Instruction Selection In LLVM
Instruction Selection Method
- FastISel
- SelectionDAGISel
- GlobalISel
related hooks, like how to specify which isel method to use
- in
TargetConfig::addCoreISelPasseswill choose a valid isel method
FastISel
- llvm ir based
- fast
- used in O0
workflow
llvm ir -> machine instruction with virtual register
how to impl
- define a class inherit from
FastISel - 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
- define a class inherit from
SelectionDAGISel - override the entry of selection isel
Select() - override most of all
selectXXXvirtual 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