A SelectionDAG representation shows data and instructions in the form of nodes. Similar to the InstCombine pass in the LLVM IR, these nodes can be combined and optimized to form a minimized SelectionDAG. But, it's not just a DAGCombine operation that optimizes the SelectionDAG. A DAGLegalize phase may generate some unnecessary DAG nodes, which are cleaned up by subsequent runs of the DAG optimization pass. This finally represents the SelectionDAG in a more simple and elegant way.
There are lots and lots of function members (most of them are named like this: visit**()) provided in the DAGCombiner class to perform optimizations by folding, reordering, combining, and modifying SDNode nodes. Note that, from the DAGCombiner constructor, we can guess that some optimizations require alias analysis information:
class DAGCombiner {
SelectionDAG &DAG;
const TargetLowering &TLI;
CombineLevel Level;
CodeGenOpt::Level OptLevel;
bool LegalOperations;
bool LegalTypes...