[Bug libstdc++/100133] New: std::copy extremely slow for random access iterator

[Bug libstdc++/100133] New: std::copy extremely slow for random access iterator

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100133

            Bug ID: 100133
           Summary: std::copy extremely slow for random access iterator
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

HI, I found that libstdc++‘s std::copy is extremely slow for random access
iterator. For the following example, if libc++'s std::copy is used, copy from
std::vector will be 10 times faster than from std::list with -O3, but for
libstdc++'s std::copy, copying from std::vector is twice as slow as std::list.

#include <vector>
#include <list>
#include <algorithm>

const std::vector<int> v(100, 42);
const std::list<int> l(100, 42);

static void copy_from_vector(benchmark::State& state)
{
  for (auto _ : state) {
    std::vector<int> to(v.size());
    std::copy(
      v.begin(), v.end(), 
      to.begin()
    );
  }
}
BENCHMARK(copy_from_vector);

static void copy_from_list(benchmark::State& state)
{
  for (auto _ : state) {
    std::vector<int> to(l.size());
    std::copy(
      l.begin(), l.end(), 
      to.begin()
    );
  }
}
BENCHMARK(copy_from_list);

you can see libc++‘s std::copy benchmark here:
https://quick-bench.com/q/OPrNMQpJf6jKPBT5G-Uq-4U6XHc
and libstdc++'s std::copy here:
https://quick-bench.com/q/cuKCK_VE-KJgD3xA-ogXNCj9UCQ.

[Bug libstdc++/100133] std::copy extremely slow for random access iterator

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100133

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
And the libstdc++‘s std::copy is also 8.9 times slower than the equivalent
std::tranfrom:

#include <vector>
#include <list>
#include <algorithm>

const std::vector<int> v(100, 42);

static void copy_from_vector(benchmark::State& state)
{
  for (auto _ : state) {
    std::vector<int> to(v.size());
    std::copy(
      v.begin(), v.end(), 
      to.begin()
    );
  }
}
BENCHMARK(copy_from_vector);

static void transform_from_vector(benchmark::State& state)
{
  for (auto _ : state) {
    std::vector<int> to(v.size());
    std::transform(
      v.begin(), v.end(), 
      to.begin(),
      [](int x) { return x; }
    );
  }
}
BENCHMARK(transform_from_vector);

you can see the benchmark here:
https://quick-bench.com/q/yuGf3npWpeU2EQhzzGud-aYNlNE. If using libc++,
std::copy is 2.2 times faster than std::transform as expected, see
https://quick-bench.com/q/3_V0EaU2jxnx4kSjWdarq7FOSh0.

[Bug libstdc++/100133] std::copy extremely slow for random access iterator

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100133

--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> ---
After actually executing the same code on my local and remote servers, I did
not produce such a result. In both cases, std::copy achieved the expected
high-efficiency performance, so I think this should only be Quick C++ Benchmark
Own question, thank you for your time

[Bug libstdc++/100133] std::copy extremely slow for random access iterator

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100133

康桓瑋 <hewillk at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to 康桓瑋 from comment #2)
> After actually executing the same code on my local and remote servers, I did
> not produce such a result. In both cases, std::copy achieved the expected
> high-efficiency performance, so I think this should only be Quick C++
> Benchmark
> Own question, thank you for your time

Close.

[Bug libstdc++/100133] std::copy extremely slow for random access iterator

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100133

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |INVALID

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This should be RESOLVED INVALID not RESOLVED FIXED, because nothing needed to
be fixed.