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

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Message-ID: <20070627121948.GN21478@ftp.linux.org.uk>
Date:	Wed, 27 Jun 2007 13:19:48 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Neil Booth <neil@...kokuya.co.uk>
Cc:	Josh Triplett <josh@...edesktop.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 08:52:45PM +0900, Neil Booth wrote:
> Al Viro wrote:-
> 
> > 
> > sparse simply doesn't check that.  We don't have anything resembling
> > support of VLA.  Note that check for integer constant expression
> > has nothing to do with that;
> > 
> > 	int x[(int)(0.6 + 0.6)];
> > 
> > is valid (if stupid).
> 
> It isn't valid; it fails the test twice.  Both 0.6 are not "immediate
> operands of casts".  Their sum is, but that's irrelevant.
> Therefore the dimension is not an ICE and a diagnostic is required.

Egads...  After rereading that...  What a mess.

int foo(void)
{
	static int a[1][0,2];
}

is, AFAICS, allowed.  Reason:
	int a[0,2]
is a VLA due to 6.7.5.2[4] (0,2 is not an ICE).  However, due to the language
in the same section,
	int a[1][0,2]
is *not* a VLA, since (a) 2 is an ICE and (b) its element type "has a known
constant size" (it does - the value of 0,2 is certainly guaranteed to be 2).
I.e., it's VM type, but not a VLA.  I.e. only the first part of 6.7.5.2[2]
applies and we are actually fine.

	So we can have a static single-element array of int [0,2], but
not a plain static int [0,2].  Lovely, that...
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