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

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Message-Id: <20230330151846.fdbc8edbfbaa6eaddb056dc7@linux-foundation.org>
Date:   Thu, 30 Mar 2023 15:18:46 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Laight <David.Laight@...LAB.COM>
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

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?