linux-kernel - [PATCH] Add a "rounddown_pow_of

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Date:	Wed, 29 Aug 2007 04:12:19 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
cc:	Andrew Morton <akpm@...l.org>
Subject: [PATCH] Add a "rounddown_pow_of_two" routine to log2.h.


To go along with the existing "roundup_pow_of_two" routine, add one
for rounding down since that operation appears to crop up on a regular
basis in the source tree.

Signed-off-by: Robert P. J. Day <rpjday@...dspring.com>

---

  some brief tests indicate that these routines return the correct
answer.  i'm not sure if i've officially submitted this before or if
it's already been accepted in a tree somewhere.


diff --git a/include/linux/log2.h b/include/linux/log2.h
index 1b8a2c1..ff8835b 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -63,6 +63,15 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 	return 1UL << fls_long(n - 1);
 }

+/*
+ * round down to nearest power of two
+ */
+static inline __attribute__((const))
+unsigned long __rounddown_pow_of_two(unsigned long n)
+{
+	return 1UL << (fls_long(n) - 1);
+}
+
 /**
  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
  * @n - parameter
@@ -165,4 +174,20 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 	__roundup_pow_of_two(n)			\
  )

+/**
+ * rounddown_pow_of_two - round the given value down to nearest power of two
+ * @n - parameter
+ *
+ * round the given value down to the nearest power of two
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define rounddown_pow_of_two(n)			\
+(						\
+	__builtin_constant_p(n) ? (		\
+		(n == 1) ? 0 :			\
+		(1UL << ilog2(n)) :		\
+	__rounddown_pow_of_two(n)		\
+ )
+
 #endif /* _LINUX_LOG2_H */
-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://crashcourse.ca
========================================================================
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