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

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Date:	Tue, 20 Oct 2009 16:24:53 +0900
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	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

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
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