[PATCH] trivial: the memset operation on a automatic array variable should be optimized out by data initialization
- From: Denis Cheng <crquan@xxxxxxxxx>
- Date: Fri, 22 Jun 2007 22:15:49 -0700 (PDT)
From: Denis Cheng <crquan@xxxxxxxxx>
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@xxxxxxxxx>
---
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@xxxxxxxxxxxxxxx> 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] = {};
JanI 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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