linux-kernel - Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

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Message-ID: <7943.1186807115@turing-police.cc.vt.edu>
Date:	Sat, 11 Aug 2007 00:38:35 -0400
From:	Valdis.Kletnieks@...edu
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	paulmck@...ux.vnet.ibm.com, heiko.carstens@...ibm.com,
	horms@...ge.net.au, linux-kernel@...r.kernel.org,
	csnook@...hat.com, rpjday@...dspring.com, netdev@...r.kernel.org,
	ak@...e.de, cfriesen@...tel.com, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, schwidefsky@...ibm.com,
	davem@...emloft.net, wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

On Sat, 11 Aug 2007 02:38:40 +0200, Segher Boessenkool said:
> >> That means GCC cannot compile Linux; it already optimises
> >> some accesses to scalars to smaller accesses when it knows
> >> it is allowed to.  Not often though, since it hardly ever
> >> helps in the cost model it employs.
> >
> > Please give an example code snippet + gcc version + arch
> > to back this up.
> 
> 	unsigned char f(unsigned long *p)
> 	{
> 	        return *p & 1;
> 	}

Not really valid, because it's still able to do one atomic access to
compute the result.

Now, if you had found an example where it converts a 32-bit atomic access into
2 separate 16-bit accesses that weren't atomic as a whole....

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