[gcc r12-6907] libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]

[Jonathan Wakely]

https://gcc.gnu.org/g:eae41b4d2cc30327f9f15c7390438c46aa09ed3f

commit r12-6907-geae41b4d2cc30327f9f15c7390438c46aa09ed3f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 27 22:31:26 2022 +0000

    libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]
    
    The compiler warns about the loop in deque::_M_range_initialize because
    it doesn't know that the number of nodes has already been correctly
    sized to match the size of the input. Use __builtin_unreachable to tell
    it that the loop will never be entered if the number of elements is
    smaller than a single node.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100516
            * include/bits/deque.tcc (_M_range_initialize<ForwardIterator>):
            Add __builtin_unreachable to loop.
            * testsuite/23_containers/deque/100516.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/deque.tcc                  |  3 +++
 libstdc++-v3/testsuite/23_containers/deque/100516.cc | 14 ++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
index 80f1813bc76..03e0a505e14 100644
--- a/libstdc++-v3/include/bits/deque.tcc
+++ b/libstdc++-v3/include/bits/deque.tcc
@@ -454,6 +454,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 		 __cur_node < this->_M_impl._M_finish._M_node;
 		 ++__cur_node)
 	      {
+		if (__n < _S_buffer_size())
+		  __builtin_unreachable(); // See PR 100516
+
 		_ForwardIterator __mid = __first;
 		std::advance(__mid, _S_buffer_size());
 		std::__uninitialized_copy_a(__first, __mid, *__cur_node,
diff --git a/libstdc++-v3/testsuite/23_containers/deque/100516.cc b/libstdc++-v3/testsuite/23_containers/deque/100516.cc
new file mode 100644
index 00000000000..ef32ae10545
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/deque/100516.cc
@@ -0,0 +1,14 @@
+// { dg-options "-O2 -Wstringop-overread" }
+// { dg-do compile { target c++11 } }
+
+// Bug 100516
+// Unexpected -Wstringop-overread in deque<char> initialization from empty
+// initializer_list
+
+#include <deque>
+
+void f()
+{
+    std::initializer_list<char> il{};
+    std::deque<char>{il};
+}