From: Boaz Harrosh <bharrosh@panasas.com>
To: adobriyan@gmail.com
Cc: Ingo Molnar <mingo@elte.hu>,
Rusty Russell <rusty@rustcorp.com.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions
Date: Wed, 20 Aug 2008 16:07:22 +0300 [thread overview]
Message-ID: <48AC170A.2070702@panasas.com> (raw)
In-Reply-To: <20080820123929.GA29972@x200.localdomain>
adobriyan@gmail.com wrote:
> On Wed, Aug 20, 2008 at 03:31:53PM +0300, Boaz Harrosh wrote:
>> Ingo Molnar wrote:
>>> * Boaz Harrosh <bharrosh@panasas.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
next prev parent reply other threads:[~2008-08-20 13:08 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-08-16 10:09 [PATCH] BUILD_BUG_ON sucks Alexey Dobriyan
2008-08-16 10:55 ` Rusty Russell
2008-08-16 20:07 ` Linus Torvalds
2008-08-17 10:32 ` [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions Ingo Molnar
2008-08-17 16:56 ` Linus Torvalds
2008-08-17 17:33 ` Ingo Molnar
2008-08-17 17:53 ` Ingo Molnar
2008-08-17 18:39 ` Linus Torvalds
2008-08-17 18:45 ` Ingo Molnar
2008-08-18 1:09 ` Rusty Russell
2008-08-18 7:54 ` Ingo Molnar
2008-08-18 9:55 ` Boaz Harrosh
2008-08-18 12:32 ` Boaz Harrosh
2008-08-19 13:34 ` Ingo Molnar
2008-08-19 16:33 ` Boaz Harrosh
2008-08-20 10:59 ` Ingo Molnar
2008-08-20 12:31 ` Boaz Harrosh
2008-08-20 12:39 ` adobriyan
2008-08-20 13:07 ` Boaz Harrosh [this message]
2008-08-21 12:17 ` Ingo Molnar
2008-08-25 1:19 ` Rusty Russell
2008-08-20 13:21 ` Boaz Harrosh
2008-08-16 17:46 ` [PATCH] BUILD_BUG_ON sucks Andrew Morton
2008-08-17 12:19 ` Theodore Tso
2008-08-17 16:33 ` Andrew Morton
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