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

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Message-ID: <20070624233314.GB10398@flower.upol.cz>
Date:	Mon, 25 Jun 2007 01:33:14 +0200
From:	Oleg Verych <olecom@...wer.upol.cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	rae l <crquan@...il.com>, trivial@...nel.org,
	linux-kernel@...r.kernel.org
Subject: memset() with zeroes (Re: [PATCH] trivial: the memset operation on a automatic array variable should be optimized out by data initialization)

On Sun, Jun 24, 2007 at 03:15:17PM -0700, Arjan van de Ven wrote:
> 
> > > I think all these benefits are the gcc's __builtin_memset optimization
> > > than the explicit call to memset.
> > 
> > ... or from complex memset() implementation (some chips even didn't do
> > `rep' fast enough somehow). Maybe code like below will be acceptable for
> > both optimizers and maintainers?
> 
> 
> we should just alias our memset to the __builtin one, and then provide a
> generic one from lib/ for the cases gcc needs to do a fallback.

In x86_64 there's infrastructure to check and select right memset().
Therefor it's need, i think.

But if one will took a look at usage, zero memset() optimization becomes
obvious, one argument off -- one reg is free from clobbering.

|-*-
flower-:22-rc4-mm2/arch/x86_64$ grep memset -R . | grep "[ 0,]0," | wc -l
42
flower-:22-rc4-mm2/arch/x86_64$
flower-:22-rc4-mm2/arch/x86_64$ cd ..
flower-:22-rc4-mm2/arch$ grep memset -R . | grep "[ 0,]0," | wc -l
735
flower-:22-rc4-mm2/arch$
flower-:22-rc4-mm2$ grep memset -R . | grep "[ 0,]0," | wc -l
6679
flower-:22-rc4-mm2$  
|-*-
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