[Bug tree-optimization/107087] [12/13 Regression] bits/stl_algobase.h:431: warning: 'void* __builtin_memcpy(void*, const void*, unsigned int)' reading between 8 and 2147483644 bytes from a region of size 4 [-Wstringop-overread]

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107087

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I wonder if some other hints about properties of the empty rep would help
codegen:

--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -204,6 +204,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        bool
        _M_is_leaked() const _GLIBCXX_NOEXCEPT
        {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 && __OPTIMIZE__
+         if (_S_empty_rep()._M_refcount != 0)
+           __builtin_unreachable();
+#endif
+
 #if defined(__GTHREADS)
          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
          // so we need to use an atomic load. However, _M_is_leaked
@@ -218,6 +223,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        bool
        _M_is_shared() const _GLIBCXX_NOEXCEPT
        {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 && __OPTIMIZE__
+         if (_S_empty_rep()._M_refcount != 0)
+           __builtin_unreachable();
+#endif
+
 #if defined(__GTHREADS)
          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
          // so we need to use an atomic load. Another thread can drop last
@@ -907,17 +917,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

     public:
       // Capacity:
+
       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 && __OPTIMIZE__
+       if (_S_empty_rep()._M_length != 0)
+         __builtin_unreachable();
+#endif
+       return _M_rep()->_M_length;
+      }

       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
       size_type
       length() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
+      { return size(); }

       ///  Returns the size() of the largest possible %string.
       size_type