SL-19110 Fast hashing classes for use in place of the slow LLMD5, where speed matters. (#64)

This commit adds the HBXX64 and HBXX128 classes for use as a drop-in
replacement for the slow LLMD5 hashing class, where speed matters and
backward compatibility (with standard hashing algorithms) and/or
cryptographic hashing qualities are not required.
It also replaces LLMD5 with HBXX* in a few existing hot (well, ok, just
"warm" for some) paths meeting the above requirements, while paving the way for
future use cases, such as in the DRTVWR-559 and sibling branches where the slow
LLMD5 is used (e.g. to hash materials and vertex buffer cache entries), and
could be use such a (way) faster algorithm with very significant benefits and
no negative impact.

Here is the comment I added in indra/llcommon/hbxx.h:

// HBXXH* classes are to be used where speed matters and cryptographic quality
// is not required (no "one-way" guarantee, though they are likely not worst in
// this respect than MD5 which got busted and is now considered too weak). The
// xxHash code they are built upon is vectorized and about 50 times faster than
// MD5. A 64 bits hash class is also provided for when 128 bits of entropy are
// not needed. The hashes collision rate is similar to MD5's.
// See https://github.com/Cyan4973/xxHash#readme for details.

1 files changed, 8 insertions, 0 deletions

diff --git a/indra/cmake/xxHash.cmake b/indra/cmake/xxHash.cmake
new file mode 100644
index 0000000000..a7c1cba62c
--- /dev/null
+++ b/indra/cmake/xxHash.cmake
@@ -0,0 +1,8 @@
+# -*- cmake -*-
+if (XXHASH_CMAKE_INCLUDED)
+  return()
+endif (XXHASH_CMAKE_INCLUDED)
+set (XXHASH_CMAKE_INCLUDED TRUE)
+
+include(Prebuilt)
+use_prebuilt_binary(xxhash)