Writing an LLVM Backend

Introduction

This document describes techniques for writing compiler backends that convert the LLVM Intermediate Representation (IR) to code for a specified machine or other languages. Code intended for a specific machine can take the form of either assembly code or binary code (usable for a JIT compiler).

The backend of LLVM features a target-independent code generator that may create output for several types of target CPUs — including X86, PowerPC, ARM, and SPARC. The backend may also be used to generate code targeted at SPUs of the Cell processor or GPUs to support the execution of compute kernels.

The document focuses on existing examples found in subdirectories of llvm/lib/Target in a downloaded LLVM release. In particular, this document focuses on the example of creating a static compiler (one that emits text assembly) for a SPARC target, because SPARC has fairly standard characteristics, such as a RISC instruction set and straightforward calling conventions.

Audience

The audience for this document is anyone who needs to write an LLVM backend to generate code for a specific hardware or software target.

Prerequisite Reading

These essential documents must be read before reading this document:

  • LLVM Language Reference Manual — a reference manual for the LLVM assembly language.
  • The LLVM Target-Independent Code Generator — a guide to the components (classes and code generation algorithms) for translating the LLVM internal representation into machine code for a specified target. Pay particular attention to the descriptions of code generation stages: Instruction Selection, Scheduling and Formation, SSA-based Optimization, Register Allocation, Prolog/Epilog Code Insertion, Late Machine Code Optimizations, and Code Emission.
  • TableGen — a document that describes the TableGen (tblgen) application that manages domain-specific information to support LLVM code generation. TableGen processes input from a target description file (.td suffix) and generates C++ code that can be used for code generation.
  • Writing an LLVM Pass — The assembly printer is a FunctionPass, as are several SelectionDAG processing steps.

To follow the SPARC examples in this document, have a copy of The SPARC Architecture Manual, Version 8 for reference. For details about the ARM instruction set, refer to the ARM Architecture Reference Manual. For more about the GNU Assembler format (GAS), see Using As, especially for the assembly printer. “Using As” contains a list of target machine dependent features.

Basic Steps

To write a compiler backend for LLVM that converts the LLVM IR to code for a specified target (machine or other language), follow these steps:

  • Create a subclass of the TargetMachine class that describes characteristics of your target machine. Copy existing examples of specific TargetMachine class and header files; for example, start with SparcTargetMachine.cpp and SparcTargetMachine.h, but change the file names for your target. Similarly, change code that references “Sparc” to reference your target.
  • Describe the register set of the target. Use TableGen to generate code for register definition, register aliases, and register classes from a target-specific RegisterInfo.td input file. You should also write additional code for a subclass of the TargetRegisterInfo class that represents the class register file data used for register allocation and also describes the interactions between registers.
  • Describe the instruction set of the target. Use TableGen to generate code for target-specific instructions from target-specific versions of TargetInstrFormats.td and TargetInstrInfo.td. You should write additional code for a subclass of the TargetInstrInfo class to represent machine instructions supported by the target machine.
  • Describe the selection and conversion of the LLVM IR from a Directed Acyclic Graph (DAG) representation of instructions to native target-specific instructions. Use TableGen to generate code that matches patterns and selects instructions based on additional information in a target-specific version of TargetInstrInfo.td. Write code for XXXISelDAGToDAG.cpp, where XXX identifies the specific target, to perform pattern matching and DAG-to-DAG instruction selection. Also write code in XXXISelLowering.cpp to replace or remove operations and data types that are not supported natively in a SelectionDAG.
  • Write code for an assembly printer that converts LLVM IR to a GAS format for your target machine. You should add assembly strings to the instructions defined in your target-specific version of TargetInstrInfo.td. You should also write code for a subclass of AsmPrinter that performs the LLVM-to-assembly conversion and a trivial subclass of TargetAsmInfo.
  • Optionally, add support for subtargets (i.e., variants with different capabilities). You should also write code for a subclass of the TargetSubtarget class, which allows you to use the -mcpu= and -mattr= command-line options.
  • Optionally, add JIT support and create a machine code emitter (subclass of TargetJITInfo) that is used to emit binary code directly into memory.

In the .cpp and .h. files, initially stub up these methods and then implement them later. Initially, you may not know which private members that the class will need and which components will need to be subclassed.

Preliminaries

To actually create your compiler backend, you need to create and modify a few files. The absolute minimum is discussed here. But to actually use the LLVM target-independent code generator, you must perform the steps described in the LLVM Target-Independent Code Generator document.

First, you should create a subdirectory under lib/Target to hold all the files related to your target. If your target is called “Dummy”, create the directory lib/Target/Dummy.

In this new directory, create a Makefile. It is easiest to copy a Makefile of another target and modify it. It should at least contain the LEVEL, LIBRARYNAME and TARGET variables, and then include $(LEVEL)/Makefile.common. The library can be named LLVMDummy (for example, see the MIPS target). Alternatively, you can split the library into LLVMDummyCodeGen and LLVMDummyAsmPrinter, the latter of which should be implemented in a subdirectory below lib/Target/Dummy (for example, see the PowerPC target).

Note that these two naming schemes are hardcoded into llvm-config. Using any other naming scheme will confuse llvm-config and produce a lot of (seemingly unrelated) linker errors when linking llc.

To make your target actually do something, you need to implement a subclass of TargetMachine. This implementation should typically be in the file lib/Target/DummyTargetMachine.cpp, but any file in the lib/Target directory will be built and should work. To use LLVM’s target independent code generator, you should do what all current machine backends do: create a subclass of LLVMTargetMachine. (To create a target from scratch, create a subclass of TargetMachine.)

To get LLVM to actually build and link your target, you need to add it to the TARGETS_TO_BUILD variable. To do this, you modify the configure script to know about your target when parsing the --enable-targets option. Search the configure script for TARGETS_TO_BUILD, add your target to the lists there (some creativity required), and then reconfigure. Alternatively, you can change autoconf/configure.ac and regenerate configure by running ./autoconf/AutoRegen.sh.

Target Machine

LLVMTargetMachine is designed as a base class for targets implemented with the LLVM target-independent code generator. The LLVMTargetMachine class should be specialized by a concrete target class that implements the various virtual methods. LLVMTargetMachine is defined as a subclass of TargetMachine in include/llvm/Target/TargetMachine.h. The TargetMachine class implementation (TargetMachine.cpp) also processes numerous command-line options.

