[PATCH] libstdc++: Fix bogus use of memcmp in ranges::lexicographical_compare (PR 93972)

[Jonathan Wakely]

On 28/02/20 14:59 -0500, Patrick Palka wrote:
>We were enabling the memcmp optimization in ranges::lexicographical_compare for
>signed integral types and for integral types larger than a byte.  But memcmp
>gives the wrong answer for arrays of such types.  This patch fixes this issue by
>refining the condition that enables the memcmp optimization.  It's now
>consistent with the corresponding condition used in
>std::lexicographical_compare.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/93972
>	* include/bits/ranges_algo.h (__lexicographical_compare_fn::operator()):
>	Fix condition for when to use memcmp, making it consistent with the
>	corresponding condition used in std::lexicographical_compare.
>	* testsuite/25_algorithms/lexicographical_compare/93972.cc: New test.
>---
> libstdc++-v3/include/bits/ranges_algo.h       |   8 +-
> .../lexicographical_compare/93972.cc          | 169 ++++++++++++++++++
> 2 files changed, 175 insertions(+), 2 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/25_algorithms/lexicographical_compare/93972.cc
>
>diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
>index 05c0851d411..8fa4a8a9161 100644
>--- a/libstdc++-v3/include/bits/ranges_algo.h
>+++ b/libstdc++-v3/include/bits/ranges_algo.h
>@@ -3466,9 +3466,13 @@ namespace ranges
> 	      {
> 		using _ValueType1 = iter_value_t<_Iter1>;
> 		using _ValueType2 = iter_value_t<_Iter2>;
>+		// This condition is consistent with the one in
>+		// __lexicographical_compare_aux in <bits/stl_algobase.h>.
> 		constexpr bool __use_memcmp
>-		  = ((is_integral_v<_ValueType1> || is_pointer_v<_ValueType1>)
>-		     && is_same_v<_ValueType1, _ValueType2>
>+		  = (__is_byte<_ValueType1>::__value
>+		     && __is_byte<_ValueType2>::__value
>+		     && !__gnu_cxx::__numeric_traits<_ValueType1>::__is_signed
>+		     && !__gnu_cxx::__numeric_traits<_ValueType2>::__is_signed

I think this could be:

                      && !is_signed_v<_ValueType1>
                      && !is_signed_v<_ValueType2>

because this code doesn't need to be valid for C++98. But on the other
hand, there's value in being consistent with the condition in
std::lexicographical_compare.

OK for master, thanks for the quick fix.