[Mesa-dev] [PATCH 3/3] util: Use 32 bit integer hash function for pointers

[Thomas Helland]

Since a pointer is basically just an int we can use integer hashing.
This one is taken from https://gist.github.com/badboy/6267743
A google search seems to suggest this is a common and good algorithm.
Since it operates 32 bits at a time it is really effective.
assert that we are hashing 32bit aligned pointers.

Signed-off-by: Thomas Helland <thomashelland90 at gmail.com>
---
 src/util/hash_table.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/util/hash_table.c b/src/util/hash_table.c
index 24184c0..54d04ef 100644
--- a/src/util/hash_table.c
+++ b/src/util/hash_table.c
@@ -393,6 +393,15 @@ _mesa_hash_table_random_entry(struct hash_table *ht,
    return NULL;
 }
 
+static inline uint32_t
+hash_32bit_int(uint32_t a) {
+   a = (a ^ 61) ^ (a >> 16);
+   a = a + (a << 3);
+   a = a ^ (a >> 4);
+   a = a * 0x27d4eb2d;
+   a = a ^ (a >> 15);
+   return a;
+}
 
 /**
  * Quick FNV-1a hash implementation based on:
@@ -406,8 +415,19 @@ _mesa_hash_table_random_entry(struct hash_table *ht,
 uint32_t
 _mesa_hash_data(const void *data, size_t size)
 {
-   return _mesa_fnv32_1a_accumulate_block(_mesa_fnv32_1a_offset_bias,
-                                          data, size);
+   uint32_t hash = _mesa_fnv32_1a_offset_bias;
+   const uint32_t *ints = (const uint32_t *) data;
+
+   assert((size % 4) == 0);
+
+   uint32_t i = size / 4;
+
+   while (i-- != 0) {
+      hash ^= hash_32bit_int(*ints);
+      ints++;
+   }
+
+   return hash;
 }
 
 /** FNV-1a string hash implementation */
-- 
2.3.4