Yes, I'm seeing clang++ slightly faster too, but only for the limited length fixed string case.

Same here.

I also fiddled with some of the many compiler parameters but felt overwhelmed by the sheer number and complexity of them, so just stuck to the basic ones for now.

I tried various parameters not realizing no improvements. The following performs similarly, without the extra CFLAGS.

$ clang++ -o llil2vec -std=c++11 -Wall -O3 llil2vec.cpp

$ ./llil2vec big1.txt big2.txt big3.txt >out.txt
llil2vec (fixed string length=6) start
get_properties      CPU time : 1.65488 secs
emplace set sort    CPU time : 0.470765 secs
write stdout        CPU time : 0.850356 secs
total               CPU time : 2.97605 secs
total        wall clock time : 3 secs
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