[committed] libstdc++: Extend std::equal memcmp optimization to std::byte [PR101485]

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

We optimize std::equal to memcmp for integers and pointers, which means
that std::byte comparisons generate bigger code than char comparisons.

We can't use memcmp for arbitrary enum types, because they could have an
overloaded operator== that has custom semantics, but we know that
std::byte doesn't do that.

libstdc++-v3/ChangeLog:

        PR libstdc++/101485
        * include/bits/stl_algobase.h (__equal_aux1): Check for
        std::byte as well.
        * testsuite/25_algorithms/equal/101485.cc: New test.
---
 libstdc++-v3/include/bits/stl_algobase.h             |  6 +++++-
 libstdc++-v3/testsuite/25_algorithms/equal/101485.cc | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/25_algorithms/equal/101485.cc

diff --git a/libstdc++-v3/include/bits/stl_algobase.h 
b/libstdc++-v3/include/bits/stl_algobase.h
index 57ff2f7cb08..dec1e4c79d8 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1257,7 +1257,11 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
       typedef typename iterator_traits<_II1>::value_type _ValueType1;
       const bool __simple = ((__is_integer<_ValueType1>::__value
 #if _GLIBCXX_USE_BUILTIN_TRAIT(__is_pointer)
-                             || __is_pointer(_ValueType1)
+                               || __is_pointer(_ValueType1)
+#endif
+#if __glibcxx_byte && __glibcxx_type_trait_variable_templates
+                               // bits/cpp_type_traits.h declares std::byte
+                               || is_same_v<_ValueType1, byte>
 #endif
                             ) && __memcmpable<_II1, _II2>::__value);
       return std::__equal<__simple>::equal(__first1, __last1, __first2);
diff --git a/libstdc++-v3/testsuite/25_algorithms/equal/101485.cc 
b/libstdc++-v3/testsuite/25_algorithms/equal/101485.cc
new file mode 100644
index 00000000000..1fbb40acae9
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/equal/101485.cc
@@ -0,0 +1,11 @@
+// { dg-options "-O0" }
+// { dg-do compile { target c++17 } }
+// { dg-final { scan-assembler "memcmp" } }
+
+#include <algorithm>
+#include <cstddef>
+
+bool eq(std::byte const* p, std::byte const* q, unsigned n)
+{
+  return std::equal(p, p + n, q);
+}
-- 
2.45.2