[PATCH] Use more gcc extensions in the Linux headers

[PATCH] Use more gcc extensions in the Linux headers

[Rusty Russell]

__builtin_types_compatible_p() has been around since gcc 2.95, and we
don't use it anywhere.  This patch quietly fixes that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r f0ff8138f993 include/linux/kernel.h
--- a/include/linux/kernel.h	Fri Mar 09 16:40:25 2007 +1100
+++ b/include/linux/kernel.h	Fri Mar 09 16:44:04 2007 +1100
@@ -35,7 +35,9 @@ extern const char linux_proc_banner[];
 #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
 #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
 
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0])			      \
+	+ sizeof(typeof(int[1 - 2*!!__builtin_types_compatible_p(typeof(arr), \
+		 typeof(&arr[0]))]))*0)
 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
 #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))

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

[Stephen Rothwell]

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On Fri, 09 Mar 2007 16:56:32 +1100 Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.

After staring at this for about 2 minutes, how about a commit message like:

Make ARRAY_SIZE complain strangely if passed a pointer instead of an
array.

:-)
--
Cheers,
Stephen Rothwell                    sfr@canb.auug.org.au
http://www.canb.auug.org.au/~sfr/

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

[Linus Torvalds]

On Fri, 9 Mar 2007, Rusty Russell wrote:
>
> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.

Whee.

Rusty, that's a work of art.

However, I would suggest that you never show it to anybody ever again. I'm 
sure that in fifty years, it will be worth much more. So please keep it 
tightly under wraps, to keep people from gouging their eyes out^W^W^W^W^W^W^W 
make a killing in the art market.

Please.

		Linus

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

[Andrey Panin]

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On 068, 03 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
> __builtin_types_compatible_p() has been around since gcc 2.95,

but it's not available in Intel C compiler IIRC :(

> and we don't use it anywhere.  This patch quietly fixes that.
>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> diff -r f0ff8138f993 include/linux/kernel.h
> --- a/include/linux/kernel.h	Fri Mar 09 16:40:25 2007 +1100
> +++ b/include/linux/kernel.h	Fri Mar 09 16:44:04 2007 +1100
> @@ -35,7 +35,9 @@ extern const char linux_proc_banner[];
>  #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>  #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
>  
> -#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> +#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0])			      \
> +	+ sizeof(typeof(int[1 - 2*!!__builtin_types_compatible_p(typeof(arr), \
> +		 typeof(&arr[0]))]))*0)
>  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
>  #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
>  #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 

-- 
Andrey Panin		| Linux and UNIX system administrator
pazke@donpac.ru		| PGP key: wwwkeys.pgp.net

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

[Christoph Hellwig]

On Fri, Mar 09, 2007 at 09:50:56AM +0300, Andrey Panin wrote:
> On 068, 03 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
> > __builtin_types_compatible_p() has been around since gcc 2.95,
> 
> but it's not available in Intel C compiler IIRC :(

So what?

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

[Andrey Panin]

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On 068, 03 09, 2007 at 07:53:08AM +0000, Christoph Hellwig wrote:
> On Fri, Mar 09, 2007 at 09:50:56AM +0300, Andrey Panin wrote:
> > On 068, 03 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
> > > __builtin_types_compatible_p() has been around since gcc 2.95,
> > 
> > but it's not available in Intel C compiler IIRC :(
> 
> So what?

Kernel compilation with Intel compiler is (was ?) supported.
This patch will break it.

-- 
Andrey Panin		| Linux and UNIX system administrator
pazke@donpac.ru		| PGP key: wwwkeys.pgp.net

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

[Christoph Hellwig]

On Fri, Mar 09, 2007 at 12:02:19PM +0300, Andrey Panin wrote:
> Kernel compilation with Intel compiler is (was ?) supported.
> This patch will break it.

It was only put in under the premise that they'll fix whatever breaks,
we're not going to put any maintaince border on us to hack around
broken propritary compilers.

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

[Linus Torvalds]

On Fri, 9 Mar 2007, Christoph Hellwig wrote:
> 
> It was only put in under the premise that they'll fix whatever breaks,
> we're not going to put any maintaince border on us to hack around
> broken propritary compilers.

Well, since Rusty's macro was hoddible *anyway*, I don't think I'd apply 
it as-is. Breaking icc for something that ugly and not-very-important 
simply makes no sense.

There are better ways to do this. 

For one, you could (and should!) abstract these kinds of things out, 
rather than put them in another macro that really does something totally 
different. Then, the macro could have become

	#define ARRAY_SIZE (sizeof_expression + 0*error_if_not_array)

which would already be a hell of a lot more readable. But more 
importantly, it's also now suddenly much easiler to abstract out for 
different compilers.

