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 ...