A SelectionDAG representation is a target-independent representation of instructions and operands. However, a target may not always support the instruction or data type represented by SelectionDAG. In that sense, the initial SelectionDAG graph constructed can be called illegal. The DAG legalize phase converts the illegal DAG into a legal DAG supported by the target architecture.

A DAG legalize phase can follow two ways to convert unsupported data types into supported data types—by promoting smaller data types to larger data types, or by truncating larger data types into smaller ones. For example, suppose that a type of target architecture supports only i32 data types. In that case, smaller data types such as i8 and i16 need to be promoted to the i32 type. A larger data type, such as i64, can be expanded to give two i32 data types. The Sign and Zero extensions can be added so that the result remains consistent in the process of promoting or expanding data types.

Similarly...