We *already* support different compilers through <linux/compiler.h>, and 
there just isn't any reason for bad code just for bad codes sake!

				Linus

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

[Trent Piepho]

On Fri, 9 Mar 2007, Linus Torvalds wrote:
> On Fri, 9 Mar 2007, Christoph Hellwig wrote:
> Well, since Rusty's macro was hoddible *anyway*, I don't think I'd apply
> it as-is. Breaking icc for something that ugly and not-very-important
> simply makes no sense.
>
> There are better ways to do this.
>
> For one, you could (and should!) abstract these kinds of things out,
> rather than put them in another macro that really does something totally
> different. Then, the macro could have become
>
> 	#define ARRAY_SIZE (sizeof_expression + 0*error_if_not_array)

/* Error if X is a pointer, 0 otherwise */
#define ERROR_IF_POINTER(x) \
	sizeof(int[-__builtin_types_compatible_p(typeof(x), typeof(&x[0]))])

/* Warning (div by zero) if x is a pointer, 0 otherwise */
#define WARN_IF_POINTER(x) \
	(0/!__builtin_types_compatible_p(typeof(x), typeof(&x[0])))

The gcc docs say __builtin_types_compatible_p returns 1 or 0, so the !!
isn't necessary.  And my gcc at least returns 0 for sizeof(int[0]).

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

[Christoph Hellwig]

On Fri, Mar 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> diff -r f0ff8138f993 include/linux/kernel.h
> --- a/include/linux/kernel.h	Fri Mar 09 16:40:25 2007 +1100
> +++ b/include/linux/kernel.h	Fri Mar 09 16:44:04 2007 +1100
> @@ -35,7 +35,9 @@ extern const char linux_proc_banner[];
>  #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>  #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
>  
> -#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> +#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0])			      \
> +	+ sizeof(typeof(int[1 - 2*!!__builtin_types_compatible_p(typeof(arr), \
> +		 typeof(&arr[0]))]))*0)

This needs a comment explaning why we're doing this, and maybe a little
explanation of the combination of gcc magic and C trickery used to implement
it to the brave non-uberhacker people trying to understand linux headers.

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

[Jarek Poplawski]

On 09-03-2007 08:52, Christoph Hellwig wrote:
> On Fri, Mar 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
...
>> +#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0])			      \
>> +	+ sizeof(typeof(int[1 - 2*!!__builtin_types_compatible_p(typeof(arr), \
>> +		 typeof(&arr[0]))]))*0)
> 
> This needs a comment explaning why we're doing this, and maybe a little
> explanation of the combination of gcc magic and C trickery used to implement
> it to the brave non-uberhacker people trying to understand linux headers.

...and to deprive them of all the pleasure of contemplating...

Jarek P.

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

[Theodore Ts'o]

On Fri, Mar 09, 2007 at 04:56:32PM +1100, Rusty Russell wrote:
> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Is your clock set correctly?  Looks like this mail was sent 23 days
early.  :-)

						- Ted

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

[Andi Kleen]

Rusty Russell <rusty@rustcorp.com.au> writes:

> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.

Using BUILD_BUG_ON_ZERO() would have been somewhat cleaner.

-Andi

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

[Rusty Russell]

On Fri, 2007-03-09 at 16:56 +1100, Rusty Russell wrote:
> __builtin_types_compatible_p() has been around since gcc 2.95, and we
> don't use it anywhere.  This patch quietly fixes that.

OK, many people complained that it needed a comment.  Good point!
==
Add comment to ARRAY_SIZE macro.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r 933e410f204f include/linux/kernel.h
--- a/include/linux/kernel.h	Sat Mar 10 09:55:31 2007 +1100
+++ b/include/linux/kernel.h	Sat Mar 10 09:55:53 2007 +1100
@@ -35,6 +35,7 @@ extern const char linux_proc_banner[];
 #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
 #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
 
+/* 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)

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

[Randy Dunlap]

On Sat, 10 Mar 2007 09:57:32 +1100 Rusty Russell wrote:

> On Fri, 2007-03-09 at 16:56 +1100, Rusty Russell wrote:
> > __builtin_types_compatible_p() has been around since gcc 2.95, and we
> > don't use it anywhere.  This patch quietly fixes that.

Bah.  Just because gcc has a "feature" doesn't mean we should use it
(in this form).


