[PATCH] LOG2: Implement a general integer log2 facility in the kernel

This facility provides three entry points:

	ilog2()		Log base 2 of unsigned long
	ilog2_u32()	Log base 2 of u32
	ilog2_u64()	Log base 2 of u64

These facilities can either be used inside functions on dynamic data:

	int do_something(long q)
	{
		...;
		y = ilog2(x)
		...;
	}

Or can be used to statically initialise global variables with constant values:

	unsigned n = ilog2(27);

When performing static initialisation, the compiler will report "error:
initializer element is not constant" if asked to take a log of zero or of
something not reducible to a constant.  They treat negative numbers as
unsigned.

When not dealing with a constant, they fall back to using fls() which permits
them to use arch-specific log calculation instructions - such as BSR on
x86/x86_64 or SCAN on FRV - if available.

[akpm@osdl.org: MMC fix]
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: David Howells <dhowells@redhat.com>
Cc: Wojtek Kaniewski <wojtekka@toxygen.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 132 insertions, 8 deletions

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 6738283ac385..3710cce16642 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -13,6 +13,7 @@
 #include <linux/types.h>
 #include <linux/compiler.h>
 #include <linux/bitops.h>
+#include <linux/log2.h>
 #include <asm/byteorder.h>
 #include <asm/bug.h>
 
@@ -157,14 +158,6 @@ static inline int printk(const char *s, ...) { return 0; }
 
 unsigned long int_sqrt(unsigned long);
 
-static inline int __attribute_pure__ long_log2(unsigned long x)
-{
-        int r = 0;
-        for (x >>= 1; x > 0; x >>= 1)
-                r++;
-        return r;
-}
-
 static inline unsigned long
 __attribute_const__ roundup_pow_of_two(unsigned long x)
 {
diff --git a/include/linux/log2.h b/include/linux/log2.h
new file mode 100644
index 000000000000..3979c60325ff
--- /dev/null
+++ b/include/linux/log2.h
@@ -0,0 +1,131 @@
+/* Integer base 2 logarithm calculation
+ *
+ * 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
+#define _LINUX_LOG2_H
+
+#include <linux/types.h>
+#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()
+ * - the arch is not required to handle n==0 if implementing the fallback
+ */
+#ifndef CONFIG_ARCH_HAS_ILOG2_U32
+static inline __attribute__((const))
+int __ilog2_u32(u32 n)
+{
+        return fls(n) - 1;
+}
+#endif
+
+#ifndef CONFIG_ARCH_HAS_ILOG2_U64
+static inline __attribute__((const))
+int __ilog2_u64(u64 n)
+{
+        return fls64(n) - 1;
+}
+#endif
+
+/**
+ * ilog2 - log of 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
+ *
+ * selects the appropriately-sized optimised version depending on sizeof(n)
+ */
+#define ilog2(n)                                \
+(                                               \
+        __builtin_constant_p(n) ? (             \
+                (n) < 1 ? ____ilog2_NaN() :     \
+                (n) & (1ULL << 63) ? 63 :       \
+                (n) & (1ULL << 62) ? 62 :       \
+                (n) & (1ULL << 61) ? 61 :       \
+                (n) & (1ULL << 60) ? 60 :       \
+                (n) & (1ULL << 59) ? 59 :       \
+                (n) & (1ULL << 58) ? 58 :       \
+                (n) & (1ULL << 57) ? 57 :       \
+                (n) & (1ULL << 56) ? 56 :       \
+                (n) & (1ULL << 55) ? 55 :       \
+                (n) & (1ULL << 54) ? 54 :       \
+                (n) & (1ULL << 53) ? 53 :       \
+                (n) & (1ULL << 52) ? 52 :       \
+                (n) & (1ULL << 51) ? 51 :       \
+                (n) & (1ULL << 50) ? 50 :       \
+                (n) & (1ULL << 49) ? 49 :       \
+                (n) & (1ULL << 48) ? 48 :       \
+                (n) & (1ULL << 47) ? 47 :       \
+                (n) & (1ULL << 46) ? 46 :       \
+                (n) & (1ULL << 45) ? 45 :       \
+                (n) & (1ULL << 44) ? 44 :       \
+                (n) & (1ULL << 43) ? 43 :       \
+                (n) & (1ULL << 42) ? 42 :       \
+                (n) & (1ULL << 41) ? 41 :       \
+                (n) & (1ULL << 40) ? 40 :       \
+                (n) & (1ULL << 39) ? 39 :       \
+                (n) & (1ULL << 38) ? 38 :       \
+                (n) & (1ULL << 37) ? 37 :       \
+                (n) & (1ULL << 36) ? 36 :       \
+                (n) & (1ULL << 35) ? 35 :       \
+                (n) & (1ULL << 34) ? 34 :       \
+                (n) & (1ULL << 33) ? 33 :       \
+                (n) & (1ULL << 32) ? 32 :       \
+                (n) & (1ULL << 31) ? 31 :       \
+                (n) & (1ULL << 30) ? 30 :       \
+                (n) & (1ULL << 29) ? 29 :       \
+                (n) & (1ULL << 28) ? 28 :       \
+                (n) & (1ULL << 27) ? 27 :       \
+                (n) & (1ULL << 26) ? 26 :       \
+                (n) & (1ULL << 25) ? 25 :       \
+                (n) & (1ULL << 24) ? 24 :       \
+                (n) & (1ULL << 23) ? 23 :       \
+                (n) & (1ULL << 22) ? 22 :       \
+                (n) & (1ULL << 21) ? 21 :       \
+                (n) & (1ULL << 20) ? 20 :       \
+                (n) & (1ULL << 19) ? 19 :       \
+                (n) & (1ULL << 18) ? 18 :       \
+                (n) & (1ULL << 17) ? 17 :       \
+                (n) & (1ULL << 16) ? 16 :       \
+                (n) & (1ULL << 15) ? 15 :       \
+                (n) & (1ULL << 14) ? 14 :       \
+                (n) & (1ULL << 13) ? 13 :       \
+                (n) & (1ULL << 12) ? 12 :       \
+                (n) & (1ULL << 11) ? 11 :       \
+                (n) & (1ULL << 10) ? 10 :       \
+                (n) & (1ULL <<  9) ?  9 :       \
+                (n) & (1ULL <<  8) ?  8 :       \
+                (n) & (1ULL <<  7) ?  7 :       \
+                (n) & (1ULL <<  6) ?  6 :       \
+                (n) & (1ULL <<  5) ?  5 :       \
+                (n) & (1ULL <<  4) ?  4 :       \
+                (n) & (1ULL <<  3) ?  3 :       \
+                (n) & (1ULL <<  2) ?  2 :       \
+                (n) & (1ULL <<  1) ?  1 :       \
+                (n) & (1ULL <<  0) ?  0 :       \
+                ____ilog2_NaN()                 \
+                                   ) :          \
+        (sizeof(n) <= 4) ?                      \
+        __ilog2_u32(n) :                        \
+        __ilog2_u64(n)                          \
+ )
+
+#endif /* _LINUX_LOG2_H */