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

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Message-ID: <37671dff9b6b4e6bb07862c11cb69874@AcuMS.aculab.com>
Date:   Fri, 31 Mar 2023 07:33:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrew Morton' <akpm@...ux-foundation.org>
CC:     Jani Nikula <jani.nikula@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Christian König <christian.koenig@....com>,
        David Gow <davidgow@...gle.com>
Subject: RE: [PATCH 0/4] log2: make is_power_of_2() more generic

From: Andrew Morton
> Sent: 30 March 2023 23:19
> 
> 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?

Try:
static int x = is_power_of_2(43);

I suspect that some (all?) of the compile-time assert checks won't
like ({...}) either.

	David

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