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From: "Mark_B53 at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/108487] New: ~20-30x slowdown in populating
 std::vector from std::ranges::iota_view
Date: Fri, 20 Jan 2023 22:20:32 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 12.2.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: Mark_B53 at yahoo dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108487

            Bug ID: 108487
           Summary: ~20-30x slowdown in populating std::vector from
                    std::ranges::iota_view
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Mark_B53 at yahoo dot com
  Target Milestone: ---

Using -std=3Dc++20 -O3, comparing gcc 12.2 vs. gcc 10.3:
 * fn2 is 20-30x slower on gcc 12.2 (i.e. 2000-3000% more)=20
 * fn1 is ~20% slower on gcc 12.2=20

This test was run on an 52 core Intel Xeon Gold 6278C CPU.  Tests on
www.godbolt.org directionally align with these findings.  It seems the slow=
down
was introduced in 10.4 & 11.1.  The trunk has identical performance to 12.2.

#include <vector>
#include <ranges>
#include <ctime>
#include <iostream>

__attribute__((noinline)) std::vector<int> fn1(int n)
{
    auto v =3D std::vector<int>(n);
    for(int i =3D 0; i !=3D n; ++i)
        v[i] =3D i;
    return v;
}

__attribute__((noinline)) std::vector<int> fn2(int n)
{
    auto rng =3D std::ranges::iota_view{0, n};
    return std::vector<int>{rng.begin(), rng.end()};
}

int main() {
    int n =3D 100'000;
    int times =3D 100'000;

    auto t0 =3D std::clock();
    for (int i =3D 0; i < times; ++i)
        fn1(n);=20=20=20=20=20=20=20=20=20=20=20=20
    auto t1 =3D std::clock();
    for (int i =3D 0; i < times; ++i)
        fn2(n);=20=20=20=20=20=20=20=20=20=20=20=20
    auto t2 =3D std::clock();
    std::cout << t1 - t0 << '\n';
    std::cout << t2 - t1 << '\n';
    return 0;
}

P.S. 20% slowdown for a common vector population is still significant IMO. =
 I
am not sure that qualifies as a bug.  I did not file one on account of the
'fn1' slowdown.=