linux-kernel - Re: [PATCH 0/4] log2: make is_power_of

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Message-ID: <8905800b-a977-e821-01ea-a43333f46904@arm.com>
Date:   Wed, 5 Apr 2023 16:27:50 +0100
From:   Steven Price <steven.price@....com>
To:     Jani Nikula <jani.nikula@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Laight <David.Laight@...LAB.COM>
Cc:     David Gow <davidgow@...gle.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH 0/4] log2: make is_power_of_2() more generic

On 31/03/2023 09:31, Jani Nikula wrote:
> On Thu, 30 Mar 2023, Andrew Morton <akpm@...ux-foundation.org> wrote:
>> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@...LAB.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