[PATCH 2/12] generic __ffs()

[Akinobu Mita]

This patch introduces the C-language equivalent of the function:
unsigned long __ffs(unsigned long word);

HAVE_ARCH___FFS_BITOPS is defined when the architecture has its own
version of these functions.

This code largely copied from:
include/asm-sparc64/bitops.h


Index: 2.6-git/include/asm-generic/bitops.h
===================================================================
--- 2.6-git.orig/include/asm-generic/bitops.h	2006-01-25 19:14:08.000000000 +0900
+++ 2.6-git/include/asm-generic/bitops.h	2006-01-25 19:14:09.000000000 +0900
@@ -193,6 +193,43 @@
 
 #endif /* HAVE_ARCH_NON_ATOMIC_BITOPS */
 
+#ifndef HAVE_ARCH___FFS_BITOPS
+
+/**
+ * __ffs - find first bit in word.
+ * @word: The word to search
+ *
+ * Returns 0..BITS_PER_LONG-1
+ * Undefined if no bit exists, so code should check against 0 first.
+ */
+static inline unsigned long __ffs(unsigned long word)
+{
+	int b = 0, s;
+
+#if BITS_PER_LONG == 32
+	s = 16; if (word << 16 != 0) s = 0; b += s; word >>= s;
+	s =  8; if (word << 24 != 0) s = 0; b += s; word >>= s;
+	s =  4; if (word << 28 != 0) s = 0; b += s; word >>= s;
+	s =  2; if (word << 30 != 0) s = 0; b += s; word >>= s;
+	s =  1; if (word << 31 != 0) s = 0; b += s;
+
+	return b;
+#elif BITS_PER_LONG == 64
+	s = 32; if (word << 32 != 0) s = 0; b += s; word >>= s;
+	s = 16; if (word << 48 != 0) s = 0; b += s; word >>= s;
+	s =  8; if (word << 56 != 0) s = 0; b += s; word >>= s;
+	s =  4; if (word << 60 != 0) s = 0; b += s; word >>= s;
+	s =  2; if (word << 62 != 0) s = 0; b += s; word >>= s;
+	s =  1; if (word << 63 != 0) s = 0; b += s;
+
+	return b;
+#else
+#error BITS_PER_LONG not defined
+#endif
+}
+
+#endif /* HAVE_ARCH___FFS_BITOPS */
+
 /*
  * fls: find last bit set.
  */
-
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