linux-kernel - Re: [PATCH] trivial: the memset operation on a automatic array variable should be optimized out by data initialization

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Date:	Mon, 18 Jun 2007 22:24:32 +0800
From:	WANG Cong <xiyou.wangcong@...il.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	Denis Cheng <crquan@...il.com>, trivial@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] trivial: the memset operation on a automatic array variable should be optimized out by data initialization

On Mon, Jun 18, 2007 at 06:25:17AM -0400, Robert P. J. Day wrote:
>On Mon, 18 Jun 2007, Denis Cheng wrote:
>
>> From: Denis Cheng <crquan@...il.com>
>>
>> the explicit memset call could be optimized out by data initialization,
>> thus all the fill working can be done by the compiler implicitly.
>>
>> Signed-off-by: Denis Cheng <crquan@...il.com>
>>
>> ---
>> Is there some comments on this?
>>
>> --- arch/x86_64/mm/init.c.orig	2007-06-07 10:08:04.000000000 +0800
>> +++ arch/x86_64/mm/init.c	2007-06-18 14:43:15.000000000 +0800
>> @@ -406,8 +406,7 @@ void __cpuinit zap_low_mappings(int cpu)
>>  #ifndef CONFIG_NUMA
>>  void __init paging_init(void)
>>  {
>> -	unsigned long max_zone_pfns[MAX_NR_ZONES];
>> -	memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
>> +	unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
>
>the drawback i see to this is that it's *visually* misleading.  it
>*appears* that what the programmer is trying to do is just initialize
>the first element, not all of them -- regardless of the actual effect.
>

C standard can guarantee all of them are initialized, and this is _not_
hard to understand for a good C programmer.

Ack for Denis's change.

Regards!

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