SL-19159 Faster LLUUID and LLMaterialID hashing for std and boost containers keys (#70)

LLUUID and LLMaterialID already have an excellent entropy and value dispersion; there is therefore strictly no need to further (slowly) hash their value for use with std and boost libraries containers.

This commit adds a trivial getDigest64() method to both LLUUID and LLMaterialID (which simply returns the XOR of the two 64 bits long words their value is made of), and uses it in std::hash and hash_value() specializations for use with containers.

1 files changed, 21 insertions, 29 deletions

diff --git a/indra/llcommon/lluuid.h b/indra/llcommon/lluuid.h
index 86a396ab06..2c36a1d222 100644
--- a/indra/llcommon/lluuid.h
+++ b/indra/llcommon/lluuid.h
@@ -119,6 +119,14 @@ public:
 	U16 getCRC16() const;
 	U32 getCRC32() const;
 
+	// Returns a 64 bits digest of the UUID, by XORing its two 64 bits long
+	// words. HB
+	inline U64 getDigest64() const
+	{
+		U64* tmp = (U64*)mData;
+		return tmp[0] ^ tmp[1];
+	}
+
 	static BOOL validate(const std::string& in_string); // Validate that the UUID string is legal.
 
 	static const LLUUID null;
@@ -165,36 +173,20 @@ public:
 	LLAssetID makeAssetID(const LLUUID& session) const;
 };
 
-// Generate a hash of an LLUUID object using the boost hash templates. 
-template <>
-struct boost::hash<LLUUID>
-{
-    typedef LLUUID argument_type;
-    typedef std::size_t result_type;
-    result_type operator()(argument_type const& s) const
-    {
-        result_type seed(0);
-
-        for (S32 i = 0; i < UUID_BYTES; ++i)
-        {
-            boost::hash_combine(seed, s.mData[i]);
-        }
-
-        return seed;
-    }
-};
-
-// Adapt boost hash to std hash
+// std::hash implementation for LLUUID
 namespace std
 {
-    template<> struct hash<LLUUID>
-    {
-        std::size_t operator()(LLUUID const& s) const noexcept
-        {
-            return boost::hash<LLUUID>()(s);
-        }
-    };
+	template<> struct hash<LLUUID>
+	{
+		inline size_t operator()(const LLUUID& id) const noexcept
+		{
+			return (size_t)id.getDigest64();
+		}
+	};
 }
-#endif
-
 
+// For use with boost containers.
+inline size_t hash_value(const LLUUID& id) noexcept
+{
+	return (size_t)id.getDigest64();
+}