Re: [PATCH] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]
On Thu, 14 Apr 2022, Jonathan Wakely wrote: > On Thu, 14 Apr 2022 at 16:21, Patrick Palka via Libstdc++ > 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> __result = {*__first, > > *__first}; > > + minmax_result> __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. Indeed, it seems to do the right thing, practically speaking at least :) FWIW the alternative approach - minmax_result> __result = {*__first, *__first}; + minmax_result> __result; + __result.max = __result.min = *__first; wouldn't be right because the value type is not necessarily default constructible. I beefed up the new testcase to verify we don't demand default constructibility here.
Re: [PATCH] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]
On Thu, 14 Apr 2022 at 16:21, Patrick Palka via Libstdc++ 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> __result = {*__first, *__first}; > + minmax_result> __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 >
[PATCH] libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]
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> __result = {*__first, *__first}; + minmax_result> __result = {*__first, __result.min}; 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