linux-kernel - [PATCH] Introduce a boolean "single_bit

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Message-ID: <alpine.LFD.2.00.0904231339080.13089@localhost.localdomain>
Date:	Thu, 23 Apr 2009 13:43:34 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
cc:	Andrew Morton <akpm@...l.org>
Subject: [PATCH] Introduce a boolean "single_bit_set" function.


A boolean single_bit_set() routine would simplify the numerous
constructs of the form (((n & (n - 1)) == 0)) when testing for
single-bitness.

Signed-off-by: Robert P. J. Day <rpjday@...shcourse.ca>

---

This is similar to the current is_power_of_2() routine defined in
include/linux/log2.h, which is mathematically identical but,
semantically, should be defined independently just so the code is more
readable.

I'm open to an alternative function name.

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 6182913..1c0c840 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -45,6 +45,13 @@ static inline unsigned long hweight_long(unsigned long w)
 	return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
 }

+static inline __attribute__((const))
+bool single_bit_set(unsigned long n)
+{
+        return (n != 0 && ((n & (n - 1)) == 0));
+}
+
+
 /**
  * rol32 - rotate a 32-bit value left
  * @word: value to rotate


========================================================================
Robert P. J. Day                               Waterloo, Ontario, CANADA

        Linux Consulting, Training and Annoying Kernel Pedantry.

Web page:                                          http://crashcourse.ca
Linked In:                             http://www.linkedin.com/in/rpjday
Twitter:                                       http://twitter.com/rpjday
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