Hi Matt, The best thing to do here is file a bug report with the code to reproduce it: https://gcc.gnu.org/bugzill
Thanks On Sat, 8 Aug 2020 at 23:01, Soul Studios <m...@soulstudios.co.nz> wrote: > > 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 > >
