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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [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]
Date: Fri, 31 Mar 2023 12:43:43 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107087

--- 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 =3D=3D 0 && __OPTIMIZE__
+         if (_S_empty_rep()._M_refcount !=3D 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 =3D=3D 0 && __OPTIMIZE__
+         if (_S_empty_rep()._M_refcount !=3D 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 a=
ny
       ///  null-termination.
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING =3D=3D 0 && __OPTIMIZE__
+       if (_S_empty_rep()._M_length !=3D 0)
+         __builtin_unreachable();
+#endif
+       return _M_rep()->_M_length;
+      }

       ///  Returns the number of characters in the string, not including a=
ny
       ///  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=