What is the difference between std::vector and llvm::SmallVector? which one to use when?

Question

I am trying to understand the use of SmallVector container in LLVM. I think std::vector can be used in place of small vector. Also what happens if we push more elements in llvm::SmallVector than its size?

Answer 1

llvm::SmallVector is a vector optimized for small arrays. This optimization comes from not performing heap allocations for a limited number of elements.

In the event that you add more elements than are described to be allocated using automatic storage it will fall back to the behavior of std::vector and allocate larger and larger arrays.

llvm::SmallVector<int, 10> smallVector;
for(int i = 0; i < 10; i++)
{
    smallVector.push_back(i);
}

// No heap allocations have been performed up to this point.

smallVector.push_back(11);

// Only 10 spaces for non heap allocated elements, 
// so the push_back above causes a heap allocation.

SmallVector can have a performance benefit when you know you will consistently have a small number of elements and won't run into heap allocations. This performance benefit comes at the cost of exception safety and a dependency on the llvm libraries.