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std::make_heap is currently implemented by iteratively applying a siftup-type algorithm. Since sift-up is O(ln n), this gives std::make_heap a worst case time complexity of O(n ln n). The C++ standard mandates that std::make_heap make no more than O(3n) comparisons, this makes our std::make_heap out of spec. Fix this by introducing an implementation of __sift_down and switch std::make_heap to create the heap using it. This gives std::make_heap linear time complexity in the worst case. This fixes PR20161. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@213615 91177308-0d34-0410-b5e6-96231b3b80d8