linux-kernel - Re: [PATCH] Use more gcc extensions in the Linux headers

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Date:	Sat, 10 Mar 2007 15:01:32 +0100 (MET)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Nigel Cunningham <nigel@...el.suspend2.net>
cc:	Valdis.Kletnieks@...edu, Rusty Russell <rusty@...tcorp.com.au>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Use more gcc extensions in the Linux headers

On Mar 10 2007 16:19, Nigel Cunningham wrote:
>On Fri, 2007-03-09 at 23:03 -0500, Valdis.Kletnieks@...edu wrote:
>> On Sat, 10 Mar 2007 09:57:32 +1100, Rusty Russell said:
>> 
>> > +/* GCC is awesome. */
>> >  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0])			      \
>> >  	+ sizeof(typeof(int[1 - 2*!!__builtin_types_compatible_p(typeof(arr), \
>> >  		 typeof(&arr[0]))]))*0)
>> 
>> -/* GCC is awesome. */
>> +/* GCC leaves me speechless. */
>
>A speechless Rusty would be horrible. That said, it would be nice if the
>comment was something more like the normal Rusty pearl of wisdom. I
>understand the first part, but have no idea was + sizeof(typeof(int[....
>does...

IOCCC entry candidate.

Simple. !!__builtin_types_compatible_p, as it sounds, will return 1 if
both types are compatible. If they are, then '1-2*!!_builtin..' will
produce -1, then - surprisingly - sizeof(typeof(int[-1])) seems strange to
me and should throw the error.

If the types are not compatible, compat_p returns 0. !! will turn it into
0. 2* will turn it into 0. 1-0 is 1. sizeof(typeof(int[1])) is valid, and
*0 will compile it away.

So in case they _ARE_ compatible, we get the compile error, as far as I
can see it. There's a ! too much in the !!_builtin line. Now we know why
such patches are dangerous.

What's more, a test program does not even fail when the types are
incompatible. (Or it's also wrong):

#include <stdio.h>
struct foo {
        int x, y, z;
};
struct bar {
        const char *fol;
};
#define c(x, y)  \
        sizeof(typeof(int[1 - \
         2*!!__builtin_types_compatible_p(typeof(x), typeof(y))]))
int main(void)
{
        printf("%d\n", c(struct foo *, struct bar *));
	printf("%d\n", c(int*, int*));
}

Both printf()s throw a compile time error.

Jan
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