core-kernel: use multiply instead of shifts in hash_64

hash_64(val) = val * (a 64-bit constant). It is "optimized" by replacing the multiply by a bunch of shifts and adds. On modern machines, this is not an optimization; remove it.

Running this hash function in a independent benchmark, it's about three times as fast (1ns vs 3ns) with a multiply as with a shift on Westmere. It's also considerably smaller (and since we inline this function often, that matters.)

---
 include/linux/hash.h |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

index b80506b..daabc3d 100644
@@ -34,7 +34,9 @@
 static inline u64 hash_64(u64 val, unsigned int bits)
 {
 	u64 hash = val;
-
+#if BITS_PER_LONG == 64
+	hash *= GOLDEN_RATIO_PRIME_64;
+#else
 	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
 	u64 n = hash;
 	n <<= 18;

@@ -49,7 +51,7 @@ static inline u64 hash_64(u64 val, unsigned int bits)
 	hash += n;
 	n <<= 2;
 	hash += n;
-
+#endif
 	/* High bits are more random, so use them. */
 	return hash >> (64 - bits);
 }

1.7.7.3

I haven't checked what output gcc provides for 32-bit kernels with this or a literal multiply. It's not even clear what optimization is _asked_ for here (possibly the reduction of strength that we probably don't even want.)