To create a concrete target-specific subclass of LLVMTargetMachine, start by copying an existing TargetMachine class and header. You should name the files that you create to reflect your specific target. For instance, for the SPARC target, name the files SparcTargetMachine.h and SparcTargetMachine.cpp.

For a target machine XXX, the implementation of XXXTargetMachine must have access methods to obtain objects that represent target components. These methods are named get*Info, and are intended to obtain the instruction set (getInstrInfo), register set (getRegisterInfo), stack frame layout (getFrameInfo), and similar information. XXXTargetMachine must also implement the getDataLayout method to access an object with target-specific data characteristics, such as data type size and alignment requirements.

For instance, for the SPARC target, the header file SparcTargetMachine.h declares prototypes for several get*Info and getDataLayout methods that simply return a class member.

namespace llvm {

class Module;

class SparcTargetMachine : public LLVMTargetMachine {
  const DataLayout DataLayout;       // Calculates type size & alignment
  SparcSubtarget Subtarget;
  SparcInstrInfo InstrInfo;
  TargetFrameInfo FrameInfo;

protected:
  virtual const TargetAsmInfo *createTargetAsmInfo() const;

public:
  SparcTargetMachine(const Module &M, const std::string &FS);

  virtual const SparcInstrInfo *getInstrInfo() const {return &InstrInfo; }
  virtual const TargetFrameInfo *getFrameInfo() const {return &FrameInfo; }
  virtual const TargetSubtarget *getSubtargetImpl() const{return &Subtarget; }
  virtual const TargetRegisterInfo *getRegisterInfo() const {
    return &InstrInfo.getRegisterInfo();
  }
  virtual const DataLayout *getDataLayout() const { return &DataLayout; }
  static unsigned getModuleMatchQuality(const Module &M);

  // Pass Pipeline Configuration
  virtual bool addInstSelector(PassManagerBase &PM, bool Fast);
  virtual bool addPreEmitPass(PassManagerBase &PM, bool Fast);
};

} // end namespace llvm
  • getInstrInfo()
  • getRegisterInfo()
  • getFrameInfo()
  • getDataLayout()
  • getSubtargetImpl()

For some targets, you also need to support the following methods:

  • getTargetLowering()
  • getJITInfo()

Some architectures, such as GPUs, do not support jumping to an arbitrary program location and implement branching using masked execution and loop using special instructions around the loop body. In order to avoid CFG modifications that introduce irreducible control flow not handled by such hardware, a target must call setRequiresStructuredCFG(true) when being initialized.

In addition, the XXXTargetMachine constructor should specify a TargetDescription string that determines the data layout for the target machine, including characteristics such as pointer size, alignment, and endianness. For example, the constructor for SparcTargetMachine contains the following:

SparcTargetMachine::SparcTargetMachine(const Module &M, const std::string &FS)
  : DataLayout("E-p:32:32-f128:128:128"),
    Subtarget(M, FS), InstrInfo(Subtarget),
    FrameInfo(TargetFrameInfo::StackGrowsDown, 8, 0) {
}

Hyphens separate portions of the TargetDescription string.

  • An upper-case “E” in the string indicates a big-endian target data model. A lower-case “e” indicates little-endian.
  • “p:” is followed by pointer information: size, ABI alignment, and preferred alignment. If only two figures follow “p:”, then the first value is pointer size, and the second value is both ABI and preferred alignment.
  • Then a letter for numeric type alignment: “i”, “f”, “v”, or “a” (corresponding to integer, floating point, vector, or aggregate). “i”, “v”, or “a” are followed by ABI alignment and preferred alignment. “f” is followed by three values: the first indicates the size of a long double, then ABI alignment, and then ABI preferred alignment.

Target Registration

You must also register your target with the TargetRegistry, which is what other LLVM tools use to be able to lookup and use your target at runtime. The TargetRegistry can be used directly, but for most targets there are helper templates which should take care of the work for you.

All targets should declare a global Target object which is used to represent the target during registration. Then, in the target’s TargetInfo library, the target should define that object and use the RegisterTarget template to register the target. For example, the Sparc registration code looks like this:

Target llvm::TheSparcTarget;

extern "C" void LLVMInitializeSparcTargetInfo() {
  RegisterTarget<Triple::sparc, /*HasJIT=*/false>
    X(TheSparcTarget, "sparc", "Sparc");
}

This allows the TargetRegistry to look up the target by name or by target triple. In addition, most targets will also register additional features which are available in separate libraries. These registration steps are separate, because some clients may wish to only link in some parts of the target — the JIT code generator does not require the use of the assembler printer, for example. Here is an example of registering the Sparc assembly printer:

extern "C" void LLVMInitializeSparcAsmPrinter() {
  RegisterAsmPrinter<SparcAsmPrinter> X(TheSparcTarget);
}

For more information, see “llvm/Target/TargetRegistry.h”.

Register Set and Register Classes

You should describe a concrete target-specific class that represents the register file of a target machine. This class is called XXXRegisterInfo (where XXX identifies the target) and represents the class register file data that is used for register allocation. It also describes the interactions between registers.

You also need to define register classes to categorize related registers. A register class should be added for groups of registers that are all treated the same way for some instruction. Typical examples are register classes for integer, floating-point, or vector registers. A register allocator allows an instruction to use any register in a specified register class to perform the instruction in a similar manner. Register classes allocate virtual registers to instructions from these sets, and register classes let the target-independent register allocator automatically choose the actual registers.

Much of the code for registers, including register definition, register aliases, and register classes, is generated by TableGen from XXXRegisterInfo.td input files and placed in XXXGenRegisterInfo.h.inc and XXXGenRegisterInfo.inc output files. Some of the code in the implementation of XXXRegisterInfo requires hand-coding.

