netdev - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-ID: <Pine.LNX.4.64.0708170840270.25474@anakin>
Date:	Fri, 17 Aug 2007 08:42:22 +0200 (CEST)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, satyam@...radead.org,
	clameter@....com, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert.xu@...hat.com>, davem@...emloft.net,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

On Thu, 16 Aug 2007, Linus Torvalds wrote:
> On Fri, 17 Aug 2007, Paul Mackerras wrote:
> > I'm really surprised it's as much as a few K.  I tried it on powerpc
> > and it only saved 40 bytes (10 instructions) for a G5 config.
> 
> One of the things that "volatile" generally screws up is a simple
> 
> 	volatile int i;
> 
> 	i++;
> 
> which a compiler will generally get horribly, horribly wrong.
> 
> 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.

Apart from having to fetch more bytes for the instructions (which does
matter), execution time is probably the same on modern processors, as they
convert the single instruction to RISC-style load, modify, store anyway.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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