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

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.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
(cherry picked from commit 878812b6f6905774ab37cb78903e3e11bf1c508c)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 0bd1ae411028dd6e3a99dfe4c3a4ac521e22c692..200a75a19c3e9636905216a18de82ba8c7ed0e45 100644 (file)
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -1005,6 +1005,8 @@ _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 (file)
index 0000000..e6027a6
--- /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());
+  }
+}