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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100070] Standard library container iterator-pair
 constructors should check C++20 iterator concepts
Date: Wed, 14 Apr 2021 16:41:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100070

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Note to self/Patrick:

Measure whether it helps to specialize transform_view's iterator so that wh=
en
_Base_iter is __normal_iterator we unwrap it and store a raw pointer.

Also, I suspect the indirections in

  return std::__invoke(*_M_parent->_M_fun, *_M_current);

are making the optimizer give up.

We have an indirection to the parent to access the semi-regular box, which =
has
its own indirections. Maybe we could just get rid of the semi-regular box f=
or a
function pointer and store a function pointer (i.e. decay_t<Fp>) directly. =
That
would have the same syntax (i.e. operator*) to access it as the semi-regular
box, but would be less abstraction to un-abstract.

And maybe store a function pointer directly in the transform_view iterator,=
 so
we don't need to go to the parent to get it on every dereference.

Barry pointed out that range-v3 elides the use of semi-regular box for some
cases, and he confirmed that storing a function pointer in the iterator hel=
ps.=