On Wed, 5 May 2021, Patrick Palka wrote:
> On Wed, 5 May 2021, Jonathan Wakely wrote:
>
> > On 04/05/21 21:42 -0400, Patrick Palka via Libstdc++ wrote:
> > > This rewrites ranges::minmax and ranges::minmax_element so that it
> > > performs at most 3*N/2 many comparisons, as required by the standard.
> > > In passing, this also fixes PR100387 by avoiding a premature std::move
> > > in ranges::minmax and in std::shift_right.
> > >
> > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk and perhaps
> > > 10/11?
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > PR libstdc++/100387
> > > * include/bits/ranges_algo.h (__minmax_fn::operator()): Rewrite
> > > to limit comparison complexity to 3*N/2. Avoid premature std::move.
> > > (__minmax_element_fn::operator()): Likewise.
> > > (shift_right): Avoid premature std::move of __result.
> > > * testsuite/25_algorithms/minmax/constrained.cc (test04, test05):
> > > New tests.
> > > * testsuite/25_algorithms/minmax_element/constrained.cc (test02):
> > > Likewise.
> > > ---
> > > libstdc++-v3/include/bits/ranges_algo.h | 87 ++++++++++++++-----
> > > .../25_algorithms/minmax/constrained.cc | 31 +++++++
> > > .../minmax_element/constrained.cc | 19 ++++
> > > 3 files changed, 113 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> > > b/libstdc++-v3/include/bits/ranges_algo.h
> > > index cda3042c11f..bbd29127e89 100644
> > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > @@ -3291,18 +3291,39 @@ namespace ranges
> > > auto __first = ranges::begin(__r);
> > > auto __last = ranges::end(__r);
> > > __glibcxx_assert(__first != __last);
> > > + auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
> > > minmax_result<range_value_t<_Range>> __result = {*__first, *__first};
> > > while (++__first != __last)
> > > {
> > > - auto __tmp = *__first;
> > > - if (std::__invoke(__comp,
> > > - std::__invoke(__proj, __tmp),
> > > - std::__invoke(__proj, __result.min)))
> > > - __result.min = std::move(__tmp);
> > > - if (!(bool)std::__invoke(__comp,
> > > - std::__invoke(__proj, __tmp),
> > > - std::__invoke(__proj, __result.max)))
> > > - __result.max = std::move(__tmp);
> > > + // Process two elements at a time so that we perform at most
> > > + // 3*N/2 many comparisons in total (each of the N/2 iterations
> >
> > Is "many" a typo here?
>
> Just a bad habit of mine to usually write "<count> many" instead of just
> "<count>" :) Consider the "many" removed.
>
> >
> > > + // of this loop performs three comparisions).
> > > + auto __val1 = *__first;
> >
> > Can we avoid making this copy if the range satisfies forward_range, by
> > keeping copies of the min/max iterators, or just forwarding to
> > ranges::minmax_element?
>
> Maybe we can make __val1 and __val2 universal references? Ah, but then
> __val1 would potentially be invalidated after incrementing __first. I
> think it should be safe to make __val2 a universal reference though.
> I've done this in v2 below.
>
> Forwarding to ranges::minmax_element seems like it would be profitable
> in some situations, e.g if the value type isn't trivially copyable. I
> can do this in a followup patch for ranges::max/max_element and
> ranges::min/min_element too, they should all use the same heuristic.
>
> >
> >
> > > + if (++__first == __last)
> > > + {
> > > + // N is odd; in this final iteration, we perform a just one
> >
> > s/perform a just one/perform just one/
>
> Fixed.
>
> >
> > > + // comparison, for a total of 3*(N-1)/2 + 1 < 3*N/2
> > > comparisons.
> >
> > I find this a bit hard to parse with the inequality there.
>
> Removed.
>
> >
> > > + if (__comp_proj(__val1, __result.min))
> > > + __result.min = std::move(__val1);
> > > + else if (!__comp_proj(__val1, __result.max))
> > > + __result.max = std::move(__val1);
> >
> > This can be two comparisons, can't it? Would this be better...
