x86-64: word-at-a-time: improve byte count calculations

This switches x86-64 over to using 'tzcount' instead of the integer
multiply trick to turn the bytemask information into actual byte counts.

We even had a comment saying that a fast bit count instruction is better
than a multiply, but x86 bit counting has traditionally been
"questionably fast", and so avoiding it was the right thing back in the
days.

Now, on any half-way modern core, using bit counting is cheaper and
smaller than the large constant multiply, so let's just switch over.

Note that as part of switching over to counting bits, we also do it at a
different point.  We used to create the byte count from the final byte
mask, but once you use the 'tzcount' instruction (aka 'bsf' on older
CPU's), you can actually count the leading zeroes using a value we have
available earlier.

In fact, we can just use the very first mask of bits that tells us
whether we have any zero bytes at all.  The zero bytes in the word will
have the high bit set, so just doing 'tzcount' on that value and
dividing by 8 will give the number of bytes that precede the first NUL
character, which is exactly what we want.

Note also that the input value to the tzcount is by definition not zero,
since that is the condition that we already used to check the whole "do
we have any zero bytes at all".  So we don't need to worry about the
legacy instruction behavior of pre-lzcount days when 'bsf' didn't have a
result for zero input.

The 32-bit code continues to use the bimple bit op trick that is faster
even on newer cores, but particularly on the older 32-bit-only ones.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 34 deletions

diff --git a/arch/x86/include/asm/word-at-a-time.h b/arch/x86/include/asm/word-at-a-time.h
index e8d7d4941c4c..422a47746657 100644
--- a/arch/x86/include/asm/word-at-a-time.h
+++ b/arch/x86/include/asm/word-at-a-time.h
@@ -5,45 +5,12 @@
 #include <linux/bitops.h>
 #include <linux/wordpart.h>
 
-/*
- * This is largely generic for little-endian machines, but the
- * optimal byte mask counting is probably going to be something
- * that is architecture-specific. If you have a reliably fast
- * bit count instruction, that might be better than the multiply
- * and shift, for example.
- */
 struct word_at_a_time {
 	const unsigned long one_bits, high_bits;
 };
 
 #define WORD_AT_A_TIME_CONSTANTS { REPEAT_BYTE(0x01), REPEAT_BYTE(0x80) }
 
-#ifdef CONFIG_64BIT
-
-/*
- * Jan Achrenius on G+: microoptimized version of
- * the simpler "(mask & ONEBYTES) * ONEBYTES >> 56"
- * that works for the bytemasks without having to
- * mask them first.
- */
-static inline long count_masked_bytes(unsigned long mask)
-{
-	return mask*0x0001020304050608ul >> 56;
-}
-
-#else	/* 32-bit case */
-
-/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
-static inline long count_masked_bytes(long mask)
-{
-	/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
-	long a = (0x0ff0001+mask) >> 23;
-	/* Fix the 1 for 00 case */
-	return a & mask;
-}
-
-#endif
-
 /* Return nonzero if it has a zero */
 static inline unsigned long has_zero(unsigned long a, unsigned long *bits, const struct word_at_a_time *c)
 {
@@ -57,6 +24,22 @@ static inline unsigned long prep_zero_mask(unsigned long a, unsigned long bits,
 	return bits;
 }
 
+#ifdef CONFIG_64BIT
+
+/* Keep the initial has_zero() value for both bitmask and size calc */
+#define create_zero_mask(bits) (bits)
+
+static inline unsigned long zero_bytemask(unsigned long bits)
+{
+	bits = (bits - 1) & ~bits;
+	return bits >> 7;
+}
+
+#define find_zero(bits) (__ffs(bits) >> 3)
+
+#else
+
+/* Create the final mask for both bytemask and size */
 static inline unsigned long create_zero_mask(unsigned long bits)
 {
 	bits = (bits - 1) & ~bits;
@@ -66,11 +49,17 @@ static inline unsigned long create_zero_mask(unsigned long bits)
 /* The mask we created is directly usable as a bytemask */
 #define zero_bytemask(mask) (mask)
 
+/* Carl Chatfield / Jan Achrenius G+ version for 32-bit */
 static inline unsigned long find_zero(unsigned long mask)
 {
-	return count_masked_bytes(mask);
+	/* (000000 0000ff 00ffff ffffff) -> ( 1 1 2 3 ) */
+	long a = (0x0ff0001+mask) >> 23;
+	/* Fix the 1 for 00 case */
+	return a & mask;
 }
 
+#endif
+
 /*
  * Load an unaligned word from kernel space.
  *