Implementing the DAG instruction selection classes

A large portion of the DAG instruction selector is generated by the llvm-tblgen tool. We still need to create classes using the generated code and put everything together. Let's begin with a part of the initialization process.

Initializing the target machine

Each backend has to provide at least one TargetMachine class, usually a subclass of the LLVMTargetMachine class. The M88kTargetMachine class holds a lot of the details required for code generation, and it also acts as a factory for other backend classes, most notably for the Subtarget class and the TargetPassConfig class. The Subtarget class holds the configuration for the code generation, such as which features are enabled. The TargetPassConfig class configures the machine passes of the backend. The declaration for our M88kTargetMachine class is in the M88ktargetMachine.h file and looks like this:

class M88kTargetMachine : public LLVMTargetMachine {
public:
 ...