[gcc r13-8193] libstdc++: Fix std::char_traits<C>::move [PR113200]

[gcc r13-8193] libstdc++: Fix std::char_traits<C>::move [PR113200]

[Jonathan Wakely]

https://gcc.gnu.org/g:22601c1c25652c71c8bab4707866c020d6dad79a

commit r13-8193-g22601c1c25652c71c8bab4707866c020d6dad79a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jan 3 15:01:09 2024 +0000

    libstdc++: Fix std::char_traits<C>::move [PR113200]
    
    The current constexpr implementation of std::char_traits<C>::move relies
    on being able to compare the pointer parameters, which is not allowed
    for unrelated pointers. We can use __builtin_constant_p to determine
    whether it's safe to compare the pointers directly. If not, then we know
    the ranges must be disjoint and so we can use char_traits<C>::copy to
    copy forwards from the first character to the last. If the pointers can
    be compared directly, then we can simplify the condition for copying
    backwards to just two pointer comparisons.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/113200
            * include/bits/char_traits.h (__gnu_cxx::char_traits::move): Use
            __builtin_constant_p to check for unrelated pointers that cannot
            be compared during constant evaluation.
            * testsuite/21_strings/char_traits/requirements/113200.cc: New
            test.
    
    (cherry picked from commit 15cc291887dc9dd92b2c93f4545e20eb6c190122)

Diff:
---
 libstdc++-v3/include/bits/char_traits.h              | 16 +++-------------
 .../21_strings/char_traits/requirements/113200.cc    | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index 68ed827f982..67f507538ca 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -227,19 +227,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 202002L
       if (std::__is_constant_evaluated())
 	{
-	  if (__s1 == __s2) // unlikely, but saves a lot of work
-	    return __s1;
-	  const auto __end = __s2 + __n - 1;
-	  bool __overlap = false;
-	  for (std::size_t __i = 0; __i < __n - 1; ++__i)
-	    {
-	      if (__s1 + __i == __end)
-		{
-		  __overlap = true;
-		  break;
-		}
-	    }
-	  if (__overlap)
+	  // Use __builtin_constant_p to avoid comparing unrelated pointers.
+	  if (__builtin_constant_p(__s2 < __s1)
+		&& __s1 > __s2 && __s1 < (__s2 + __n))
 	    {
 	      do
 		{
diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc
new file mode 100644
index 00000000000..0fe765d53bc
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++20 } }
+
+// PR libstdc++/113200
+// char_traits::move is not constexpr when the argument is a string literal
+
+#include <string_view>
+
+template<std::size_t N> struct S
+{
+  char data_[ N ];
+
+  constexpr S( char const* p ): data_{}
+  {
+    std::char_traits<char>::move( data_, p, N );
+  }
+};
+
+template<std::size_t N> S( char const(&)[N] ) -> S<N>;
+
+constexpr S s( "test" );