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.
