From: David Wells <drwells@email.unc.edu>
Date: Tue, 14 Jun 2022 20:23:48 +0000 (-0400)
Subject: Fix the MPI window functions with clang.
X-Git-Tag: v9.4.0-rc1~19^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F13975%2Fhead;p=dealii.git

Fix the MPI window functions with clang.

libc++ defines std::unique_ptr::reset() as

    _LIBCPP_INLINE_VISIBILITY
    void reset(nullptr_t = nullptr) _NOEXCEPT {
      pointer __tmp = __ptr_.first();
      __ptr_.first() = nullptr;
      if (__tmp)
        __ptr_.second()(__tmp);
    }

The result of this is that aligned_vector->elements.get() is nullptr at the
point at which we call the deleter - hence this deleter won't work correctly.

We can work around this by avoiding std::unique_ptr::get() and just using our
own member variables or ptr, which are guaranteed to be valid since they will
not be set to nullptr by std::unique_ptr::reset().
---

diff --git a/include/deal.II/base/aligned_vector.h b/include/deal.II/base/aligned_vector.h
index 7913f06a31..e27b872b3e 100644
--- a/include/deal.II/base/aligned_vector.h
+++ b/include/deal.II/base/aligned_vector.h
@@ -1128,13 +1128,18 @@ AlignedVector<T>::Deleter::MPISharedMemDeleterAction::delete_array(
   T *                     ptr)
 {
   (void)ptr;
-  Assert(aligned_vector->elements.get() == ptr, ExcInternalError());
+  // It would be nice to assert that aligned_vector->elements.get() equals ptr,
+  // but it is not guaranteed to work: clang, for example, sets elements.get()
+  // to nullptr and then calls the deleter on a previously made copy. Hence we
+  // must assume here that elements.get() (which is managed by the unique_ptr)
+  // may be nullptr at this point.
+  //
+  // used_elements_end is a member variable of AlignedVector (i.e., we control
+  // it, not unique_ptr) so it is still set to its correct value.
 
   if (is_shmem_root)
     if (std::is_trivial<T>::value == false)
-      for (T *p = aligned_vector->used_elements_end - 1;
-           p >= aligned_vector->elements.get();
-           --p)
+      for (T *p = aligned_vector->used_elements_end - 1; p >= ptr; --p)
         p->~T();
 
   int ierr;