Defining a Register

The XXXRegisterInfo.td file typically starts with register definitions for a target machine. The Register class (specified in Target.td) is used to define an object for each register. The specified string n becomes the Name of the register. The basic Register object does not have any subregisters and does not specify any aliases.

class Register<string n> {
  string Namespace = "";
  string AsmName = n;
  string Name = n;
  int SpillSize = 0;
  int SpillAlignment = 0;
  list<Register> Aliases = [];
  list<Register> SubRegs = [];
  list<int> DwarfNumbers = [];
}

For example, in the X86RegisterInfo.td file, there are register definitions that utilize the Register class, such as:

def AL : Register<"AL">, DwarfRegNum<[0, 0, 0]>;

This defines the register AL and assigns it values (with DwarfRegNum) that are used by gcc, gdb, or a debug information writer to identify a register. For register AL, DwarfRegNum takes an array of 3 values representing 3 different modes: the first element is for X86-64, the second for exception handling (EH) on X86-32, and the third is generic. -1 is a special Dwarf number that indicates the gcc number is undefined, and -2 indicates the register number is invalid for this mode.

From the previously described line in the X86RegisterInfo.td file, TableGen generates this code in the X86GenRegisterInfo.inc file:

static const unsigned GR8[] = { X86::AL, ... };

const unsigned AL_AliasSet[] = { X86::AX, X86::EAX, X86::RAX, 0 };

const TargetRegisterDesc RegisterDescriptors[] = {
  ...
{ "AL", "AL", AL_AliasSet, Empty_SubRegsSet, Empty_SubRegsSet, AL_SuperRegsSet }, ...

From the register info file, TableGen generates a TargetRegisterDesc object for each register. TargetRegisterDesc is defined in include/llvm/Target/TargetRegisterInfo.h with the following fields:

struct TargetRegisterDesc {
  const char     *AsmName;      // Assembly language name for the register
  const char     *Name;         // Printable name for the reg (for debugging)
  const unsigned *AliasSet;     // Register Alias Set
  const unsigned *SubRegs;      // Sub-register set
  const unsigned *ImmSubRegs;   // Immediate sub-register set
  const unsigned *SuperRegs;    // Super-register set
};

TableGen uses the entire target description file (.td) to determine text names for the register (in the AsmName and Name fields of TargetRegisterDesc) and the relationships of other registers to the defined register (in the other TargetRegisterDesc fields). In this example, other definitions establish the registers “AX”, “EAX”, and “RAX” as aliases for one another, so TableGen generates a null-terminated array (AL_AliasSet) for this register alias set.

The Register class is commonly used as a base class for more complex classes. In Target.td, the Register class is the base for the RegisterWithSubRegs class that is used to define registers that need to specify subregisters in the SubRegs list, as shown here:

class RegisterWithSubRegs<string n, list<Register> subregs> : Register<n> {
  let SubRegs = subregs;
}

In SparcRegisterInfo.td, additional register classes are defined for SPARC: a Register subclass, SparcReg, and further subclasses: Ri, Rf, and Rd. SPARC registers are identified by 5-bit ID numbers, which is a feature common to these subclasses. Note the use of “let” expressions to override values that are initially defined in a superclass (such as SubRegs field in the Rd class).

class SparcReg<string n> : Register<n> {
  field bits<5> Num;
  let Namespace = "SP";
}
// Ri - 32-bit integer registers
class Ri<bits<5> num, string n> :
SparcReg<n> {
  let Num = num;
}
// Rf - 32-bit floating-point registers
class Rf<bits<5> num, string n> :
SparcReg<n> {
  let Num = num;
}
// Rd - Slots in the FP register file for 64-bit floating-point values.
class Rd<bits<5> num, string n, list<Register> subregs> : SparcReg<n> {
  let Num = num;
  let SubRegs = subregs;
}

In the SparcRegisterInfo.td file, there are register definitions that utilize these subclasses of Register, such as:

def G0 : Ri< 0, "G0">, DwarfRegNum<[0]>;
def G1 : Ri< 1, "G1">, DwarfRegNum<[1]>;
...
def F0 : Rf< 0, "F0">, DwarfRegNum<[32]>;
def F1 : Rf< 1, "F1">, DwarfRegNum<[33]>;
...
def D0 : Rd< 0, "F0", [F0, F1]>, DwarfRegNum<[32]>;
def D1 : Rd< 2, "F2", [F2, F3]>, DwarfRegNum<[34]>;

The last two registers shown above (D0 and D1) are double-precision floating-point registers that are aliases for pairs of single-precision floating-point sub-registers. In addition to aliases, the sub-register and super-register relationships of the defined register are in fields of a register’s TargetRegisterDesc.

Defining a Register Class

The RegisterClass class (specified in Target.td) is used to define an object that represents a group of related registers and also defines the default allocation order of the registers. A target description file XXXRegisterInfo.td that uses Target.td can construct register classes using the following class:

class RegisterClass<string namespace,
list<ValueType> regTypes, int alignment, dag regList> {
  string Namespace = namespace;
  list<ValueType> RegTypes = regTypes;
  int Size = 0;  // spill size, in bits; zero lets tblgen pick the size
  int Alignment = alignment;

  // CopyCost is the cost of copying a value between two registers
  // default value 1 means a single instruction
  // A negative value means copying is extremely expensive or impossible
  int CopyCost = 1;
  dag MemberList = regList;

  // for register classes that are subregisters of this class
  list<RegisterClass> SubRegClassList = [];

  code MethodProtos = [{}];  // to insert arbitrary code
  code MethodBodies = [{}];
}

To define a RegisterClass, use the following 4 arguments:

  • The first argument of the definition is the name of the namespace.
  • The second argument is a list of ValueType register type values that are defined in include/llvm/CodeGen/ValueTypes.td. Defined values include integer types (such as i16, i32, and i1 for Boolean), floating-point types (f32, f64), and vector types (for example, v8i16 for an 8 x i16 vector). All registers in a RegisterClass must have the same ValueType, but some registers may store vector data in different configurations. For example a register that can process a 128-bit vector may be able to handle 16 8-bit integer elements, 8 16-bit integers, 4 32-bit integers, and so on.
  • The third argument of the RegisterClass definition specifies the alignment required of the registers when they are stored or loaded to memory.
  • The final argument, regList, specifies which registers are in this class. If an alternative allocation order method is not specified, then regList also defines the order of allocation used by the register allocator. Besides simply listing registers with (add R0, R1, ...), more advanced set operators are available. See include/llvm/Target/Target.td for more information.

