Selecting an instruction

An IR instruction in DAG needs to be lowered to a target-specific instruction. The SDAG node contains IR, which needs to be mapped on machine-specific DAG nodes. The outcome of the selection phase is ready for scheduling.

Getting ready

1. For selecting a machine-specific instruction, a separate class, TOYDAGToDAGISel, needs to be defined. To compile the file containing this class definition, add the filename to the CMakeLists.txt file in the TOY folder:

   $ vi CMakeLists .txt
   add_llvm_target(...
   ...
   TOYISelDAGToDAG.cpp
   ...
   )

2. A pass entry needs to be added in the TOYTargetMachine.h and TOYTargetMachine.cpp files:

   $ vi TOYTargetMachine.h
   const TOYInstrInfo *getInstrInfo() const override {
   return getSubtargetImpl()->getInstrInfo();
   }

3. The following code in TOYTargetMachine.cpp will create a pass in the instruction selection stage:

   class TOYPassConfig : public TargetPassConfig {
   public:
   ...
   virtual bool addInstSelector();
   };
   ...
   bool TOYPassConfig::addInstSelector() {
   addPass...