From: Jani Nikula <jani.nikula@intel.com>
To: Steven Price <steven.price@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
David Laight <David.Laight@ACULAB.COM>
Cc: "dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"intel-gfx@lists.freedesktop.org"
<intel-gfx@lists.freedesktop.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"David Gow" <davidgow@google.com>,
"Christian König" <christian.koenig@amd.com>
Subject: Re: [PATCH 0/4] log2: make is_power_of_2() more generic
Date: Fri, 12 Apr 2024 13:01:36 +0300 [thread overview]
Message-ID: <87jzl2527z.fsf@intel.com> (raw)
In-Reply-To: <8905800b-a977-e821-01ea-a43333f46904@arm.com>
On Wed, 05 Apr 2023, Steven Price <steven.price@arm.com> wrote:
> On 31/03/2023 09:31, Jani Nikula wrote:
>> On Thu, 30 Mar 2023, Andrew Morton <akpm@linux-foundation.org> wrote:
>>> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
>>>
>>>>> But wouldn't all these issues be addressed by simply doing
>>>>>
>>>>> #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
>>>>>
>>>>> ?
>>>>>
>>>>> (With suitable tweaks to avoid evaluating `n' more than once)
>>>>
>>>> I think you need to use the 'horrid tricks' from min() to get
>>>> a constant expression from constant inputs.
>>>
>>> This
>>>
>>> --- a/include/linux/log2.h~a
>>> +++ a/include/linux/log2.h
>>> @@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
>>> * *not* considered a power of two.
>>> * Return: true if @n is a power of 2, otherwise false.
>>> */
>>> -static inline __attribute__((const))
>>> -bool is_power_of_2(unsigned long n)
>>> -{
>>> - return (n != 0 && ((n & (n - 1)) == 0));
>>> -}
>>> +#define is_power_of_2(_n) \
>>> + ({ \
>>> + typeof(_n) n = (_n); \
>>> + n != 0 && ((n & (n - 1)) == 0); \
>>> + })
>>>
>>> /**
>>> * __roundup_pow_of_two() - round up to nearest power of two
>>> _
>>>
>>> worked for me in a simple test.
>>>
>>> --- a/fs/open.c~b
>>> +++ a/fs/open.c
>>> @@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
>>> }
>>>
>>> EXPORT_SYMBOL(stream_open);
>>> +
>>> +#include <linux/log2.h>
>>> +
>>> +int foo(void)
>>> +{
>>> + return is_power_of_2(43);
>>> +}
>>> _
>>>
>>>
>>> foo:
>>> # fs/open.c:1573: }
>>> xorl %eax, %eax #
>>> ret
>>>
>>>
>>> Is there some more tricky situation where it breaks?
>>
>> It doesn't work with BUILD_BUG_ON_ZERO().
>
> Like most programming problems, you just need another layer of
> indirection! The below works for me in all the cases I could think of
> (including __uint128_t).
>
>
> #define __IS_POWER_OF_2(n) (n != 0 && ((n & (n - 1)) == 0))
>
> #define _IS_POWER_OF_2(n, unique_n) \
> ({ \
> typeof(n) unique_n = (n); \
> __IS_POWER_OF_2(unique_n); \
> })
>
> #define is_power_of_2(n) \
> __builtin_choose_expr(__is_constexpr((n)), \
> __IS_POWER_OF_2((n)), \
> _IS_POWER_OF_2(n, __UNIQUE_ID(_n)))
>
>
> Although Jani's original might be easier to understand.
I dropped the ball since I couldn't make heads or tails what I should be
doing. And a year has passed. I'll note that the kernel has a number of
helpers for "is power of 2" for u64 and for constant expressions,
outside of log2.h.
I tried to make is_power_of_2() work for all the cases. Would it be more
palatable if I just added all the variants separately to log2.h?
- Leave is_power_of_2() as is
- Add is_power_of_2_u64() for 32-bit build compatible 64-bit checks
- Add IS_POWER_OF_2() macro for constant expressions
Please just tell me what to do and I'll do it.
BR,
Jani.
--
Jani Nikula, Intel
next prev parent reply other threads:[~2024-04-12 10:01 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-30 10:42 [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 1/4] log2: add helper __IS_POWER_OF_2() Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 2/4] log2: have is_power_of_2() support bigger types than unsigned long Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 3/4] log2: allow use of is_power_of_2() in constant expressions Jani Nikula
2023-03-30 10:42 ` [Intel-gfx] [PATCH 4/4] drm/i915/reg: use is_power_of_2() from log2.h Jani Nikula
2023-03-30 10:59 ` [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic Christian König
2023-03-30 11:10 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for " Patchwork
2023-03-30 11:10 ` [Intel-gfx] ✗ Fi.CI.SPARSE: " Patchwork
2023-03-30 11:23 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2023-03-30 19:50 ` [Intel-gfx] [PATCH 0/4] " Andrew Morton
2023-03-30 21:53 ` David Laight
2023-03-30 22:18 ` Andrew Morton
2023-03-31 7:33 ` David Laight
2023-03-31 8:31 ` Jani Nikula
2023-04-05 15:27 ` Steven Price
2024-04-12 10:01 ` Jani Nikula [this message]
2023-03-31 5:58 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork
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