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