Unroll loop in lookup_2_lanes (#3364)

Summary:
The current loop goes from 0 to 31.  It has an if statement to do an assignment for j < 16 and a different assignment for j >= 16.  By unrolling the loop to do the j < 16 and the j >= 16 iterations in parallel the if j < 16 is eliminated and the number of loop iterations is reduced in half.

Then unroll the loop for the j < 16 and the j >=16 to a depth of 2.

This change results in approximately a 55% reduction in the execution time for the bench_ivf_fastscan.py workload on Power 10 when compiled with CMAKE_INSTALL_CONFIG_NAME=Release.

The removal of the if (j < 16) statement and the unrolling of the loop removes branch cycle stall and register dependencies on instruction issue. The result is the unrolled code is able issue instructions earlier thus reducing the total number of cycles required to execute the function.

Pull Request resolved: facebookresearch/faiss#3364

Reviewed By: kuarora

Differential Revision: D56455690

Pulled By: mdouze

fbshipit-source-id: 490a17a40d9d4439b1a8ea22e991e706d68fb2fa

Author: Carl Love

faiss/utils/simdlib.h

@@ -27,6 +27,10 @@
 
 #include <faiss/utils/simdlib_neon.h>
 
+#elif defined(__PPC64__)
+
+#include <faiss/utils/simdlib_ppc64.h>
+
 #else
 
 // emulated = all operations are implemented as scalars