linux-kernel - Re: [PATCH 16/16] fix handling of integer constant expressions

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Message-ID: <20070627180408.GC21478@ftp.linux.org.uk>
Date:	Wed, 27 Jun 2007 19:04:09 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Neil Booth <neil@...kokuya.co.uk>,
	Josh Triplett <josh@...edesktop.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

On Wed, Jun 27, 2007 at 10:45:55AM -0700, Linus Torvalds wrote:
> 
> 
> On Wed, 27 Jun 2007, Al Viro wrote:
> > 
> > Eh...  I'd say that my variant for offsetof() is simply better - it usually
> > directly turns into EXPR_VALUE, right in place, without rather convoluted
> > work.  Aside of "should such cast be a constant integer expression"...
> 
> Umm. But sparse is meant to parse C code. Which very much includes *other* 
> projects.
> 
> The kernel, for example, has its own offsetof. And yes, these days we use 
> "__compiler_offsetof()", but we used to do
> 
> 	#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
> 
> and I seriously doubt that the kernel is the only one doing things like 
> that.

You can't have it both way, really.  If we are talking about annotating
a codebase we _can_ annotate, that one is not a problem at all.  If we
are talking about vanilla C project that never heard about sparse...
We can define whatever extensions we like, but such project has to
cope with whatever C compilers they had been using.

So "sparse believes that this defintion of offsetof can be used as
array size" will mean fsck-all outside of #ifdef __CHECKER__ and
under such ifdef we can always define it to builtin; if anything,
that will be faster and easier on sparse.
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