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.
- 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 theCMakeLists.txtfile in theTOYfolder:$ vi CMakeLists .txt add_llvm_target(... ... TOYISelDAGToDAG.cpp ... )
- A pass entry needs to be added in the
TOYTargetMachine.handTOYTargetMachine.cppfiles:$ vi TOYTargetMachine.h const TOYInstrInfo *getInstrInfo() const override { return getSubtargetImpl()->getInstrInfo(); } - The following code in
TOYTargetMachine.cppwill create a pass in the instruction selection stage:class TOYPassConfig : public TargetPassConfig { public: ... virtual bool addInstSelector(); }; ... bool TOYPassConfig::addInstSelector() { addPass(createTOYISelDag(getTOYTargetMachine())); return false; }
To define an instruction selection function, proceed with the following steps:
- Create a file called
TOYISelDAGToDAG.cpp:$ vi TOYISelDAGToDAG.cpp - Include the following files:
#include "TOY.h" #include "TOYTargetMachine.h" #include "llvm/CodeGen/SelectionDAGISel.h" #include "llvm/Support/Compiler.h" #include "llvm/Support/Debug.h" #include "TOYInstrInfo.h"
- Define a new class called
TOYDAGToDAGISelas follows, which will inherit from theSelectionDAGISelclass:class TOYDAGToDAGISel : public SelectionDAGISel { const TOYSubtarget &Subtarget; public: explicit TOYDAGToDAGISel(TOYTargetMachine &TM, CodeGenOpt::Level OptLevel) : SelectionDAGISel(TM, OptLevel), Subtarget(*TM.getSubtargetImpl()) {} }; - The most important function to define in this class is
Select(), which will return anSDNodeobject specific to the machine instruction:Declare it in the class:
SDNode *Select(SDNode *N);Define it further as follows:
SDNode *TOYDAGToDAGISel::Select(SDNode *N) { return SelectCode(N); }
- Another important function is used to define the address selection function, which will calculate the base and offset of the address for load and store operations.
Declare it as shown here:
bool SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset);
Define it further, like this:
bool TOYDAGToDAGISel::SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset) { if (FrameIndexSDNode *FIN = dyn_cast<FrameIndexSDNode>(Addr)) { Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), getTargetLowering()- >getPointerTy()); Offset = CurDAG->getTargetConstant(0, MVT::i32); return true; } if (Addr.getOpcode() == ISD::TargetExternalSymbol || Addr.getOpcode() == ISD::TargetGlobalAddress || Addr.getOpcode() == ISD::TargetGlobalTLSAddress) { return false; // direct calls. } Base = Addr; Offset = CurDAG->getTargetConstant(0, MVT::i32); return true; } - The
createTOYISelDagpass converts a legalized DAG into a toy-specific DAG, ready for instruction scheduling in the same file:FunctionPass *llvm::createTOYISelDag(TOYTargetMachine &TM, CodeGenOpt::Level OptLevel) { return new TOYDAGToDAGISel(TM, OptLevel); }
The TOYDAGToDAGISel::Select() function of TOYISelDAGToDAG.cpp is used for the selection of the OP code DAG node, while TOYDAGToDAGISel::SelectAddr() is used for the selection of the DATA DAG node with the addr type. Note that if the address is global or external, we return false for the address, since its address is calculated in the global context.