In SparcRegisterInfo.td, three RegisterClass objects are defined: FPRegs, DFPRegs, and IntRegs. For all three register classes, the first argument defines the namespace with the string “SP”. FPRegs defines a group of 32 single-precision floating-point registers (F0 to F31); DFPRegs defines a group of 16 double-precision registers (D0-D15).

// F0, F1, F2, ..., F31
def FPRegs : RegisterClass<"SP", [f32], 32, (sequence "F%u", 0, 31)>;

def DFPRegs : RegisterClass<"SP", [f64], 64,
                            (add D0, D1, D2, D3, D4, D5, D6, D7, D8,
                                 D9, D10, D11, D12, D13, D14, D15)>;

def IntRegs : RegisterClass<"SP", [i32], 32,
    (add L0, L1, L2, L3, L4, L5, L6, L7,
         I0, I1, I2, I3, I4, I5,
         O0, O1, O2, O3, O4, O5, O7,
         G1,
         // Non-allocatable regs:
         G2, G3, G4,
         O6,        // stack ptr
         I6,        // frame ptr
         I7,        // return address
         G0,        // constant zero
         G5, G6, G7 // reserved for kernel
    )>;

Using SparcRegisterInfo.td with TableGen generates several output files that are intended for inclusion in other source code that you write. SparcRegisterInfo.td generates SparcGenRegisterInfo.h.inc, which should be included in the header file for the implementation of the SPARC register implementation that you write (SparcRegisterInfo.h). In SparcGenRegisterInfo.h.inc a new structure is defined called SparcGenRegisterInfo that uses TargetRegisterInfo as its base. It also specifies types, based upon the defined register classes: DFPRegsClass, FPRegsClass, and IntRegsClass.

SparcRegisterInfo.td also generates SparcGenRegisterInfo.inc, which is included at the bottom of SparcRegisterInfo.cpp, the SPARC register implementation. The code below shows only the generated integer registers and associated register classes. The order of registers in IntRegs reflects the order in the definition of IntRegs in the target description file.

// IntRegs Register Class...
static const unsigned IntRegs[] = {
  SP::L0, SP::L1, SP::L2, SP::L3, SP::L4, SP::L5,
  SP::L6, SP::L7, SP::I0, SP::I1, SP::I2, SP::I3,
  SP::I4, SP::I5, SP::O0, SP::O1, SP::O2, SP::O3,
  SP::O4, SP::O5, SP::O7, SP::G1, SP::G2, SP::G3,
  SP::G4, SP::O6, SP::I6, SP::I7, SP::G0, SP::G5,
  SP::G6, SP::G7,
};

// IntRegsVTs Register Class Value Types...
static const MVT::ValueType IntRegsVTs[] = {
  MVT::i32, MVT::Other
};

namespace SP {   // Register class instances
  DFPRegsClass    DFPRegsRegClass;
  FPRegsClass     FPRegsRegClass;
  IntRegsClass    IntRegsRegClass;
...
  // IntRegs Sub-register Classess...
  static const TargetRegisterClass* const IntRegsSubRegClasses [] = {
    NULL
  };
...
  // IntRegs Super-register Classess...
  static const TargetRegisterClass* const IntRegsSuperRegClasses [] = {
    NULL
  };
...
  // IntRegs Register Class sub-classes...
  static const TargetRegisterClass* const IntRegsSubclasses [] = {
    NULL
  };
...
  // IntRegs Register Class super-classes...
  static const TargetRegisterClass* const IntRegsSuperclasses [] = {
    NULL
  };

  IntRegsClass::IntRegsClass() : TargetRegisterClass(IntRegsRegClassID,
    IntRegsVTs, IntRegsSubclasses, IntRegsSuperclasses, IntRegsSubRegClasses,
    IntRegsSuperRegClasses, 4, 4, 1, IntRegs, IntRegs + 32) {}
}

The register allocators will avoid using reserved registers, and callee saved registers are not used until all the volatile registers have been used. That is usually good enough, but in some cases it may be necessary to provide custom allocation orders.

Implement a subclass of TargetRegisterInfo

The final step is to hand code portions of XXXRegisterInfo, which implements the interface described in TargetRegisterInfo.h (see The TargetRegisterInfo class). These functions return 0, NULL, or false, unless overridden. Here is a list of functions that are overridden for the SPARC implementation in SparcRegisterInfo.cpp:

  • getCalleeSavedRegs — Returns a list of callee-saved registers in the order of the desired callee-save stack frame offset.
  • getReservedRegs — Returns a bitset indexed by physical register numbers, indicating if a particular register is unavailable.
  • hasFP — Return a Boolean indicating if a function should have a dedicated frame pointer register.
  • eliminateCallFramePseudoInstr — If call frame setup or destroy pseudo instructions are used, this can be called to eliminate them.
  • eliminateFrameIndex — Eliminate abstract frame indices from instructions that may use them.
  • emitPrologue — Insert prologue code into the function.
  • emitEpilogue — Insert epilogue code into the function.

Instruction Set

During the early stages of code generation, the LLVM IR code is converted to a SelectionDAG with nodes that are instances of the SDNode class containing target instructions. An SDNode has an opcode, operands, type requirements, and operation properties. For example, is an operation commutative, does an operation load from memory. The various operation node types are described in the include/llvm/CodeGen/SelectionDAGNodes.h file (values of the NodeType enum in the ISD namespace).

TableGen uses the following target description (.td) input files to generate much of the code for instruction definition:

  • Target.td — Where the Instruction, Operand, InstrInfo, and other fundamental classes are defined.
  • TargetSelectionDAG.td — Used by SelectionDAG instruction selection generators, contains SDTC* classes (selection DAG type constraint), definitions of SelectionDAG nodes (such as imm, cond, bb, add, fadd, sub), and pattern support (Pattern, Pat, PatFrag, PatLeaf, ComplexPattern.
  • XXXInstrFormats.td — Patterns for definitions of target-specific instructions.
  • XXXInstrInfo.td — Target-specific definitions of instruction templates, condition codes, and instructions of an instruction set. For architecture modifications, a different file name may be used. For example, for Pentium with SSE instruction, this file is X86InstrSSE.td, and for Pentium with MMX, this file is X86InstrMMX.td.

There is also a target-specific XXX.td file, where XXX is the name of the target. The XXX.td file includes the other .td input files, but its contents are only directly important for subtargets.

