Summary:
F14 is a 14-way probing hash table that resolves collisions by double
hashing.  Up to 14 keys are stored in a chunk at a single hash table
position.  SSE2 vector instructions are used to filter within a chunk;
intra-chunk search takes only a handful of instructions.  "F14" refers
to the fact that the algorithm "F"ilters up to "14" keys at a time.
This strategy allows the hash table to be operated at a high maximum
load factor (12/14) while still keeping probe chains very short.

F14 provides compelling replacements for most of the hash tables we use in
production at Facebook.  Switching to it can improve memory efficiency
and performance at the same time.  The hash table implementations
widely deployed in C++ at Facebook exist along a spectrum of space/time
tradeoffs.  The fastest is the least memory efficient, and the most
memory efficient is much slower than the rest.  F14 moves the curve,
simultaneously improving memory efficiency and performance when compared
to the existing algorithms, especially for complex keys and large maps.

Reviewed By: yfeldblum

Differential Revision: D7154343

fbshipit-source-id: 42ebd11b353285855c0fed5dd4b3af4620d39e98