[PATCH] libstdc++: optimize ranges::for_each for segmented iterators

[Yuao Ma]

On Thu, Aug 20, 2026 at 6:31 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Thu, 20 Aug 2026 at 11:30, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > On Wed, 19 Aug 2026 at 17:23, Yuao Ma <addr2line@gmail.com> wrote:
> > >
> > > Hi!
> > >
> > > Similar to std::for_each, this patch optimizes ranges::for_each for
> > > segmented iterators.
> >
> > If I understand correctly, this will break cases that require
> > std::invoke to invoke the function object, e.g.
> >
> > ranges::for_each(r, &T::f);
>
> A more concrete example:
>
> struct T { void f() { } };
> std::deque<T> d;
> ranges::for_each(d, &T::f);
>
> deque's _S_for_each_segment just uses __func without std::invoke, doesn't it?
>

Actually this will compile and run without error, and my local check
verifies this. I think the reason is that what we passed to the __func
is the internal lambda of the std::__for_each_segmented, rather than
the &T::f. The only place which will be called with member function is
correctly handled with std::invoke.

> >
> > >
> > > Fully tested on x86_64-linux with no regressions.
> > >
> > > Using the newly added benchmark, it shows a 3x improvement when using
> > > ranges::for_each with std::deque.
> > >
> > > === Wed Aug 19 03:28:22 PM UTC 2026 ===
> > > for_each.cc               std::for_each vector<int>   2r    1u    0s
> > >       0mem    0pf
> > > for_each.cc               std::for_each deque<int>   2r    2u    0s
> > >      0mem    0pf
> > > for_each.cc               std::for_each list<int>    13r   14u    0s
> > >       0mem    0pf
> > > for_each.cc               std::ranges::for_each vector<int>   2r    2u
> > >    0s         0mem    0pf
> > > for_each.cc               std::ranges::for_each deque<int>   6r    5u
> > >   0s         0mem    0pf
> > > for_each.cc               std::ranges::for_each list<int>  13r   14u
> > >  0s         0mem    0pf
> > > === Wed Aug 19 04:09:51 PM UTC 2026 ===
> > > for_each.cc               std::for_each vector<int>   2r    1u    0s
> > >       0mem    0pf
> > > for_each.cc               std::for_each deque<int>   2r    2u    0s
> > >      0mem    0pf
> > > for_each.cc               std::for_each list<int>    13r   14u    0s
> > >       0mem    0pf
> > > for_each.cc               std::ranges::for_each vector<int>   2r    2u
> > >    0s         0mem    0pf
> > > for_each.cc               std::ranges::for_each deque<int>   2r    1u
> > >   0s         0mem    0pf
> > > for_each.cc               std::ranges::for_each list<int>  13r   14u
> > >  0s         0mem    0pf
> > >
> > > Please take a look when you are available, thanks!
> > >
> > > Note: after preparing this patch I found the -std=gnu++11 in the check
> > > performance script based on Jonathan's guidance. I can prepare a patch
> > > for this tomorrow and get rid of the STD in the benchmark.
> > >
> > > Yuao
>