>
> Whoops, yeah...
>
> >
> > // N is odd; in this final iteration, we perform at most two
> > // comparisons, for a total of 3*(N-1)/2 + 2 comparisons,
> > // which is not more than 3*N/2, as required.
> >
> > ?
>
> Ah, but then the total is more than 3*N/2 :( And I think we reach this
> case really when N is even, not odd (sorry, I really botched this
> patch).
>
> And when N=2 in particular, we perform up two comparisons instead of
> three, but actually a single comparison should suffice in this case. I
> think all this is fixed in v2 below by handling the second element in
> the range specially.
>
> >
> > > + break;
> > > + }
> > > + auto __val2 = *__first;
> > > + if (!__comp_proj(__val2, __val1))
> > > + {
> > > + if (__comp_proj(__val1, __result.min))
> > > + __result.min = std::move(__val1);
> > > + if (!__comp_proj(__val2, __result.max))
> > > + __result.max = std::move(__val2);
> > > + }
> > > + else
> > > + {
> > > + if (__comp_proj(__val2, __result.min))
> > > + __result.min = std::move(__val2);
> > > + if (!__comp_proj(__val1, __result.max))
> > > + __result.max = std::move(__val1);
> > > + }
> >
> > I thought we might be able to simplify this to something like:
> >
> > auto __val2 = *__first;
> > auto&& [__min, __max] = (*this)(__val1, __val2, __comp, __proj);
> > if (__comp_proj(__min, __result.min))
> > __result.min = __min;
> > if (__comp_proj(__result.max, __max))
> > __result.max = __max;
> >
> > But it doesn't work because we need to move from __min and __max, but
> > the (*this)(...) returns minmax_result<const T&> and can't be moved
> > from.
> >
> > We could get around that but it's not much of a simplification:
> >
> > range_value_t<Range> __val2 = *__first;
> > auto [__min, __max] = (*this)(std::addressof(__val1),
> > std::addressof(__val2),
> > __comp,
> > [](auto __p) -> const auto& {
> > return *__p;
> > });
> > if (__comp_proj(*__min, __result.min))
> > __result.min = std::move(*__min);
> > if (__comp_proj(__result.max, *__max))
> > __result.max = std::move(*__max);
>
> Hmm, now that __val2 was made a universal reference, this simplifcation
> might not work.
>
> >
> > > }
> > > return __result;
> > > }
> > > @@ -3408,21 +3429,40 @@ namespace ranges
> > > operator()(_Iter __first, _Sent __last,
> > > _Comp __comp = {}, _Proj __proj = {}) const
> > > {
> > > - if (__first == __last)
> > > - return {__first, __first};
> > > -
> > > + auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
> > > minmax_element_result<_Iter> __result = {__first, __first};
> > > - auto __i = __first;
> > > - while (++__i != __last)
> > > + if (__first == __last)
> > > + return __result;
> > > + while (++__first != __last)
> > > {
> > > - if (std::__invoke(__comp,
> > > - std::__invoke(__proj, *__i),
> > > - std::__invoke(__proj, *__result.min)))
> > > - __result.min = __i;
> > > - if (!(bool)std::__invoke(__comp,
> > > - std::__invoke(__proj, *__i),
> > > - std::__invoke(__proj, *__result.max)))
> > > - __result.max = __i;
> > > + // Process two elements at a time so that we perform at most
> > > + // 3*N/2 many comparisons in total (each of the N/2 iterations
> > > + // of this loop performs three comparisions).
> > > + auto __prev = __first;
> > > + if (++__first == __last)
> > > + {
> > > + // N is odd; in this final iteration, we perform a just one
> > > + // comparison, for a total of 3*(N-1)/2 + 1 < 3*N/2
> > > comparisons.
> >
> > Same comments on the comments as above.
>
> Fixed.