You should describe a concrete target-specific class XXXInstrInfo that represents machine instructions supported by a target machine. XXXInstrInfo contains an array of XXXInstrDescriptor objects, each of which describes one instruction. An instruction descriptor defines:

  • Opcode mnemonic
  • Number of operands
  • List of implicit register definitions and uses
  • Target-independent properties (such as memory access, is commutable)
  • Target-specific flags

The Instruction class (defined in Target.td) is mostly used as a base for more complex instruction classes.

class Instruction {
  string Namespace = "";
  dag OutOperandList;    // A dag containing the MI def operand list.
  dag InOperandList;     // A dag containing the MI use operand list.
  string AsmString = ""; // The .s format to print the instruction with.
  list<dag> Pattern;     // Set to the DAG pattern for this instruction.
  list<Register> Uses = [];
  list<Register> Defs = [];
  list<Predicate> Predicates = [];  // predicates turned into isel match code
  ... remainder not shown for space ...
}

A SelectionDAG node (SDNode) should contain an object representing a target-specific instruction that is defined in XXXInstrInfo.td. The instruction objects should represent instructions from the architecture manual of the target machine (such as the SPARC Architecture Manual for the SPARC target).

A single instruction from the architecture manual is often modeled as multiple target instructions, depending upon its operands. For example, a manual might describe an add instruction that takes a register or an immediate operand. An LLVM target could model this with two instructions named ADDri and ADDrr.

You should define a class for each instruction category and define each opcode as a subclass of the category with appropriate parameters such as the fixed binary encoding of opcodes and extended opcodes. You should map the register bits to the bits of the instruction in which they are encoded (for the JIT). Also you should specify how the instruction should be printed when the automatic assembly printer is used.

As is described in the SPARC Architecture Manual, Version 8, there are three major 32-bit formats for instructions. Format 1 is only for the CALL instruction. Format 2 is for branch on condition codes and SETHI (set high bits of a register) instructions. Format 3 is for other instructions.

Each of these formats has corresponding classes in SparcInstrFormat.td. InstSP is a base class for other instruction classes. Additional base classes are specified for more precise formats: for example in SparcInstrFormat.td, F2_1 is for SETHI, and F2_2 is for branches. There are three other base classes: F3_1 for register/register operations, F3_2 for register/immediate operations, and F3_3 for floating-point operations. SparcInstrInfo.td also adds the base class Pseudo for synthetic SPARC instructions.

SparcInstrInfo.td largely consists of operand and instruction definitions for the SPARC target. In SparcInstrInfo.td, the following target description file entry, LDrr, defines the Load Integer instruction for a Word (the LD SPARC opcode) from a memory address to a register. The first parameter, the value 3 (112), is the operation value for this category of operation. The second parameter (0000002) is the specific operation value for LD/Load Word. The third parameter is the output destination, which is a register operand and defined in the Register target description file (IntRegs).

def LDrr : F3_1 <3, 0b000000, (outs IntRegs:$dst), (ins MEMrr:$addr),
                 "ld [$addr], $dst",
                 [(set i32:$dst, (load ADDRrr:$addr))]>;

The fourth parameter is the input source, which uses the address operand MEMrr that is defined earlier in SparcInstrInfo.td:

def MEMrr : Operand<i32> {
  let PrintMethod = "printMemOperand";
  let MIOperandInfo = (ops IntRegs, IntRegs);
}

The fifth parameter is a string that is used by the assembly printer and can be left as an empty string until the assembly printer interface is implemented. The sixth and final parameter is the pattern used to match the instruction during the SelectionDAG Select Phase described in The LLVM Target-Independent Code Generator. This parameter is detailed in the next section, Instruction Selector.

Instruction class definitions are not overloaded for different operand types, so separate versions of instructions are needed for register, memory, or immediate value operands. For example, to perform a Load Integer instruction for a Word from an immediate operand to a register, the following instruction class is defined:

def LDri : F3_2 <3, 0b000000, (outs IntRegs:$dst), (ins MEMri:$addr),
                 "ld [$addr], $dst",
                 [(set i32:$dst, (load ADDRri:$addr))]>;

Writing these definitions for so many similar instructions can involve a lot of cut and paste. In .td files, the multiclass directive enables the creation of templates to define several instruction classes at once (using the defm directive). For example in SparcInstrInfo.td, the multiclass pattern F3_12 is defined to create 2 instruction classes each time F3_12 is invoked:

multiclass F3_12 <string OpcStr, bits<6> Op3Val, SDNode OpNode> {
  def rr  : F3_1 <2, Op3Val,
                 (outs IntRegs:$dst), (ins IntRegs:$b, IntRegs:$c),
                 !strconcat(OpcStr, " $b, $c, $dst"),
                 [(set i32:$dst, (OpNode i32:$b, i32:$c))]>;
  def ri  : F3_2 <2, Op3Val,
                 (outs IntRegs:$dst), (ins IntRegs:$b, i32imm:$c),
                 !strconcat(OpcStr, " $b, $c, $dst"),
                 [(set i32:$dst, (OpNode i32:$b, simm13:$c))]>;
}

So when the defm directive is used for the XOR and ADD instructions, as seen below, it creates four instruction objects: XORrr, XORri, ADDrr, and ADDri.

defm XOR   : F3_12<"xor", 0b000011, xor>;
defm ADD   : F3_12<"add", 0b000000, add>;

SparcInstrInfo.td also includes definitions for condition codes that are referenced by branch instructions. The following definitions in SparcInstrInfo.td indicate the bit location of the SPARC condition code. For example, the 10th bit represents the “greater than” condition for integers, and the 22nd bit represents the “greater than” condition for floats.

def ICC_NE  : ICC_VAL< 9>;  // Not Equal
def ICC_E   : ICC_VAL< 1>;  // Equal
def ICC_G   : ICC_VAL<10>;  // Greater
...
def FCC_U   : FCC_VAL<23>;  // Unordered
def FCC_G   : FCC_VAL<22>;  // Greater
def FCC_UG  : FCC_VAL<21>;  // Unordered or Greater
...

(Note that Sparc.h also defines enums that correspond to the same SPARC condition codes. Care must be taken to ensure the values in Sparc.h correspond to the values in SparcInstrInfo.td. I.e., SPCC::ICC_NE = 9, SPCC::FCC_U = 23 and so on.)

