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From: "pobrn at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113835] New: compiling std::vector with const size in C++20
 is slow
Date: Thu, 08 Feb 2024 17:11:36 +0000
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X-Bugzilla-Version: 13.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113835

            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 =3D 1'000'000;
    std::vector<int> x(N);
    int main() {}

Then:

    $ hyperfine 'g++ -std=3Dc++20 -O2 x.cpp' 'g++ -std=3Dc++17 -O2 x.cpp'
    Benchmark 1: g++ -std=3Dc++20 -O2 x.cpp
      Time (mean =C2=B1 =CF=83):      4.945 s =C2=B1  0.116 s    [User: 4.6=
76 s, System: 0.229
s]
      Range (min =E2=80=A6 max):    4.770 s =E2=80=A6  5.178 s    10 runs

    Benchmark 2: g++ -std=3Dc++17 -O2 x.cpp
      Time (mean =C2=B1 =CF=83):     491.3 ms =C2=B1  24.0 ms    [User: 440=
.9 ms, System: 46.3
ms]
      Range (min =E2=80=A6 max):   465.6 ms =E2=80=A6 538.0 ms    10 runs

    Summary
      g++ -std=3Dc++17 -O2 x.cpp ran
       10.07 =C2=B1 0.55 times faster than g++ -std=3Dc++20 -O2 x.cpp

If you remove the `const` from `N`, the runtime will be closer to C++17 lev=
els.

`-ftime-report` suggests that "constant expression evaluation" is the reaso=
n. I
imagine this is related to C++20 making std::vector constexpr.=