On Wed, Jan 25, 2006 at 08:32:06PM +0900, Akinobu Mita wrote:

+#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	mask;
+	int b = 0, s;
 
-	addr += nr >> 5;
-	mask = 1 << (nr & 0x1f);
-	return ((mask & *addr) != 0);
+#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

This code generates more expensive shifts than our (ARMs) existing C
version.  This is a backward step.

Basically, shifts which depend on a variable are more expensive than
constant-based shifts.

I've not really looked at the rest because I haven't figured out which
bits will be used on ARM and which won't - which I think is another
problem with this patch set.  I'll look again later tonight.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core