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[committed] libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]

Message ID 20220127233117.1112432-1-jwakely@redhat.com
State Committed
Commit eae41b4d2cc30327f9f15c7390438c46aa09ed3f
Headers
Series [committed] libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516] |

Commit Message

Jonathan Wakely Jan. 27, 2022, 11:31 p.m. UTC 
  Tested powerpc64le-linux, pushed to trunk.


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.
---
 libstdc++-v3/include/bits/deque.tcc                |  3 +++
 .../testsuite/23_containers/deque/100516.cc        | 14 ++++++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/23_containers/deque/100516.cc
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Patch

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};
+}