10-12% performance decrease in benchmark going from GCC8 to GCC9

[Soul Studios]

Hi all,
recently have been working on a new version of the plf::colony container 
(plflib.org) and found GCC9 was giving 10-12% worse performance on a 
given benchmark than GCC8.

Previous versions of the colony container did not experience this 
performance loss going from GCC8 to GCC9.
However Clang 6 and MSVC2019 show no performance loss going from the old 
colony version to the new version.

The effect is repeatable across architectures - I've tested on xubuntu, 
windows running nuwen mingw, and on Core2 and Haswell CPUs, with and 
without -march=native specified.

Compiler flags are: -O2;-march=native;-std=c++17

Code is attached with an absolute minimum use-case - other benchmarks 
have not shown such strong performance differences - including both 
simpler and more complex tests.
So I cannot reduce further, please do not ask me to do so.

The benchmark in question inserts into a container initially then 
iterates over container elements repeatedly, randomly erasing and/or 
inserting new elements.


In addition I've attached the assembly output under both GCC8 and GCC9. 
In this case I have output from 8.2 and 9.2 respectively, but the same 
effects apply to 8.4 and 9.3. The output for 8 is a lot larger than 9, 
wondering if there's more unrolling occurring.

Any questions let me know. I will help where I can, but my knowledge of 
assembly is limited. If supplying the older version of colony is useful 
I'm happy to do so.

Nanotimer is a ~nanosecond-precision sub-timeslice cross-platform timer.
Colony is a bucket-array-like unordered sequence container.
Thanks,
Matt


<<attachment: benchmark_code_and_results.zip>>