[gcc r13-9486] libstdc++: Fix bogus -Wstringop-overflow in std::vector::insert [PR117983]

[Jonathan Wakely]

https://gcc.gnu.org/g:8f633fe7c7d03ed6c0ce029a831037a6274a0882

commit r13-9486-g8f633fe7c7d03ed6c0ce029a831037a6274a0882
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 28 22:00:38 2025 +0000

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

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index c762dbf798f6..ddbfde447f62 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -799,6 +799,8 @@ _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 000000000000..e6027a677eed
--- /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());
+  }
+}