[PATCH][gcc-14] libstdc++: Fix bogus -Wstringop-overflow in std::vector::insert [PR117983]

[Jonathan Wakely]

This was fixed on trunk by r15-4473-g3abe751ea86e34, but that isn't
suitable for backporting. Instead, just add another unreachable
condition in std::vector::_M_range_insert so the compiler knows this
memcpy doesn't use a length originating from a negative ptrdiff_t
converted to a very positive size_t.

libstdc++-v3/ChangeLog:

        PR libstdc++/117983
        * include/bits/vector.tcc (vector::_M_range_insert): Add
        unreachable condition to tell the compiler begin() <= end().
        * testsuite/23_containers/vector/modifiers/insert/117983.cc: New
        test.

(cherry picked from commit 878812b6f6905774ab37cb78903e3e11bf1c508c)
---

Tested x86_64-linux.

This warning is a regression on 12/13/14 so this is needed on all
release branches.

 libstdc++-v3/include/bits/vector.tcc            |  2 ++
 .../vector/modifiers/insert/117983.cc           | 17 +++++++++++++++++
 2 files changed, 19 insertions(+)
 create mode 100644 
libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/117983.cc

diff --git a/libstdc++-v3/include/bits/vector.tcc 
b/libstdc++-v3/include/bits/vector.tcc
index 0458d560075..b7f472ae09c 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -1002,6 +1002,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
                // reachable.
                pointer __old_start = this->_M_impl._M_start;
                pointer __old_finish = this->_M_impl._M_finish;
+               if ((__old_finish - __old_start) < 0)
+                 __builtin_unreachable();
 
                const size_type __len =
                  _M_check_len(__n, "vector::_M_range_insert");
diff --git 
a/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/117983.cc 
b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/117983.cc
new file mode 100644
index 00000000000..e6027a677ee
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/117983.cc
@@ -0,0 +1,17 @@
+// { dg-options "-O3 -Werror=stringop-overflow" }
+// { dg-do compile }
+
+// PR libstdc++/117983
+// -Wstringop-overflow false positive for __builtin_memmove from vector::insert
+
+#include <vector>
+
+typedef std::vector<unsigned char> bytes;
+
+void push(bytes chunk, bytes& data) {
+  if (data.empty()) {
+    data.swap(chunk);
+  } else {
+    data.insert(data.end(), chunk.begin(), chunk.end());
+  }
+}
-- 
2.49.0