https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96750
Bug ID: 96750
Summary: 10-12% performance decrease in benchmark going from
GCC8 to GCC9/GCC10
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mattreecebentley at gmail dot com
Target Milestone: ---
Created attachment 49102
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49102&action=edit
Compiler output
Have recently been working on a new version of the plf::colony container
(plflib.org) and found GCC9 was giving ~10% worse performance on average in a
given benchmark than GCC8. Further investigation found GCC10 was just as bad.
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 presented is 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.
Compilers/environments used:
Xubuntu 20: GCC8.4, GCC9.3, GCC10.0.1
Windows 7: Nuwen mingw GCC8.2, nuwen mingw GCC9.2
The attached code output is from the Xubuntu environment.
Any questions let me know. I will help where I can, but my knowledge of
assembly is limited.
Information on code components:
Nanotimer is a ~nanosecond-precision sub-timeslice cross-platform timer.
Colony is a bucket-array-like unordered sequence container.
The attached zip contains the build logs and compiler preprocessed outputs for
GCC 8.4, 9.3 and 10.0.1
Thanks-
Mat
