https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98465
--- Comment #28 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Actually tested version.
The above testcase with [2, INF] range doesn't make really much sense,
but adjusted testcase where n has [0, 2] range doesn't warn anymore like the
one with constant 2.
diff --git a/libstdc++-v3/include/bits/c++config
b/libstdc++-v3/include/bits/c++config
index b57ff339990..69336a32bc6 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -731,6 +731,10 @@ namespace std
# define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
#endif
+#if _GLIBCXX_HAS_BUILTIN(__builtin_object_size)
+# define _GLIBCXX_HAVE_BUILTIN_OBJECT_SIZE 1
+#endif
+
#undef _GLIBCXX_HAS_BUILTIN
#if _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED && __cplusplus >= 201402L
diff --git a/libstdc++-v3/include/bits/basic_string.tcc
b/libstdc++-v3/include/bits/basic_string.tcc
index 5beda8b829b..bc6e0b98186 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -477,7 +477,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
if (__s + __len2 <= __p + __len1)
this->_S_move(__p, __s, __len2);
+#if defined(_GLIBCXX_HAVE_BUILTIN_OBJECT_SIZE) && defined(__OPTIMIZE__)
+ /* Help the optimizers rule out impossible cases and
+ get rid of false positive warnings at the same time.
+ If we know the maximum size of the __s object and
+ it is shorter than 2 * __len2 - __len1, then
+ __s >= __p + __len1 case is impossible. */
+ else if (!(__builtin_constant_p(__builtin_object_size(__s, 0)
+ < ((2 * __len2 - __len1)
+ * sizeof(_CharT)))
+ && (__builtin_object_size(__s, 0)
+ < (2 * __len2 - __len1) * sizeof(_CharT)))
+ && __s >= __p + __len1)
+#else
else if (__s >= __p + __len1)
+#endif
this->_S_copy(__p, __s + __len2 - __len1, __len2);
else
{
