include: update log2 header from the Linux kernel

Without the patch gcc 8 produces:
warning: ignoring attribute ‘noreturn’ because it conflicts with
attribute ‘const’ [-Wattributes]
 int ____ilog2_NaN(void);

So let's update the include from Linux kernel v4.16.

This removes static checks of ilog2() arguments.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

include/linux/log2.h

@@ -3,6 +3,11 @@
  *
  * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
  */
 
 #ifndef _LINUX_LOG2_H
@@ -12,12 +17,6 @@
 #include <linux/bitops.h>
 
 /*
- * deal with unrepresentable constant logarithms
- */
-extern __attribute__((const, noreturn))
-int ____ilog2_NaN(void);
-
-/*
  * non-constant log of base 2 calculators
  * - the arch may override these in asm/bitops.h if they can be implemented
  *   more efficiently than using fls() and fls64()
@@ -39,19 +38,23 @@ int __ilog2_u64(u64 n)
 }
 #endif
 
-/*
- *  Determine whether some value is a power of two, where zero is
+/**
+ * is_power_of_2() - check if a value is a power of two
+ * @n: the value to check
+ *
+ * Determine whether some value is a power of two, where zero is
  * *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));
 }
 
-/*
- * round up to nearest power of two
+/**
+ * __roundup_pow_of_two() - round up to nearest power of two
+ * @n: value to round up
  */
 static inline __attribute__((const))
 unsigned long __roundup_pow_of_two(unsigned long n)
@@ -59,8 +62,9 @@ unsigned long __roundup_pow_of_two(unsigned long n)
 	return 1UL << fls_long(n - 1);
 }
 
-/*
- * round down to nearest power of two
+/**
+ * __rounddown_pow_of_two() - round down to nearest power of two
+ * @n: value to round down
  */
 static inline __attribute__((const))
 unsigned long __rounddown_pow_of_two(unsigned long n)
@@ -69,19 +73,19 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 }
 
 /**
- * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
- * @n - parameter
+ * ilog2 - log base 2 of 32-bit or a 64-bit unsigned value
+ * @n: parameter
  *
  * constant-capable log of base 2 calculation
  * - this can be used to initialise global variables from constant data, hence
- *   the massive ternary operator construction
+ * the massive ternary operator construction
  *
  * selects the appropriately-sized optimised version depending on sizeof(n)
  */
 #define ilog2(n)				\
 (						\
 	__builtin_constant_p(n) ? (		\
-		(n) < 1 ? ____ilog2_NaN() :	\
+		(n) < 2 ? 0 :			\
 		(n) & (1ULL << 63) ? 63 :	\
 		(n) & (1ULL << 62) ? 62 :	\
 		(n) & (1ULL << 61) ? 61 :	\
@@ -144,10 +148,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 		(n) & (1ULL <<  4) ?  4 :	\
 		(n) & (1ULL <<  3) ?  3 :	\
 		(n) & (1ULL <<  2) ?  2 :	\
-		(n) & (1ULL <<  1) ?  1 :	\
-		(n) & (1ULL <<  0) ?  0 :	\
-		____ilog2_NaN()			\
-				   ) :		\
+		1) :				\
 	(sizeof(n) <= 4) ?			\
 	__ilog2_u32(n) :			\
 	__ilog2_u64(n)				\
@@ -155,7 +156,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 
 /**
  * roundup_pow_of_two - round the given value up to nearest power of two
- * @n - parameter
+ * @n: parameter
  *
  * round the given value up to the nearest power of two
  * - the result is undefined when n == 0
@@ -172,7 +173,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 
 /**
  * rounddown_pow_of_two - round the given value down to nearest power of two
- * @n - parameter
+ * @n: parameter
  *
  * round the given value down to the nearest power of two
  * - the result is undefined when n == 0
@@ -185,6 +186,12 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 	__rounddown_pow_of_two(n)		\
  )
 
+static inline __attribute_const__
+int __order_base_2(unsigned long n)
+{
+	return n > 1 ? ilog2(n - 1) + 1 : 0;
+}
+
 /**
  * order_base_2 - calculate the (rounded up) base 2 order of the argument
  * @n: parameter
@@ -198,7 +205,11 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
  *  ob2(5) = 3
  *  ... and so on.
  */
-
-#define order_base_2(n) ilog2(roundup_pow_of_two(n))
-
+#define order_base_2(n)				\
+(						\
+	__builtin_constant_p(n) ? (		\
+		((n) == 0 || (n) == 1) ? 0 :	\
+		ilog2((n) - 1) + 1) :		\
+	__order_base_2(n)			\
+)
 #endif /* _LINUX_LOG2_H */