linux-kernel - [PATCH 6/6] Alter roundup_pow_of_two() so that it can make use of long

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Message-Id: <20060913130304.32022.10732.stgit@warthog.cambridge.redhat.com>
Date:	Wed, 13 Sep 2006 14:03:04 +0100
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...l.org, akpm@...l.org
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	dhowells@...hat.com
Subject: [PATCH 6/6] Alter roundup_pow_of_two() so that it can make use of long_log2() on a constant

From: David Howells <dhowells@...hat.com>

Alter roundup_pow_of_two() so that it can make use of long_log2() on a constant
to produce a constant value, retaining the ability for an arch to override it
in the non-const case.

This permits the function to be used to initialise variables.

Signed-Off-By: David Howells <dhowells@...hat.com>
---

 include/linux/kernel.h |    6 ------
 include/linux/log2.h   |   23 +++++++++++++++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index e8c6c66..e5947fb 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -155,12 +155,6 @@ #endif
 
 unsigned long int_sqrt(unsigned long);
 
-static inline unsigned long
-__attribute_const__ roundup_pow_of_two(unsigned long x)
-{
-	return 1UL << fls_long(x - 1);
-}
-
 extern int printk_ratelimit(void);
 extern int __printk_ratelimit(int ratelimit_jiffies, int ratelimit_burst);
 
diff --git a/include/linux/log2.h b/include/linux/log2.h
index a2f6858..2b55af0 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -74,6 +74,15 @@ #endif
 #endif
 
 /*
+ * round up to nearest power of two
+ */
+static inline __attribute__((const))
+unsigned long __roundup_pow_of_two(unsigned long n)
+{
+	return 1UL << fls_long(n - 1);
+}
+
+/*
  * constant-capable 32-bit log of base 2 calculation
  * - this can be used to initialise global variables from constant data, hence
  *   the massive ternary operator construction
@@ -218,4 +227,18 @@ (									\
 	__get_order(n, PAGE_SHIFT)					\
  )
 
+/*
+ * round up to nearest power of two
+ * - the result is undefined when n == 0
+ * - this can be used to initialise global variables from constant data
+ */
+#define roundup_pow_of_two(n)				\
+(							\
+	__builtin_constant_p(n) ? (			\
+		(n == 1) ? 0 :				\
+		(1UL << (long_log2((n) - 1) + 1))	\
+				   ) :			\
+	__roundup_pow_of_two(n)				\
+ )
+
 #endif /* _LINUX_LOG2_H */
-
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