[PATCH] dwarf_loader: use a better hashing function

[Bill Wendling <morbo@google.com>]

This hashing function[1] produces better hash table bucket
distributions. The original hashing function always produced zeros in
the three least significant bits.

The new hashing funciton gives a modest performance boost.

      Original      New
       0:11.41       0:11.38
       0:11.36       0:11.34
       0:11.35       0:11.26
      -----------------------
  Avg: 0:11.373      0:11.327

for a performance improvement of 0.4%.

[1] From Numerical Recipes, 3rd Ed. 7.1.4 Random Hashes and Random Bytes

Signed-off-by: Bill Wendling <morbo@google.com>
---
 hash.h | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/hash.h b/hash.h
index d3aa416..ea201ab 100644
--- a/hash.h
+++ b/hash.h
@@ -33,22 +33,17 @@
 
 static inline uint64_t hash_64(const uint64_t val, const unsigned int bits)
 {
-	uint64_t hash = val;
+	uint64_t hash = val * 0x369DEA0F31A53F85UL + 0x255992D382208B61UL;
 
-	/*  Sigh, gcc can't optimise this alone like it does for 32 bits. */
-	uint64_t n = hash;
-	n <<= 18;
-	hash -= n;
-	n <<= 33;
-	hash -= n;
-	n <<= 3;
-	hash += n;
-	n <<= 3;
-	hash -= n;
-	n <<= 4;
-	hash += n;
-	n <<= 2;
-	hash += n;
+	hash ^= hash >> 21;
+	hash ^= hash << 37;
+	hash ^= hash >>  4;
+
+	hash *= 0x422E19E1D95D2F0DUL;
+
+	hash ^= hash << 20;
+	hash ^= hash >> 41;
+	hash ^= hash <<  5;
 
 	/* High bits are more random, so use them. */
 	return hash >> (64 - bits);
-- 
2.30.0.478.g8a0d178c01-goog