Instruction Operand Mapping

The code generator backend maps instruction operands to fields in the instruction. Operands are assigned to unbound fields in the instruction in the order they are defined. Fields are bound when they are assigned a value. For example, the Sparc target defines the XNORrr instruction as a F3_1 format instruction having three operands.

def XNORrr  : F3_1<2, 0b000111,
                   (outs IntRegs:$dst), (ins IntRegs:$b, IntRegs:$c),
                   "xnor $b, $c, $dst",
                   [(set i32:$dst, (not (xor i32:$b, i32:$c)))]>;

The instruction templates in SparcInstrFormats.td show the base class for F3_1 is InstSP.

class InstSP<dag outs, dag ins, string asmstr, list<dag> pattern> : Instruction {
  field bits<32> Inst;
  let Namespace = "SP";
  bits<2> op;
  let Inst{31-30} = op;
  dag OutOperandList = outs;
  dag InOperandList = ins;
  let AsmString   = asmstr;
  let Pattern = pattern;
}

InstSP leaves the op field unbound.

class F3<dag outs, dag ins, string asmstr, list<dag> pattern>
    : InstSP<outs, ins, asmstr, pattern> {
  bits<5> rd;
  bits<6> op3;
  bits<5> rs1;
  let op{1} = 1;   // Op = 2 or 3
  let Inst{29-25} = rd;
  let Inst{24-19} = op3;
  let Inst{18-14} = rs1;
}

F3 binds the op field and defines the rd, op3, and rs1 fields. F3 format instructions will bind the operands rd, op3, and rs1 fields.

class F3_1<bits<2> opVal, bits<6> op3val, dag outs, dag ins,
           string asmstr, list<dag> pattern> : F3<outs, ins, asmstr, pattern> {
  bits<8> asi = 0; // asi not currently used
  bits<5> rs2;
  let op         = opVal;
  let op3        = op3val;
  let Inst{13}   = 0;     // i field = 0
  let Inst{12-5} = asi;   // address space identifier
  let Inst{4-0}  = rs2;
}

F3_1 binds the op3 field and defines the rs2 fields. F3_1 format instructions will bind the operands to the rd, rs1, and rs2 fields. This results in the XNORrr instruction binding $dst, $b, and $c operands to the rd, rs1, and rs2 fields respectively.

Instruction Operand Name Mapping

TableGen will also generate a function called getNamedOperandIdx() which can be used to look up an operand’s index in a MachineInstr based on its TableGen name. Setting the UseNamedOperandTable bit in an instruction’s TableGen definition will add all of its operands to an enumeration in the llvm::XXX:OpName namespace and also add an entry for it into the OperandMap table, which can be queried using getNamedOperandIdx()

int DstIndex = SP::getNamedOperandIdx(SP::XNORrr, SP::OpName::dst); // => 0
int BIndex = SP::getNamedOperandIdx(SP::XNORrr, SP::OpName::b);     // => 1
int CIndex = SP::getNamedOperandIdx(SP::XNORrr, SP::OpName::c);     // => 2
int DIndex = SP::getNamedOperandIdx(SP::XNORrr, SP::OpName::d);     // => -1

...

The entries in the OpName enum are taken verbatim from the TableGen definitions, so operands with lowercase names will have lower case entries in the enum.

To include the getNamedOperandIdx() function in your backend, you will need to define a few preprocessor macros in XXXInstrInfo.cpp and XXXInstrInfo.h. For example:

XXXInstrInfo.cpp:

#define GET_INSTRINFO_NAMED_OPS // For getNamedOperandIdx() function
#include "XXXGenInstrInfo.inc"

XXXInstrInfo.h:

#define GET_INSTRINFO_OPERAND_ENUM // For OpName enum
#include "XXXGenInstrInfo.inc"

namespace XXX {
  int16_t getNamedOperandIdx(uint16_t Opcode, uint16_t NamedIndex);
} // End namespace XXX

Instruction Operand Types

TableGen will also generate an enumeration consisting of all named Operand types defined in the backend, in the llvm::XXX::OpTypes namespace. Some common immediate Operand types (for instance i8, i32, i64, f32, f64) are defined for all targets in include/llvm/Target/Target.td, and are available in each Target’s OpTypes enum. Also, only named Operand types appear in the enumeration: anonymous types are ignored. For example, the X86 backend defines brtarget and brtarget8, both instances of the TableGen Operand class, which represent branch target operands:

def brtarget : Operand<OtherVT>;
def brtarget8 : Operand<OtherVT>;

This results in:

namespace X86 {
namespace OpTypes {
enum OperandType {
  ...
  brtarget,
  brtarget8,
  ...
  i32imm,
  i64imm,
  ...
  OPERAND_TYPE_LIST_END
} // End namespace OpTypes
} // End namespace X86

In typical TableGen fashion, to use the enum, you will need to define a preprocessor macro:

#define GET_INSTRINFO_OPERAND_TYPES_ENUM // For OpTypes enum
#include "XXXGenInstrInfo.inc"

Instruction Scheduling

Instruction itineraries can be queried using MCDesc::getSchedClass(). The value can be named by an enumemation in llvm::XXX::Sched namespace generated by TableGen in XXXGenInstrInfo.inc. The name of the schedule classes are the same as provided in XXXSchedule.td plus a default NoItinerary class.

Instruction Relation Mapping

This TableGen feature is used to relate instructions with each other. It is particularly useful when you have multiple instruction formats and need to switch between them after instruction selection. This entire feature is driven by relation models which can be defined in XXXInstrInfo.td files according to the target-specific instruction set. Relation models are defined using InstrMapping class as a base. TableGen parses all the models and generates instruction relation maps using the specified information. Relation maps are emitted as tables in the XXXGenInstrInfo.inc file along with the functions to query them. For the detailed information on how to use this feature, please refer to How To Use Instruction Mappings.

