> For some reason, short string version is faster when compiled with clang++
> (while long string version is not)
> clang++ -o llil2vec -std=c++11 -Wall -O3 llil2vec.cpp

> ./llil2vec big1.txt big2.txt big3.txt >f.tmp
llil2vec (fixed string length=6) start
get_properties      CPU time : 1.87217 secs
emplace set sort    CPU time : 0.589238 secs
write stdout        CPU time : 0.842179 secs
total               CPU time : 3.30369 secs
total        wall clock time : 3 secs

> ./llil2vec big1.txt big2.txt big3.txt >f.tmp
llil2vec (fixed string length=6) start
get_properties      CPU time : 1.95909 secs
emplace set sort    CPU time : 0.610479 secs
write stdout        CPU time : 0.959859 secs
total               CPU time : 3.52965 secs
total        wall clock time : 4 secs

> ./llil2vec big1.txt big2.txt big3.txt >f.tmp
llil2vec (fixed string length=6) start
get_properties      CPU time : 1.86549 secs
emplace set sort    CPU time : 0.608097 secs
write stdout        CPU time : 0.857176 secs
total               CPU time : 3.33087 secs
total        wall clock time : 3 secs

> diff f.tmp vec.tmp