linux-kernel - Re: the memset operation on a automatic array variable can be removed by data Initialization

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Message-ID: <Pine.LNX.4.64.0706181341070.5731@fbirervta.pbzchgretzou.qr>
Date:	Mon, 18 Jun 2007 13:42:30 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Denis Cheng <crquan@...il.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: the memset operation on a automatic array variable can be removed
 by data Initialization


On Jun 18 2007 00:43, Denis Cheng wrote:
>Subject: the memset operation on a automatic array variable can be removed by
>    data Initialization
>
>--- arch/x86_64/mm/init.c	2007-06-07 10:08:04.000000000 +0800
>+++ /tmp/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 cost is the same. "= {0}" is transformed into a bunch of movs,
or a rep mov, (At least for x86), so is equivalent to memset (which
will get transformed to __builtin_memset anyway). So I wonder
what this really buys.

And, you do not even need the zero. Just write
	...[MAX_NR_ZONES] = {};

> 	max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
> 	max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
> 	max_zone_pfns[ZONE_NORMAL] = end_pfn;


	Jan
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