>
> >
> > > + if (__comp_proj(*__prev, *__result.min))
> > > + __result.min = __prev;
> > > + else if (!__comp_proj(*__prev, *__result.max))
> > > + __result.max = __prev;
> > > + break;
> > > + }
> > > + if (!__comp_proj(*__first, *__prev))
> > > + {
> > > + if (__comp_proj(*__prev, *__result.min))
> > > + __result.min = __prev;
> > > + if (!__comp_proj(*__first, *__result.max))
> > > + __result.max = __first;
> > > + }
> > > + else
> > > + {
> > > + if (__comp_proj(*__first, *__result.min))
> > > + __result.min = __first;
> > > + if (!__comp_proj(*__prev, *__result.max))
> > > + __result.max = __prev;
> > > + }
> >
> > We don't need to move anything here, so this could be written using
> > ranges::minmax. I'm not sure it is an improvement though (except for
> > being slightly fewer lines of code):
> >
> > auto __mm = minmax(__prev, __first, __comp,
> > [](auto&& __it) -> auto&& { return *__it; });
> >
> > if (__comp_proj(*__mm.min, *__result.min))
> > __result.min = __mm.min;
> > if (__comp_proj(*__result.max, *__mm.max))
> > __result.max = __mm.max;
>
> Hmm, it's shorter, but on the other hand it'd make the implementations
> of minmax and minmax_element no longer mirror each other as closely. So
> I didn't make this change for now.
>
> Here's v2, which makes the following additional changes:
>
> * Fixes comment typos, and some nearby indentation in the signature
> of minmax
> * Makes both minmax and minmax_element handle the second element
> specially in order to optimally perform just a single comparison
> when N=2 and to always stay below 3*N/2 comparisons
> * Avoids using a deduced return type for __val1
> * Makes __val2 a universal reference to avoid a move
> * Adds comparison complexity tests for the N=2,3 cases
>
> -- >8 --
>
> Subject: [PATCH] libstdc++: Reduce ranges::minmax/minmax_element comparison
> complexity
>
> This rewrites ranges::minmax and ranges::minmax_element so that it
> performs at most 3*N/2 many comparisons, as required by the standard.
> In passing, this also fixes PR100387 by avoiding a premature std::move
> in ranges::minmax and in std::shift_right.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk and perhaps
> 10/11?
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/100387
> * include/bits/ranges_algo.h (__minmax_fn::operator()): Rewrite
> to limit comparison complexity to 3*N/2.
> (__minmax_element_fn::operator()): Likewise.
> (shift_right): Avoid premature std::move of __result.
> * testsuite/25_algorithms/minmax/constrained.cc (test04, test05):
> New tests.
> * testsuite/25_algorithms/minmax_element/constrained.cc (test02):
> Likewise.
> ---
> libstdc++-v3/include/bits/ranges_algo.h | 113 ++++++++++++++----
> .../25_algorithms/minmax/constrained.cc | 42 +++++++
> .../minmax_element/constrained.cc | 27 +++++
> 3 files changed, 156 insertions(+), 26 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index cda3042c11f..2091cbf5b4e 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -3283,26 +3283,59 @@ namespace ranges
> template<input_range _Range, typename _Proj = identity,
> indirect_strict_weak_order<projected<iterator_t<_Range>, _Proj>>
> _Comp = ranges::less>
> - requires indirectly_copyable_storable<iterator_t<_Range>,
> - range_value_t<_Range>*>
> + requires indirectly_copyable_storable<iterator_t<_Range>,
> range_value_t<_Range>*>
> constexpr minmax_result<range_value_t<_Range>>
> operator()(_Range&& __r, _Comp __comp = {}, _Proj __proj = {}) const
> {
> auto __first = ranges::begin(__r);
> auto __last = ranges::end(__r);
> __glibcxx_assert(__first != __last);
> + auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
> minmax_result<range_value_t<_Range>> __result = {*__first, *__first};
> + if (++__first == __last)
> + return __result;
> + else
> + {
> + // At this point __result.min == __result.max, so a single
> + // comparison with the next element suffices.
