On Mon, Jul 13, 2026 at 5:22 PM Patrick Palka <[email protected]> wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk and perhaps 16?
> I'm not sure if it's worth avoiding dereferencing *__first twice in this
> special case, but I figured we might as well?
>
> -- >8 --
>
> When inserting a range of pair-like elements we can avoid constructing a
> value_type temporary and instead obtain the corresponding key and value
> to insert directly from *__first.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/flat_map (flat_map::_M_insert): Avoid constructing
> value_type temporary when the iterator already has pair-like
> elements.
> ---
> libstdc++-v3/include/std/flat_map | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
LGTM, only small suggestions.
>
> diff --git a/libstdc++-v3/include/std/flat_map
> b/libstdc++-v3/include/std/flat_map
> index b82d41b4f5e1..1815ced51225 100644
> --- a/libstdc++-v3/include/std/flat_map
> +++ b/libstdc++-v3/include/std/flat_map
> @@ -637,9 +637,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> auto __n = size();
> for (; __first != __last; ++__first)
> {
>
I do not think we need this braces for nesting.
> - value_type __value = *__first;
> - _M_cont.keys.emplace_back(std::move(__value.first));
> - _M_cont.values.emplace_back(std::move(__value.second));
> + if constexpr (__pair_like<iter_reference_t<_Iter>>)
> + {
> + auto&& __value = *__first;
> + _M_cont.keys.emplace_back
> +
> (std::get<0>(std::forward<decltype(__value)>(__value)));
> + _M_cont.values.emplace_back
> +
> (std::get<1>(std::forward<decltype(__value)>(__value)));
>
// Maybe we could alias iter_reference_t<_Iter> and replace the
decltypes(__value) and auto&&?
> + }
> + else
> + {
> + value_type __value = *__first;
> + _M_cont.keys.emplace_back(std::move(__value.first));
> + _M_cont.values.emplace_back(std::move(__value.second));
> + }
> }
> auto __zv = views::zip(_M_cont.keys, _M_cont.values);
> if (__is_sorted)
> --
> 2.55.0.122.gf85a7e6620
>
> Do you think it will be worth to have a test that will count number of
moves, when
we pass sourted_unique flag is passed?
