On Thu, 14 Apr 2022 at 16:21, Patrick Palka via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk and 11/10
> once the branch is unfrozen?
>
> PR libstdc++/104858
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_algo.h (__minmax_fn): Avoid dereferencing
> __first twice at the start.
> * testsuite/25_algorithms/minmax/constrained.cc (test06): New test.
> ---
> libstdc++-v3/include/bits/ranges_algo.h | 2 +-
> .../25_algorithms/minmax/constrained.cc | 23 +++++++++++++++++++
> 2 files changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index 62dc605080a..3d30fb1428c 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -3084,7 +3084,7 @@ namespace ranges
> 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};
> + minmax_result<range_value_t<_Range>> __result = {*__first,
> __result.min};
Clever ... I'm surprised this even works. I would have expected it to
evaluate both initializers before actually initializing the members.
TIL.
OK for trunk now, and branches once thawed.
> if (++__first == __last)
> return __result;
> else
> diff --git a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> index 90882afb6d0..306c495babe 100644
> --- a/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> +++ b/libstdc++-v3/testsuite/25_algorithms/minmax/constrained.cc
> @@ -129,6 +129,28 @@ test05()
> VERIFY( result.min == "a"s && result.max == "c"s );
> }
>
> +struct A {
> + A() = default;
> + A(const A&) = default;
> + A(A&&) { ++move_count; }
> + A& operator=(const A&) = default;
> + A& operator=(A&&) = default;
> + friend auto operator<=>(const A&, const A&) = default;
> + static inline int move_count = 0;
> +};
> +
> +void
> +test06()
> +{
> + // PR libstdc++/104858
> + // Verify ranges::minmax doesn't dereference the iterator for the first
> + // element in the range twice.
> + A a;
> + ranges::subrange r = {std::move_iterator(&a), std::move_sentinel(&a + 1)};
> + ranges::minmax(r);
> + VERIFY( A::move_count == 1 );
> +}
> +
> int
> main()
> {
> @@ -137,4 +159,5 @@ main()
> test03();
> test04();
> test05();
> + test06();
> }
> --
> 2.36.0.rc2.10.g1ac7422e39
>
