linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Date:	Wed, 20 Aug 2008 16:07:22 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	adobriyan@...il.com
CC:	Ingo Molnar <mingo@...e.hu>, Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

adobriyan@...il.com wrote:
> On Wed, Aug 20, 2008 at 03:31:53PM +0300, Boaz Harrosh wrote:
>> Ingo Molnar wrote:
>>> * Boaz Harrosh <bharrosh@...asas.com> wrote:
>>>
>>>> If the user of virtio_has_feature() must pass a compile-time constant 
>>>> then it must be converted to a MACRO, and then the BUILD_BUG_ON will 
>>>> work. Or it should be changed to a BUG_ON() if fbit is a runtime 
>>>> variable.
>> The use of __builtin_constant_p in inline functions is broken. This
>> is because it will give different results depending on the -O level
>> used. So I think that using it in the Kernel with inlines is plain
>> broken. And should be discouraged.
> 
> See how kmalloc() works.
> 
Right:

static inline void *kmalloc(size_t size, gfp_t flags)
{
	if (__builtin_constant_p(size)) {
	 	... do some stuff
	else
		return __kmalloc_xxx(..)
}

So if optimization is on we might gain some cycles
but we must make sure we have the proper else clause.

In any way virtio_has_feature() is broken and no amount
of optimization may ever fix it. Please look at it:

static inline bool virtio_has_feature(const struct virtio_device *vdev,
				      unsigned int fbit)
{
	/* Did you forget to fix assumptions on max features? */
	if (__builtin_constant_p(fbit))
		BUILD_BUG_ON(sizeof(fbit) >= 32);


the __builtin_constant_p(fbit) will never help.
sizeof(fbit) is always sizeof(unsigned int) in any case. Only
a macro can help here. Sorry

Boaz

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