The code emission phase lowers the code from code generator abstractions (such as MachineFunction class, MachineInstr class, and so on) to machine code layer abstractions (MCInst class, MCStreamer class, and so on). The important classes in this phase are the target-independent AsmPrinter class, target-specific subclasses of AsmPrinter, and the TargetLoweringObjectFile class.
The MC layer is responsible for emitting object files, which consist of labels, directives, and instructions; while the CodeGen layer consists of MachineFunctions, MachineBasicBlock and MachineInstructions. A key class used at this point in time is the MCStreamer class, which consists of assembler APIs. The MCStreamer class has functions such as EmitLabel, EmitSymbolAttribute, SwitchSection, and so on, which directly correspond to the aforementioned assembly-level directives.
There are four important things that need to be implemented for the target in order to emit code:
- Define a subclass of the
AsmPrinterclass for the target. This class implements the general lowering process, converting theMachineFunctionsfunctions into MC label constructs. TheAsmPrinterbase class methods and routines help implement a target-specificAsmPrinterclass. TheTargetLoweringObjectFileclass implements much of the common logic for theELF,COFF, orMachOtargets. - Implement an instruction printer for the target. The instruction printer takes an
MCInstclass and renders it into araw_ostreamclass as text. Most of this is automatically generated from the.tdfile (when you specify something like add$dst,$src1,$src2in the instructions), but you need to implement routines to print operands. - Implement code that lowers a
MachineInstrclass to anMCInstclass, usually implemented in<target>MCInstLower.cpp. This lowering process is often target-specific, and is responsible for turning jump table entries, constant pool indices, global variable addresses, and so on intoMCLabels, as appropriate. The instruction printer or the encoder takes theMCInststhat are generated. - Implement a subclass of
MCCodeEmitterthat lowersMCInststo machine code bytes and relocations. This is important if you want to support direct.ofile emission, or want to implement an assembler for your target.
Let's visit some important functions in the AsmPrinter base class in the lib/CodeGen/AsmPrinter/AsmPrinter.cpp file:
EmitLinkage(): This emits the linkage of the given variables or functions:void AsmPrinter::EmitLinkage(const GlobalValue *GV, MCSymbol *GVSym) const ;
EmitGlobalVariable(): This emits the specified global variable to the.sfile:void AsmPrinter::EmitGlobalVariable(const GlobalVariable *GV);
EmitFunctionHeader(): This emits the header of the current function:void AsmPrinter::EmitFunctionHeader();
EmitFunctionBody(): This method emits the body and trailer of a function:void AsmPrinter::EmitFunctionBody();
EmitJumpTableInfo(): This prints assembly representations of the jump tables used by the current function to the current output stream:void AsmPrinter::EmitJumpTableInfo();
EmitJumpTableEntry(): This emits a jump table entry for the specifiedMachineBasicBlockclass to the current stream:void AsmPrinter::EmitJumpTableEntry(const MachineJumpTableInfo *MJTI, const MachineBasicBlock *MBB, unsigned UID) const;
- Emit integer types of 8, 16, or 32 bit size:
void AsmPrinter::EmitInt8(int Value) const { OutStreamer.EmitIntValue(Value, 1); } void AsmPrinter::EmitInt16(int Value) const { OutStreamer.EmitIntValue(Value, 2); } void AsmPrinter::EmitInt32(int Value) const { OutStreamer.EmitIntValue(Value, 4); }
For detailed implementation on code emission, see the lib/CodeGen/AsmPrinter/AsmPrinter.cpp file. One important thing to note is that this class uses the OutStreamer class object to output assembly instructions. The details of target-specific code emission will be covered in later chapters.