linux-kernel - Re: Is gcc thread-unsafe?

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Message-ID: <alpine.LFD.0.999.0710261020250.30120@woody.linux-foundation.org>
Date:	Fri, 26 Oct 2007 10:25:29 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Haley <aph@...hat.com>
cc:	Bart Van Assche <bart.vanassche@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Is gcc thread-unsafe?

On Fri, 26 Oct 2007, Andrew Haley wrote:
>
> Bart Van Assche writes:
> 
>  > Andrew, do you know whether gcc currently contains any optimization
>  > that interchanges the order of accesses to non-volatile variables
>  > and function calls ?
> 
> It sure does.

Note that doing so is perfectly fine.

But only for local variables that haven't had their addresses taken.

The fact is, those kinds of variables really *are* special. They are 
provably not accessible from any other context, and re-ordering them (or 
doing anything AT ALL to them - the most basic and very important 
optimization is caching them in registers, of course) is always purely an 
internal compiler issue.

But if gcc re-orders functions calls with *other* memory accesses, gcc is 
totally broken. I doubt it does that. It would break on all but the most 
trivial programs, and it would be a clear violation of even standard C.

HOWEVER: the bug that started this thread isn't even "reordering 
accesses", it's *adding* accesses that weren't there (and please don't mix 
this up with "volatile", since volatile is a totally unrelated issue and 
has nothing what-so-ever to do with anything).

		Linus
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