[gcc r15-1715] libstdc++: Extend std::equal memcmp optimization to std::byte [PR101485]

[Jonathan Wakely]

https://gcc.gnu.org/g:fab60eaa94b50b1eea84f0d001004c851d4c781b

commit r15-1715-gfab60eaa94b50b1eea84f0d001004c851d4c781b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jun 28 11:14:39 2024 +0100

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

Diff:
---
 libstdc++-v3/include/bits/stl_algobase.h             |  6 +++++-
 libstdc++-v3/testsuite/25_algorithms/equal/101485.cc | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

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);
+}