From: Steven Price <steven.price@arm.com>
To: Jani Nikula <jani.nikula@intel.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: [Intel-gfx] [PATCH 0/4] log2: make is_power_of_2() more generic
Date: Wed, 5 Apr 2023 16:27:50 +0100 [thread overview]
Message-ID: <8905800b-a977-e821-01ea-a43333f46904@arm.com> (raw)
In-Reply-To: <87edp52ufk.fsf@intel.com>
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.
Steve
next prev parent reply other threads:[~2023-04-05 15:27 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 [this message]
2024-04-12 10:01 ` Jani Nikula
2023-03-31 5:58 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork
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