Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Jani Nikula]

On Thu, 30 Mar 2023, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@xxxxxxxxxx> 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().

test.c:
#define IS_POWER_OF_2(_n)				\
	({						\
		typeof(_n) n = (_n);			\
		n != 0 && ((n & (n - 1)) == 0);		\
	})

#define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))

#define FOO(n) ((n) + BUILD_BUG_ON_ZERO(!IS_POWER_OF_2(n)))

int main(void)
{
	return FOO(2);
}

$ gcc test.c
test.c: In function ‘main’:
test.c:16:51: error: bit-field ‘<anonymous>’ width not an integer constant
   16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
      |                                                   ^
test.c:18:23: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
   18 | #define FOO(n) ((n) + BUILD_BUG_ON_ZERO(!IS_POWER_OF_2(n)))
      |                       ^~~~~~~~~~~~~~~~~
test.c:22:9: note: in expansion of macro ‘FOO’
   22 |  return FOO(2);
      |         ^~~


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center