> + auto&& __val = *__first;
> + if (__comp_proj(__val, __result.min))
> + __result.min = std::forward<decltype(__val)>(__val);
> + else
> + __result.max = std::forward<decltype(__val)>(__val);
> + }
> while (++__first != __last)
> {
> - auto __tmp = *__first;
> - if (std::__invoke(__comp,
> - std::__invoke(__proj, __tmp),
> - std::__invoke(__proj, __result.min)))
> - __result.min = std::move(__tmp);
> - if (!(bool)std::__invoke(__comp,
> - std::__invoke(__proj, __tmp),
> - std::__invoke(__proj, __result.max)))
> - __result.max = std::move(__tmp);
> + // Now process two elements at a time so that we perform at most
> + // 3*(N-2)/2 comparisons in total (each of the (N-2)/2 iterations
Oops, this should now say "1 + 3*(N-2)/2 comparisons" because of the
initial comparison with the second element above.
> + // of this loop performs three comparisons).
> + range_value_t<_Range> __val1 = *__first;
> + if (++__first == __last)
> + {
> + // N is odd; in this final iteration, we perform at most two
> + // comparisons, for a total of 1 + 3*(N-3)/2 + 2 comparisons,
> + // which is not more than 3*N/2, as required.
> + if (__comp_proj(__val1, __result.min))
> + __result.min = std::move(__val1);
> + else if (!__comp_proj(__val1, __result.max))
> + __result.max = std::move(__val1);
> + break;
> + }
> + auto&& __val2 = *__first;
> + if (!__comp_proj(__val2, __val1))
> + {
> + if (__comp_proj(__val1, __result.min))
> + __result.min = std::move(__val1);
> + if (!__comp_proj(__val2, __result.max))
> + __result.max = std::forward<decltype(__val2)>(__val2);
> + }
> + else
> + {
> + if (__comp_proj(__val2, __result.min))
> + __result.min = std::forward<decltype(__val2)>(__val2);
> + if (!__comp_proj(__val1, __result.max))
> + __result.max = std::move(__val1);
> + }
> }
> return __result;
> }
> @@ -3408,21 +3441,50 @@ namespace ranges
> operator()(_Iter __first, _Sent __last,
> _Comp __comp = {}, _Proj __proj = {}) const
> {
> - if (__first == __last)
> - return {__first, __first};
> -
> + auto __comp_proj = __detail::__make_comp_proj(__comp, __proj);
> minmax_element_result<_Iter> __result = {__first, __first};
> - auto __i = __first;
> - while (++__i != __last)
> + if (__first == __last || ++__first == __last)
> + return __result;
> + else
> {
> - if (std::__invoke(__comp,
> - std::__invoke(__proj, *__i),
> - std::__invoke(__proj, *__result.min)))
> - __result.min = __i;
> - if (!(bool)std::__invoke(__comp,
> - std::__invoke(__proj, *__i),
> - std::__invoke(__proj, *__result.max)))
> - __result.max = __i;
> + // At this point __result.min == __result.max, so a single
> + // comparison with the next element suffices.
> + if (__comp_proj(*__first, *__result.min))
> + __result.min = __first;
> + else
> + __result.max = __first;
> + }
> + while (++__first != __last)
> + {
> + // Now process two elements at a time so that we perform at most
> + // 3*(N-2)/2 comparisons in total (each of the (N-2)/2 iterations
Here too; consider this fixed.
> + // of this loop performs three comparisons).
> + auto __prev = __first;
> + if (++__first == __last)
> + {
> + // N is odd; in this final iteration, we perform at most two
> + // comparisons, for a total of 1 + 3*(N-3)/2 + 2 comparisons,
> + // which is not more than 3*N/2, as required.
> + if (__comp_proj(*__prev, *__result.min))
> + __result.min = __prev;
> + else if (!__comp_proj(*__prev, *__result.max))
> + __result.max = __prev;
> + break;
> + }
> + if (!__comp_proj(*__first, *__prev))
> + {
> + if (__comp_proj(*__prev, *__result.min))
> + __result.min = __prev;
> + if (!__comp_proj(*__first, *__result.max))
> + __result.max = __first;
> + }
> + else
> + {
> + if (__comp_proj(*__first, *__result.min))
> + __result.min = __first;
> + if (!__comp_proj(*__prev, *__result.max))
> + __result.max = __prev;
> + }
> }
> return __result;
> }
> @@ -3749,8 +3811,7 @@ namespace ranges
> // i.e. we are shifting out at least half of the range. In
> // this case we can safely perform the shift with a single
> // move.
