[gcc r13-4525] libstdc++: Add hint to compiler about vector invariants [PR106434]

[gcc r13-4525] libstdc++: Add hint to compiler about vector invariants [PR106434]

[Jonathan Wakely]

https://gcc.gnu.org/g:4ba94abf147fe7778a7541849ce27cafee74df9b

commit r13-4525-g4ba94abf147fe7778a7541849ce27cafee74df9b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 5 12:53:42 2022 +0000

    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:
---
 libstdc++-v3/include/bits/vector.tcc | 40 +++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 27ef1a4ee7f..8ae79ffc7af 100644
--- 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);