[PATCH v2] 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 function gives a
modest performance boost:

  Original: 0:11.373s
  New:      0:11.110s

for a performance improvement of ~2%.

[1] From the hash function used in libbpf.

Signed-off-by: Bill Wendling <morbo@google.com>
---
 hash.h | 20 +-------------------
 1 file changed, 1 insertion(+), 19 deletions(-)

diff --git a/hash.h b/hash.h
index d3aa416..6f952c7 100644
--- a/hash.h
+++ b/hash.h
@@ -33,25 +33,7 @@
 
 static inline uint64_t hash_64(const uint64_t val, const unsigned int bits)
 {
-	uint64_t hash = val;
-
-	/*  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;
-
-	/* High bits are more random, so use them. */
-	return hash >> (64 - bits);
+	return (val * 11400714819323198485LLU) >> (64 - bits);
 }
 
 static inline uint32_t hash_32(uint32_t val, unsigned int bits)
-- 
2.30.0.478.g8a0d178c01-goog