> OK, many people complained that it needed a comment.  Good point!
> ==
> Add comment to ARRAY_SIZE macro.
> 
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> 
> diff -r 933e410f204f include/linux/kernel.h
> --- a/include/linux/kernel.h	Sat Mar 10 09:55:31 2007 +1100
> +++ b/include/linux/kernel.h	Sat Mar 10 09:55:53 2007 +1100
> @@ -35,6 +35,7 @@ extern const char linux_proc_banner[];
>  #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>  #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
>  
> +/* 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)


---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***

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

[Robert P. J. Day]

On Sat, 10 Mar 2007, Rusty Russell wrote:

> On Fri, 2007-03-09 at 16:56 +1100, Rusty Russell wrote:
> > __builtin_types_compatible_p() has been around since gcc 2.95, and we
> > don't use it anywhere.  This patch quietly fixes that.
>
> OK, many people complained that it needed a comment.  Good point!
> ==
> Add comment to ARRAY_SIZE macro.
>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> diff -r 933e410f204f include/linux/kernel.h
> --- a/include/linux/kernel.h	Sat Mar 10 09:55:31 2007 +1100
> +++ b/include/linux/kernel.h	Sat Mar 10 09:55:53 2007 +1100
> @@ -35,6 +35,7 @@ extern const char linux_proc_banner[];
>  #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>  #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
>
> +/* 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)

ah, but is that "universe" kind of awesome, or the "hot dogs" kind of
awesome?

http://youtube.com/watch?v=0rYT0YvQ3hs

rday

-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://fsdev.net/wiki/index.php?title=Main_Page
========================================================================

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

[Roland Dreier]

Perhaps this patch can go into Wesnoth for testing for a while before
we merge it into the kernel?

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

[Jan Engelhardt]

On Mar 10 2007 09:57, Rusty Russell wrote:
>On Fri, 2007-03-09 at 16:56 +1100, Rusty Russell wrote:
>> __builtin_types_compatible_p() has been around since gcc 2.95, and we
>> don't use it anywhere.  This patch quietly fixes that.
>
>OK, many people complained that it needed a comment.  Good point!
>==
>Add comment to ARRAY_SIZE macro.
>
>Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
>diff -r 933e410f204f include/linux/kernel.h
>--- a/include/linux/kernel.h	Sat Mar 10 09:55:31 2007 +1100
>+++ b/include/linux/kernel.h	Sat Mar 10 09:55:53 2007 +1100
>@@ -35,6 +35,7 @@ extern const char linux_proc_banner[];
> #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
> #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
> 
>+/* 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)

Getting back at the macro, how would you like to have it merged?



Jan
-- 

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

[Rusty Russell]

On Sat, 2007-03-10 at 02:04 +0100, Jan Engelhardt wrote:
> Getting back at the macro, how would you like to have it merged?

Well, this is what I sent to Linus and Andrew (many thanks to those who
made appropriately whimsical *or* useful comments):

diff -r 1ccdf46b0f41 include/linux/compiler-gcc.h
--- a/include/linux/compiler-gcc.h	Sat Mar 10 09:55:29 2007 +1100
+++ b/include/linux/compiler-gcc.h	Sat Mar 10 11:06:35 2007 +1100
@@ -22,6 +22,9 @@
     __asm__ ("" : "=r"(__ptr) : "0"(ptr));		\
     (typeof(ptr)) (__ptr + (off)); })
 
+/* &a[0] degrades to a pointer: a different type from an array */
+#define __must_be_array(a) \
+  BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
 
 #define inline		inline		__attribute__((always_inline))
 #define __inline__	__inline__	__attribute__((always_inline))
diff -r 1ccdf46b0f41 include/linux/compiler-intel.h
--- a/include/linux/compiler-intel.h	Sat Mar 10 09:55:29 2007 +1100
+++ b/include/linux/compiler-intel.h	Sat Mar 10 11:06:25 2007 +1100
@@ -21,4 +21,7 @@
      __ptr = (unsigned long) (ptr);				\
     (typeof(ptr)) (__ptr + (off)); })
 
+/* Intel ECC compiler doesn't support __builtin_types_compatible_p() */
+#define __must_be_array(a) 0
+
 #endif
diff -r 1ccdf46b0f41 include/linux/kernel.h
--- a/include/linux/kernel.h	Sat Mar 10 09:55:29 2007 +1100
+++ b/include/linux/kernel.h	Sat Mar 10 11:06:16 2007 +1100
@@ -35,7 +35,8 @@ extern const char linux_proc_banner[];
 #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
 #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
 
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
+
 #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
 #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))

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

[Jan Engelhardt]

