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

[Jonathan Wakely]

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?

>
> >
> > 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