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

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Message-ID: <20070619132718.GC12950@stusta.de>
Date:	Tue, 19 Jun 2007 15:27:18 +0200
From:	Adrian Bunk <bunk@...sta.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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 10:07:21AM -0700, Jeremy Fitzhardinge wrote:
> 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.
> 
> How does the generated code change?  Does gcc do something stupid like
> statically allocate a prototype structure full of zeros, and then memcpy
> it in?  Or does it generate a series of explicit assignments for each
> member?  Or does it generate a memset anyway?
> 
> Seems to me that this gives gcc the opportunity to be more stupid, and
> the only right answer is what we're doing anyway.

I checked with gcc 4.2, and gcc is quite clever:

If an array is big, gcc uses memset.
If an array is small, gcc does it directly in assembler.

>     J

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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