Instruction Selection In LLVM

Instruction Selection Method

• FastISel
• SelectionDAGISel
• GlobalISel

related hooks, like how to specify which isel method to use

• in TargetConfig::addCoreISelPasses will choose a valid isel method

FastISel

• llvm ir based
• fast
• used in O0

workflow

llvm ir -> machine instruction with virtual register

how to impl

1. define a class inherit from FastISel
2. override a virtual function fastSelectInstruction, then emit each opcode in llvm ir which is target dependent

SelectionDAG based ISel

• should lowering to DAG first from llvm ir
• will do several times legalize and combine in instruction selecting, includes:
  • type legalize then combine
  • vector legalize then combine
  • dag legalize then combine
• will do heuristic schdeule on DAG after selected to MachineDAG

workflow

llvm ir -> lowering to DAG -> initialize DAG -> combine -> type legalize and combine -> vector legalize and type legalize and combine -> dag legalize and combine -> insturction selection -> instruction scheduling(will introduce later)

how to impl

1. define a class inherit from SelectionDAGISel
2. override the entry of selection isel Select()
3. override most of all selectXXX virtual function to select instruction, usually named as <TargetName>DAGToDAGISel()

what is lowering

lowering define some legalization info and other rules for DAG builder when SelectionDAG is initialized

GlobalISel

TODO