[Bug libstdc++/104719] Use of `std::move` in libstdc++ leads to worsened debug performance

["vittorio.romeo at outlook dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104719

--- Comment #9 from Vittorio Romeo <vittorio.romeo at outlook dot com> ---
I have done some benchmarking for three use cases, both with `-O0` and `-Og`,
hacking my `libstdc++` headers to add `[[gnu::always_inline]]` where deemed
appropriate. 

---

The use cases were:

1. `operator[]` benchmark -- `vector_squareop` and `carray_squareop` as seen
above

2. Iterator benchmark -- `vector_iter` and `carray_iter` as seen above

3. Algorithm benchmark -- `std::accumulate` versus a raw `for` loop

---

All the benchmark results, benchmarking rig specs, and used code available
here:
https://gist.github.com/vittorioromeo/efa005d44ccd4ec7279181768a0c1f0b

---

In short, these are the results:

- For all benchmarks, when using `-O0` without any modification to `libstdc++`,
the overhead of the Standard Library can be huge (+25-400%).

- For all benchmarks, when using `-Og` without any modification to `libstdc++`,
the overhead of the Standard Library is small (+5-15%).

- For the `operator[]` benchmark, when using `-O0` after applying
`[[gnu::always_inline]]` to all the functions touched by the benchmark, we
reduce the overhead from 25% to around 10%.

- For the `operator[]` benchmark, when using `-Og` after applying
`[[gnu::always_inline]]` to all the functions touched by the benchmark, we
reduce the overhead from 34% to around 11%. 

- For the iterator benchmark, when using `-O0` after applying
`[[gnu::always_inline]]` to all the functions touched by the benchmark, we
reduce the overhead from 302% to around 186%. 

- For the iterator benchmark, when using `-Og` after applying
`[[gnu::always_inline]]` to all the functions touched by the benchmark, we
reduce the overhead from 11% to around 8%. 

- For the algorithm benchmark, when using `-O0` after applying
`[[gnu::always_inline]]` to all the functions touched by the benchmark, we
reduce the overhead from 304% to around 47%.

- For the algorithm benchmark, when using `-Og`, independently of whether we
modify `libstdc++` or not, the overhead is around 36%.