.5000206@zytor.com> (raw)
In-Reply-To: <4ADD64FC.3070006@gmail.com>
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
next prev parent reply other threads:[~2009-10-20 7:25 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-10-20 7:10 Jan Beulich
2009-10-20 7:21 ` Eric Dumazet
2009-10-20 7:24 ` H. Peter Anvin [this message]
2009-10-26 8:28 ` Andi Kleen
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