Implement a subclass of TargetInstrInfo

The final step is to hand code portions of XXXInstrInfo, which implements the interface described in TargetInstrInfo.h (see The TargetInstrInfo class). These functions return 0 or a Boolean or they assert, unless overridden. Here’s a list of functions that are overridden for the SPARC implementation in SparcInstrInfo.cpp:

  • isLoadFromStackSlot — If the specified machine instruction is a direct load from a stack slot, return the register number of the destination and the FrameIndex of the stack slot.
  • isStoreToStackSlot — If the specified machine instruction is a direct store to a stack slot, return the register number of the destination and the FrameIndex of the stack slot.
  • copyPhysReg — Copy values between a pair of physical registers.
  • storeRegToStackSlot — Store a register value to a stack slot.
  • loadRegFromStackSlot — Load a register value from a stack slot.
  • storeRegToAddr — Store a register value to memory.
  • loadRegFromAddr — Load a register value from memory.
  • foldMemoryOperand — Attempt to combine instructions of any load or store instruction for the specified operand(s).

Branch Folding and If Conversion

Performance can be improved by combining instructions or by eliminating instructions that are never reached. The AnalyzeBranch method in XXXInstrInfo may be implemented to examine conditional instructions and remove unnecessary instructions. AnalyzeBranch looks at the end of a machine basic block (MBB) for opportunities for improvement, such as branch folding and if conversion. The BranchFolder and IfConverter machine function passes (see the source files BranchFolding.cpp and IfConversion.cpp in the lib/CodeGen directory) call AnalyzeBranch to improve the control flow graph that represents the instructions.

Several implementations of AnalyzeBranch (for ARM, Alpha, and X86) can be examined as models for your own AnalyzeBranch implementation. Since SPARC does not implement a useful AnalyzeBranch, the ARM target implementation is shown below.

AnalyzeBranch returns a Boolean value and takes four parameters:

  • MachineBasicBlock &MBB — The incoming block to be examined.
  • MachineBasicBlock *&TBB — A destination block that is returned. For a conditional branch that evaluates to true, TBB is the destination.
  • MachineBasicBlock *&FBB — For a conditional branch that evaluates to false, FBB is returned as the destination.
  • std::vector<MachineOperand> &Cond — List of operands to evaluate a condition for a conditional branch.

In the simplest case, if a block ends without a branch, then it falls through to the successor block. No destination blocks are specified for either TBB or FBB, so both parameters return NULL. The start of the AnalyzeBranch (see code below for the ARM target) shows the function parameters and the code for the simplest case.

bool ARMInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                 MachineBasicBlock *&TBB,
                                 MachineBasicBlock *&FBB,
                                 std::vector<MachineOperand> &Cond) const
{
  MachineBasicBlock::iterator I = MBB.end();
  if (I == MBB.begin() || !isUnpredicatedTerminator(--I))
    return false;

If a block ends with a single unconditional branch instruction, then AnalyzeBranch (shown below) should return the destination of that branch in the TBB parameter.

if (LastOpc == ARM::B || LastOpc == ARM::tB) {
  TBB = LastInst->getOperand(0).getMBB();
  return false;
}

If a block ends with two unconditional branches, then the second branch is never reached. In that situation, as shown below, remove the last branch instruction and return the penultimate branch in the TBB parameter.

if ((SecondLastOpc == ARM::B || SecondLastOpc == ARM::tB) &&
    (LastOpc == ARM::B || LastOpc == ARM::tB)) {
  TBB = SecondLastInst->getOperand(0).getMBB();
  I = LastInst;
  I->eraseFromParent();
  return false;
}

A block may end with a single conditional branch instruction that falls through to successor block if the condition evaluates to false. In that case, AnalyzeBranch (shown below) should return the destination of that conditional branch in the TBB parameter and a list of operands in the Cond parameter to evaluate the condition.

if (LastOpc == ARM::Bcc || LastOpc == ARM::tBcc) {
  // Block ends with fall-through condbranch.
  TBB = LastInst->getOperand(0).getMBB();
  Cond.push_back(LastInst->getOperand(1));
  Cond.push_back(LastInst->getOperand(2));
  return false;
}

If a block ends with both a conditional branch and an ensuing unconditional branch, then AnalyzeBranch (shown below) should return the conditional branch destination (assuming it corresponds to a conditional evaluation of “true”) in the TBB parameter and the unconditional branch destination in the FBB (corresponding to a conditional evaluation of “false”). A list of operands to evaluate the condition should be returned in the Cond parameter.

unsigned SecondLastOpc = SecondLastInst->getOpcode();

if ((SecondLastOpc == ARM::Bcc && LastOpc == ARM::B) ||
    (SecondLastOpc == ARM::tBcc && LastOpc == ARM::tB)) {
  TBB =  SecondLastInst->getOperand(0).getMBB();
  Cond.push_back(SecondLastInst->getOperand(1));
  Cond.push_back(SecondLastInst->getOperand(2));
  FBB = LastInst->getOperand(0).getMBB();
  return false;
}

For the last two cases (ending with a single conditional branch or ending with one conditional and one unconditional branch), the operands returned in the Cond parameter can be passed to methods of other instructions to create new branches or perform other operations. An implementation of AnalyzeBranch requires the helper methods RemoveBranch and InsertBranch to manage subsequent operations.

AnalyzeBranch should return false indicating success in most circumstances. AnalyzeBranch should only return true when the method is stumped about what to do, for example, if a block has three terminating branches. AnalyzeBranch may return true if it encounters a terminator it cannot handle, such as an indirect branch.

Instruction Selector

LLVM uses a SelectionDAG to represent LLVM IR instructions, and nodes of the SelectionDAG ideally represent native target instructions. During code generation, instruction selection passes are performed to convert non-native DAG instructions into native target-specific instructions. The pass described in XXXISelDAGToDAG.cpp is used to match patterns and perform DAG-to-DAG instruction selection. Optionally, a pass may be defined (in XXXBranchSelector.cpp) to perform similar DAG-to-DAG operations for branch instructions. Later, the code in XXXISelLowering.cpp replaces or removes operations and data types not supported natively (legalizes) in a SelectionDAG.

TableGen generates code for instruction selection using the following target description input files:

  • XXXInstrInfo.td — Contains definitions of instructions in a target-specific instruction set, generates XXXGenDAGISel.inc, which is included in XXXISelDAGToDAG.cpp.
  • XXXCallingConv.td — Contains the calling and return value conventions for the target architecture, and it generates XXXGenCallingConv.inc, which is included in XXXISelLowering.cpp.

The implementation of an instruction selection pass must include a header that declares the FunctionPass class or a subclass of FunctionPass. In XXXTargetMachine.cpp, a Pass Manager (PM) should add each instruction selection pass into the queue of passes to run.

