diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -721,21 +721,35 @@
 void SmallVectorImpl<T>::swap(SmallVectorImpl<T> &RHS) {
   if (this == &RHS) return;
 
-  // We can only avoid copying elements if neither vector is small.
-  if (!this->isSmall() && !RHS.isSmall()) {
+  auto SwapBuffers = [&] {
     std::swap(this->BeginX, RHS.BeginX);
     std::swap(this->Size, RHS.Size);
     std::swap(this->Capacity, RHS.Capacity);
+  };
+
+  // We can only avoid copying elements if neither vector is small.
+  if (!this->isSmall() && !RHS.isSmall()) {
+    SwapBuffers();
     return;
   }
-  if (RHS.size() > this->capacity())
+  if (RHS.size() > this->capacity()) {
     this->grow(RHS.size());
-  if (this->size() > RHS.capacity())
+    if (!RHS.isSmall()) {
+      // Neither are small now, so swap the buffers.
+      SwapBuffers();
+      return;
+    }
+  } else if (this->size() > RHS.capacity()) {
     RHS.grow(this->size());
+    if (!this->isSmall()) {
+      // Neither are small now, so swap the buffers.
+      SwapBuffers();
+      return;
+    }
+  }
 
   // Swap the shared elements.
-  size_t NumShared = this->size();
-  if (NumShared > RHS.size()) NumShared = RHS.size();
+  size_t NumShared = std::min(this->size(), RHS.size());
   for (size_type i = 0; i != NumShared; ++i)
     std::swap((*this)[i], RHS[i]);