[ADT] Add SmallVector::pop_back_n

[njames93]

Summary

Adds a method called pop_back_n to SmallVector.
This is more readable and less error prone than the alternatives of using

Vector.resize(Vector.size() - N);
Vector.erase(Vector.end() - N, Vector.end());
for (unsigned I = 0;I<N;++I) Vector.pop_back();

[dblaikie]

More expressive in what way(s)? I hesitate a little to add more surface area compared to the standard library as it adds a bit of friction when moving between different containers. (not an outright "never" but "might be a bit higher bar than it would otherwise be")
Could/should this use naming conventions from StringPiece? (there's various drop_n, etc)

[njames93]

In D90576#2367400, @dblaikie wrote:

More expressive in what way(s)?

Vector.resize(Vector.size() - 2); This takes a little longer to reason and its more error prone. If a typo or bad refactor caused the call to size to be on a different container which just happens to have a size method, that could easily go unnoticed
Vector.pop_back_n(2) is very easy to see what its doing and it removes the aforementioned risk.

Could/should this use naming conventions from StringPiece? (there's various drop_n, etc)

I'm easy on the name, don't really know where pop_back_n came from to be honest. I considered what ArrayRef and StringRef use, drop_back, but that didn't sound quite as nice.
Overloading pop_back could work, but I don't know if anything would be gained from that.

[dblaikie]

Fair enough - might be worth updating the commit message to clarify what's meant by "expressive" (don't have a perfect suggestion, perhaps "more self-documenting"? "expressive" makes me think/read it as though this new API provides more expressive power, as though it can do something the other API cannot, for instance)

llvm/include/llvm/ADT/SmallVector.h

void resize(size_type N, const T &NV) {
     }
   }
 
   void reserve(size_type N) {
     if (this->capacity() < N)
       this->grow(N);
   }
 
+  void pop_back_n(size_type NumItems) {
+    assert(this->size() >= NumItems);
+    this->destroy_range(this->end() - NumItems, this->end());
+    this->set_size(this->size() - NumItems);
+  }
+
   LLVM_NODISCARD T pop_back_val() {
     T Result = ::std::move(this->back());
     this->pop_back();
     return Result;
   }
 
   void swap(SmallVectorImpl &RHS);
 

llvm/unittests/ADT/SmallVectorTest.cpp

TYPED_TEST(SmallVectorTest, PushPopTest) {
   this->theVector.insert(this->theVector.begin(), this->theVector[1]);
   this->assertValuesInOrder(this->theVector, 3u, 2, 1, 2);
 
   // Pop one element
   this->theVector.pop_back();
   this->assertValuesInOrder(this->theVector, 2u, 2, 1);
 
   // Pop remaining elements
-  this->theVector.pop_back();
-  this->theVector.pop_back();
+  this->theVector.pop_back_n(2);
   this->assertEmpty(this->theVector);
 
   // Check number of constructor calls. Should be 2 for each list element,
   // one for the argument to push_back, one for the argument to insert,
   // and one for the list element itself.
   if (!RequiresGrowth) {
     EXPECT_EQ(5, Constructable::getNumConstructorCalls());
     EXPECT_EQ(5, Constructable::getNumDestructorCalls());