From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ftp.linux.org.uk>
Subject: Re: fun with ?:
Date: Wed, 23 May 2007 15:32:02 +0100
Message-ID: <20070523143202.GY4095@ftp.linux.org.uk>
References: <20070519025249.GZ4095@ftp.linux.org.uk> <4653633B.3000000@freedesktop.org> <20070522224619.GI4095@ftp.linux.org.uk> <46537BC1.9000808@freedesktop.org> <20070523000234.GJ4095@ftp.linux.org.uk> <20070523142544.GB2547@daikokuya.co.uk>
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To: Neil Booth <neil@daikokuya.co.uk>
Cc: Josh Triplett <josh@freedesktop.org>, linux-sparse@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>

On Wed, May 23, 2007 at 11:25:44PM +0900, Neil Booth wrote:
> I respect you too much Al to doubt you, but I do warn you that
> getting the rules for integer constant expressions right in C is
> harder than it looks.  GCC is not very close.  The immediate
> cast of float bit painful in recursive descent parsers.  I've managed
> to find cases where Comeau's online compiler doesn't get it right,
> and they're pretty good.
> 
> I have a test suite that tests these things to an unhealthy level of
> pedantry for my own implementation (the only one I know passes the lot,
> of course 8-); happy to run sparse when you've finished if you like.

gcc integer constant expressions handling is a bad joke.

extern int n;
struct {
	int x : 1 + n - n;
} y;

passes with -pedantic -std=c99.  Replacing that with 1 + n - n + n - n
is still OK with gcc; 1 + n + n - n - n is not.

So that's hardly an example of, well, anything.