[ADT] Remove unnecessary copies in SmallVectorImpl::swap

[njames93]

Summary

If one of the containers needs to grow to accomodate the amount of items in the other container, but the other container is in big mode, We can just swap the buffers over.
This saves copying the items between 2 known heap allocated buffers.

[dblaikie]

Is this testable? It should produce fewer element copies, yes? Could you add a test for it?

llvm/include/llvm/ADT/SmallVector.h

return std::lexicographical_compare(this->begin(), this->end(),
                                         RHS.begin(), RHS.end());
   }
 };
 
 template <typename T>
 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]);
 
   // Copy over the extra elts.
   if (this->size() > RHS.size()) {
     size_t EltDiff = this->size() - RHS.size();
     this->uninitialized_copy(this->begin()+NumShared, this->end(), RHS.end());
     RHS.set_size(RHS.size() + EltDiff);