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?