> - std::move(std::move(__first), std::move(__dest_head),
> - std::move(__result));
> + std::move(std::move(__first), std::move(__dest_head),
> __result);
> return __result;
> }
> ++__dest_head;
> diff --git a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> index 786922414b5..c365152bf2b 100644
> --- a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> +++ b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> @@ -19,6 +19,8 @@
> // { dg-do run { target c++2a } }
>
> #include <algorithm>
> +#include <string>
> +#include <vector>
> #include <testsuite_hooks.h>
> #include <testsuite_iterators.h>
>
> @@ -89,10 +91,50 @@ test03()
> == res_t(1,4) );
> }
>
> +void
> +test04()
> +{
> + // Verify we perform at most 3*N/2 applications of the comparison
> predicate.
> + static int counter;
> + struct counted_less
> + { bool operator()(int a, int b) { ++counter; return a < b; } };
> +
> + ranges::minmax({1,2}, counted_less{});
> + VERIFY( counter == 1 );
> +
> + counter = 0;
> + ranges::minmax({1,2,3}, counted_less{});
> + VERIFY( counter == 3 );
> +
> + counter = 0;
> + ranges::minmax({1,2,3,4,5,6,7,8,9,10}, counted_less{});
> + VERIFY( counter <= 15 );
> +
> + counter = 0;
> + ranges::minmax({10,9,8,7,6,5,4,3,2,1}, counted_less{});
> + VERIFY( counter <= 15 );
> +}
> +
> +void
> +test05()
> +{
> + // PR libstdc++/100387
> + using namespace std::literals::string_literals;
> + auto comp = [](const auto& a, const auto& b) {
> + return a.size() == b.size() ? a.front() < b.front() : a.size() >
> b.size();
> + };
> + auto result = ranges::minmax({"b"s, "a"s}, comp);
> + VERIFY( result.min == "a"s && result.max == "b"s );
> + result = ranges::minmax({"c"s, "b"s, "a"s}, comp);
> + VERIFY( result.min == "a"s && result.max == "c"s );
> +}
> +
> int
> main()
> {
> test01();
> test02();
> test03();
> + test04();
> + test05();
> }
> diff --git
> a/libstdc++-v3/testsuite/25_algorithms/minmax_element/constrained.cc
> b/libstdc++-v3/testsuite/25_algorithms/minmax_element/constrained.cc
> index 3b11c0dd96c..0919f7dda8f 100644
> --- a/libstdc++-v3/testsuite/25_algorithms/minmax_element/constrained.cc
> +++ b/libstdc++-v3/testsuite/25_algorithms/minmax_element/constrained.cc
> @@ -61,8 +61,35 @@ test01()
> static_assert(ranges::minmax_element(y, y+3, {}, &X::i).max->j == 3);
> }
>
> +void
> +test02()
> +{
> + // Verify we perform at most 3*N/2 applications of the comparison
> predicate.
> + static int counter;
> + struct counted_less
> + { bool operator()(int a, int b) { ++counter; return a < b; } };
> +
> + int x[] = {1,2,3,4,5,6,7,8,9,10};
> + ranges::minmax_element(x, x+2, counted_less{});
> + VERIFY( counter == 1 );
> +
> + counter = 0;
> + ranges::minmax_element(x, x+3, counted_less{});
> + VERIFY( counter == 3 );
> +
> + counter = 0;
> + ranges::minmax_element(x, counted_less{});
> + VERIFY( counter <= 15 );
> +
> + ranges::reverse(x);
> + counter = 0;
> + ranges::minmax_element(x, counted_less{});
> + VERIFY( counter <= 15 );
> +}
> +
> int
> main()
> {
> test01();
> + test02();
> }
> --
> 2.31.1.442.g7e39198978
>
>
