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

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Message-ID: <20070815224433.GQ9645@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2007 15:44:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Satyam Sharma <satyam@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rpjday@...dspring.com, netdev@...r.kernel.org, ak@...e.de,
	cfriesen@...tel.com, Heiko Carstens <heiko.carstens@...ibm.com>,
	jesper.juhl@...il.com, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	clameter@....com, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Wed, Aug 15, 2007 at 11:05:35PM +0200, Segher Boessenkool wrote:
> >>No; compilation units have nothing to do with it, GCC can optimise
> >>across compilation unit boundaries just fine, if you tell it to
> >>compile more than one compilation unit at once.
> >
> >Last I checked, the Linux kernel build system did compile each .c file
> >as a separate compilation unit.
> 
> I have some patches to use -combine -fwhole-program for Linux.
> Highly experimental, you need a patched bleeding edge toolchain.
> If there's interest I'll clean it up and put it online.
> 
> David Woodhouse had some similar patches about a year ago.

Sounds exciting...  ;-)

> >>>In many cases, the compiler also has to assume that
> >>>msleep_interruptible()
> >>>might call back into a function in the current compilation unit, thus
> >>>possibly modifying global static variables.
> >>
> >>It most often is smart enough to see what compilation-unit-local
> >>variables might be modified that way, though :-)
> >
> >Yep.  For example, if it knows the current value of a given such local
> >variable, and if all code paths that would change some other variable
> >cannot be reached given that current value of the first variable.
> 
> Or the most common thing: if neither the address of the translation-
> unit local variable nor the address of any function writing to that
> variable can "escape" from that translation unit, nothing outside
> the translation unit can write to the variable.

But there is usually at least one externally callable function in
a .c file.

> >At least given that gcc doesn't know about multiple threads of 
> >execution!
> 
> Heh, only about the threads it creates itself (not relevant to
> the kernel, for sure :-) )

;-)

							Thanx, Paul
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