On Mar 11 2007 13:50, Rusty Russell wrote:
>On Sat, 2007-03-10 at 02:04 +0100, Jan Engelhardt wrote:
>> Getting back at the macro, how would you like to have it merged?
>
>Well, this is what I sent to Linus and Andrew (many thanks to those who
>made appropriately whimsical *or* useful comments):
>
>diff -r 1ccdf46b0f41 include/linux/compiler-gcc.h
>--- a/include/linux/compiler-gcc.h	Sat Mar 10 09:55:29 2007 +1100
>+++ b/include/linux/compiler-gcc.h	Sat Mar 10 11:06:35 2007 +1100
>@@ -22,6 +22,9 @@
>     __asm__ ("" : "=r"(__ptr) : "0"(ptr));		\
>     (typeof(ptr)) (__ptr + (off)); })
> 
>+/* &a[0] degrades to a pointer: a different type from an array */
>+#define __must_be_array(a) \
>+  BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))

This looks _much_ nicer! (And BUILD_BUG_ON is also appropriately
commented.)

> 
> #define inline		inline		__attribute__((always_inline))
> #define __inline__	__inline__	__attribute__((always_inline))
>diff -r 1ccdf46b0f41 include/linux/compiler-intel.h
>--- a/include/linux/compiler-intel.h	Sat Mar 10 09:55:29 2007 +1100
>+++ b/include/linux/compiler-intel.h	Sat Mar 10 11:06:25 2007 +1100
>@@ -21,4 +21,7 @@
>      __ptr = (unsigned long) (ptr);				\
>     (typeof(ptr)) (__ptr + (off)); })
> 
>+/* Intel ECC compiler doesn't support __builtin_types_compatible_p() */
>+#define __must_be_array(a) 0
>+
> #endif
>diff -r 1ccdf46b0f41 include/linux/kernel.h
>--- a/include/linux/kernel.h	Sat Mar 10 09:55:29 2007 +1100
>+++ b/include/linux/kernel.h	Sat Mar 10 11:06:16 2007 +1100
>@@ -35,7 +35,8 @@ extern const char linux_proc_banner[];
> #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
> #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
> 
>-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
>+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
>+
> #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
> #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))

80 cols *cough* :)


Jan
-- 

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

[Rusty Russell]

On Sun, 2007-03-11 at 03:58 +0100, Jan Engelhardt wrote:
> >-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
> >+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
> >+
> 80 cols *cough* :)

I think your cough added a column?

Rusty.

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

[Valdis.Kletnieks]

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On Sun, 11 Mar 2007 13:50:37 +1100, Rusty Russell said:
> Well, this is what I sent to Linus and Andrew (many thanks to those who
> made appropriately whimsical *or* useful comments):

Ahh.. much better - it's now a form that even I can get my brain wrapped around :)

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

[Valdis.Kletnieks]

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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. */



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

[Randy Dunlap]

On Fri, 09 Mar 2007 23:03:05 -0500 Valdis.Kletnieks@vt.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. */

"awesome" can mean "inspiring awe or admiration or wonder" (amazing)
or it can mean "awful" (as in terrifying).  8)

---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***

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

[Valdis.Kletnieks]

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On Fri, 09 Mar 2007 20:24:42 PST, Randy Dunlap said:
> On Fri, 09 Mar 2007 23:03:05 -0500 Valdis.Kletnieks@vt.edu wrote:
> > -/* GCC is awesome. */
> > +/* GCC leaves me speechless. */
> 
> "awesome" can mean "inspiring awe or admiration or wonder" (amazing)
> or it can mean "awful" (as in terrifying).  8)

And as those who know me well will attest, it takes something well down the
road of either definition to render me actually speechless.. :)

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

[Nigel Cunningham]

Hi.

On Fri, 2007-03-09 at 23:03 -0500, Valdis.Kletnieks@vt.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...

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

[Jan Engelhardt]

On Mar 10 2007 16:19, Nigel Cunningham wrote:
>On Fri, 2007-03-09 at 23:03 -0500, Valdis.Kletnieks@vt.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
-- 

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

[Andreas Schwab]

Jan Engelhardt <jengelh@linux01.gwdg.de> writes:

> 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.

The error case is when the types are compatible.  That means that the
argument is in fact _not_ an array.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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

[Jan Engelhardt]

On Mar 10 2007 16:18, Andreas Schwab wrote:
>Jan Engelhardt <jengelh@linux01.gwdg.de> writes:
>
>> 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.
>

	>The error case is when the types are compatible.  That means that
	>the argument is in fact _not_ an array.

That one should go in as a comment for that macro.



Jan
--