libstdc++: Add hint to compiler about vector invariants [PR106434]

The PR shows a bogus warning where jump threading generates code for the
undefined case that the insertion point is a value-initialized iterator
but _M_finish and _M_end_of_storage are unequal (so at least one must be
non-null). Using __builtin_unreachable() removes the bogus warning. Also
add an assertion to diagnose undefined misuses of a null iterator here,
so we don't just silently optimize that undefined code to something
unsafe.

libstdc++-v3/ChangeLog:

	PR c++/106434
	* include/bits/vector.tcc (insert(const_iterator, const T&)):
	Add assertion and optimization hint that the iterator for the
	insertion point must be non-null.

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 27ef1a4..8ae79ff 100644 (file)
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -139,26 +139,32 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     {
       const size_type __n = __position - begin();
       if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
-       if (__position == end())
-         {
-           _GLIBCXX_ASAN_ANNOTATE_GROW(1);
-           _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
-                                    __x);
-           ++this->_M_impl._M_finish;
-           _GLIBCXX_ASAN_ANNOTATE_GREW(1);
-         }
-       else
-         {
+       {
+         __glibcxx_assert(__position != const_iterator());
+         if (!(__position != const_iterator()))
+           __builtin_unreachable(); // PR 106434
+
+         if (__position == end())
+           {
+             _GLIBCXX_ASAN_ANNOTATE_GROW(1);
+             _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
+                                      __x);
+             ++this->_M_impl._M_finish;
+             _GLIBCXX_ASAN_ANNOTATE_GREW(1);
+           }
+         else
+           {
 #if __cplusplus >= 201103L
-           const auto __pos = begin() + (__position - cbegin());
-           // __x could be an existing element of this vector, so make a
-           // copy of it before _M_insert_aux moves elements around.
-           _Temporary_value __x_copy(this, __x);
-           _M_insert_aux(__pos, std::move(__x_copy._M_val()));
+             const auto __pos = begin() + (__position - cbegin());
+             // __x could be an existing element of this vector, so make a
+             // copy of it before _M_insert_aux moves elements around.
+             _Temporary_value __x_copy(this, __x);
+             _M_insert_aux(__pos, std::move(__x_copy._M_val()));
 #else
-           _M_insert_aux(__position, __x);
+             _M_insert_aux(__position, __x);
 #endif
-         }
+           }
+       }
       else
 #if __cplusplus >= 201103L
        _M_realloc_insert(begin() + (__position - cbegin()), __x);