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

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Date:	Fri, 22 Jun 2007 22:15:49 -0700 (PDT)
From:	Denis Cheng <crquan@...il.com>
To:	trivial@...nel.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] trivial: the memset operation on a automatic array
	variable should be optimized out by data initialization

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.

and C standard guaranteed all the unspecified data field initialized to zero.

Signed-off-by: Denis Cheng <crquan@...il.com>

---
After comments in the former threads:
http://lkml.org/lkml/2007/6/18/119
http://lkml.org/lkml/2007/6/18/48

On 6/18/07, Jan Engelhardt <jengelh@...putergmbh.de> wrote:
> 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] = {};

>         Jan
I also think this style of zero initialization would be better.
so the patch is little different:

--- arch/x86_64/mm/init.c.orig	2007-06-07 10:08:04.000000000 +0800
+++ arch/x86_64/mm/init.c	2007-06-23 13:12:26.000000000 +0800
@@ -406,8 +406,8 @@ 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] = {};
+
 	max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
 	max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
 	max_zone_pfns[ZONE_NORMAL] = end_pfn;
-
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