class SyntaxTree::YARV::ControlFlowGraph::Compiler

def find_basic_block_starts


* fallen through to from a branch
* the target of a branch
* the start of an instruction sequence

they're either:
Finds the indices of the instructions that start a basic block because 
def find_basic_block_starts
  block_starts = Set.new([0])
  insns.each do |index, insn|
    branch_targets = insn.branch_targets
    if branch_targets.any?
      branch_targets.each do |branch_target|
        block_starts.add(labels[branch_target])
      end
      block_starts.add(index + insn.length) if insn.falls_through?
    end
  end
  block_starts.to_a.sort
end