On 8/10/20 3:30 AM, Jonathan Wakely via Gcc wrote:
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
Also, be sure to follow the instructions at https://gcc.gnu.org/bugs/.
Bill
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
