linux-kernel - [PATCH 2/4] LOG2: Alter roundup_pow_of_two() so that it can use a ilog2() on a constant [try #4]

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Message-Id: <20061006133417.9972.76941.stgit@warthog.cambridge.redhat.com>
Date:	Fri, 06 Oct 2006 14:34:17 +0100
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...l.org, akpm@...l.org, sfr@...b.auug.org.au
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	dhowells@...hat.com
Subject: [PATCH 2/4] LOG2: Alter roundup_pow_of_two() so that it can use a ilog2() on a constant [try #4]

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

Alter roundup_pow_of_two() so that it can make use of ilog2() 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   |   26 ++++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 473101a..67ca3ad 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -156,12 +156,6 @@ #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 3979c60..d02e1a5 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -43,6 +43,15 @@ int __ilog2_u64(u64 n)
 }
 #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);
+}
+
 /**
  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
  * @n - parameter
@@ -128,4 +137,21 @@ (						\
 	__ilog2_u64(n)				\
  )
 
+/**
+ * roundup_pow_of_two - round the given value up to nearest power of two
+ * @n - parameter
+ *
+ * round the given balue up 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 roundup_pow_of_two(n)			\
+(						\
+	__builtin_constant_p(n) ? (		\
+		(n == 1) ? 0 :			\
+		(1UL << (ilog2((n) - 1) + 1))	\
+				   ) :		\
+	__roundup_pow_of_two(n)			\
+ )
+
 #endif /* _LINUX_LOG2_H */
-
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