https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113835
Bug ID: 113835
Summary: compiling std::vector with const size in C++20 is slow
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: pobrn at protonmail dot com
Target Milestone: ---
Consider the following code:
#include <vector>
const std::size_t N = 1'000'000;
std::vector<int> x(N);
int main() {}
Then:
$ hyperfine 'g++ -std=c++20 -O2 x.cpp' 'g++ -std=c++17 -O2 x.cpp'
Benchmark 1: g++ -std=c++20 -O2 x.cpp
Time (mean ± σ): 4.945 s ± 0.116 s [User: 4.676 s, System: 0.229
s]
Range (min … max): 4.770 s … 5.178 s 10 runs
Benchmark 2: g++ -std=c++17 -O2 x.cpp
Time (mean ± σ): 491.3 ms ± 24.0 ms [User: 440.9 ms, System: 46.3
ms]
Range (min … max): 465.6 ms … 538.0 ms 10 runs
Summary
g++ -std=c++17 -O2 x.cpp ran
10.07 ± 0.55 times faster than g++ -std=c++20 -O2 x.cpp
If you remove the `const` from `N`, the runtime will be closer to C++17 levels.
`-ftime-report` suggests that "constant expression evaluation" is the reason. I
imagine this is related to C++20 making std::vector constexpr.
