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Cc: Paul Mackerras <paulus@samba.org>, heiko.carstens@de.ibm.com,
       horms@verge.net.au, linux-kernel@vger.kernel.org, rpjday@mindspring.com,
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       akpm@linux-foundation.org, Nick Piggin <nickpiggin@yahoo.com.au>,
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       Chris Snook <csnook@redhat.com>, Herbert Xu <herbert.xu@redhat.com>,
       davem@davemloft.net, wensong@linux-vs.org, wjiang@resilience.com
From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
Date: Sat, 18 Aug 2007 00:29:30 +0200
To: Linus Torvalds <torvalds@linux-foundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> In a reasonable world, gcc should just make that be (on x86)
>
> 	addl $1,i(%rip)
>
> on x86-64, which is indeed what it does without the volatile. But with 
> the
> volatile, the compiler gets really nervous, and doesn't dare do it in 
> one
> instruction, and thus generates crap like
>
>         movl    i(%rip), %eax
>         addl    $1, %eax
>         movl    %eax, i(%rip)
>
> instead. For no good reason, except that "volatile" just doesn't have 
> any
> good/clear semantics for the compiler, so most compilers will just 
> make it
> be "I will not touch this access in any way, shape, or form". Including
> even trivially correct instruction optimization/combination.

It's just a (target-specific, perhaps) missed-optimisation kind
of bug in GCC.  Care to file a bug report?

> but is
> (again) something that gcc doesn't dare do, since "i" is volatile.

Just nobody taught it it can do this; perhaps no one wanted to
add optimisations like that, maybe with a reasoning like "people
who hit the go-slow-in-unspecified-ways button should get what
they deserve" ;-)


Segher

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