The LLVM static compiler (llc) is an excellent tool for visualizing the contents of DAGs. To display the SelectionDAG before or after specific processing phases, use the command line options for llc, described at SelectionDAG Instruction Selection Process.

To describe instruction selector behavior, you should add patterns for lowering LLVM code into a SelectionDAG as the last parameter of the instruction definitions in XXXInstrInfo.td. For example, in SparcInstrInfo.td, this entry defines a register store operation, and the last parameter describes a pattern with the store DAG operator.

def STrr  : F3_1< 3, 0b000100, (outs), (ins MEMrr:$addr, IntRegs:$src),
                 "st $src, [$addr]", [(store i32:$src, ADDRrr:$addr)]>;

ADDRrr is a memory mode that is also defined in SparcInstrInfo.td:

def ADDRrr : ComplexPattern<i32, 2, "SelectADDRrr", [], []>;

The definition of ADDRrr refers to SelectADDRrr, which is a function defined in an implementation of the Instructor Selector (such as SparcISelDAGToDAG.cpp).

In lib/Target/TargetSelectionDAG.td, the DAG operator for store is defined below:

def store : PatFrag<(ops node:$val, node:$ptr),
                    (st node:$val, node:$ptr), [{
  if (StoreSDNode *ST = dyn_cast<StoreSDNode>(N))
    return !ST->isTruncatingStore() &&
           ST->getAddressingMode() == ISD::UNINDEXED;
  return false;
}]>;

XXXInstrInfo.td also generates (in XXXGenDAGISel.inc) the SelectCode method that is used to call the appropriate processing method for an instruction. In this example, SelectCode calls Select_ISD_STORE for the ISD::STORE opcode.

SDNode *SelectCode(SDValue N) {
  ...
  MVT::ValueType NVT = N.getNode()->getValueType(0);
  switch (N.getOpcode()) {
  case ISD::STORE: {
    switch (NVT) {
    default:
      return Select_ISD_STORE(N);
      break;
    }
    break;
  }
  ...

The pattern for STrr is matched, so elsewhere in XXXGenDAGISel.inc, code for STrr is created for Select_ISD_STORE. The Emit_22 method is also generated in XXXGenDAGISel.inc to complete the processing of this instruction.

SDNode *Select_ISD_STORE(const SDValue &N) {
  SDValue Chain = N.getOperand(0);
  if (Predicate_store(N.getNode())) {
    SDValue N1 = N.getOperand(1);
    SDValue N2 = N.getOperand(2);
    SDValue CPTmp0;
    SDValue CPTmp1;

    // Pattern: (st:void i32:i32:$src,
    //           ADDRrr:i32:$addr)<<P:Predicate_store>>
    // Emits: (STrr:void ADDRrr:i32:$addr, IntRegs:i32:$src)
    // Pattern complexity = 13  cost = 1  size = 0
    if (SelectADDRrr(N, N2, CPTmp0, CPTmp1) &&
        N1.getNode()->getValueType(0) == MVT::i32 &&
        N2.getNode()->getValueType(0) == MVT::i32) {
      return Emit_22(N, SP::STrr, CPTmp0, CPTmp1);
    }
...

The SelectionDAG Legalize Phase

The Legalize phase converts a DAG to use types and operations that are natively supported by the target. For natively unsupported types and operations, you need to add code to the target-specific XXXTargetLowering implementation to convert unsupported types and operations to supported ones.

In the constructor for the XXXTargetLowering class, first use the addRegisterClass method to specify which types are supported and which register classes are associated with them. The code for the register classes are generated by TableGen from XXXRegisterInfo.td and placed in XXXGenRegisterInfo.h.inc. For example, the implementation of the constructor for the SparcTargetLowering class (in SparcISelLowering.cpp) starts with the following code:

addRegisterClass(MVT::i32, SP::IntRegsRegisterClass);
addRegisterClass(MVT::f32, SP::FPRegsRegisterClass);
addRegisterClass(MVT::f64, SP::DFPRegsRegisterClass);

You should examine the node types in the ISD namespace (include/llvm/CodeGen/SelectionDAGNodes.h) and determine which operations the target natively supports. For operations that do not have native support, add a callback to the constructor for the XXXTargetLowering class, so the instruction selection process knows what to do. The TargetLowering class callback methods (declared in llvm/Target/TargetLowering.h) are:

  • setOperationAction — General operation.
  • setLoadExtAction — Load with extension.
  • setTruncStoreAction — Truncating store.
  • setIndexedLoadAction — Indexed load.
  • setIndexedStoreAction — Indexed store.
  • setConvertAction — Type conversion.
  • setCondCodeAction — Support for a given condition code.

Note: on older releases, setLoadXAction is used instead of setLoadExtAction. Also, on older releases, setCondCodeAction may not be supported. Examine your release to see what methods are specifically supported.

These callbacks are used to determine that an operation does or does not work with a specified type (or types). And in all cases, the third parameter is a LegalAction type enum value: Promote, Expand, Custom, or Legal. SparcISelLowering.cpp contains examples of all four LegalAction values.

Promote

For an operation without native support for a given type, the specified type may be promoted to a larger type that is supported. For example, SPARC does not support a sign-extending load for Boolean values (i1 type), so in SparcISelLowering.cpp the third parameter below, Promote, changes i1 type values to a large type before loading.

setLoadExtAction(ISD::SEXTLOAD, MVT::i1, Promote);

Expand

For a type without native support, a value may need to be broken down further, rather than promoted. For an operation without native support, a combination of other operations may be used to similar effect. In SPARC, the floating-point sine and cosine trig operations are supported by expansion to other operations, as indicated by the third parameter, Expand, to setOperationAction:

setOperationAction(ISD::FSIN, MVT::f32, Expand);
setOperationAction(ISD::FCOS, MVT::f32, Expand);

Custom

For some operations, simple type promotion or operation expansion may be insufficient. In some cases, a special intrinsic function must be implemented.

For example, a constant value may require special treatment, or an operation may require spilling and restoring registers in the stack and working with register allocators.

As seen in SparcISelLowering.cpp code below, to perform a type conversion from a floating point value to a signed integer, first the setOperationAction should be called with Custom as the third parameter:

setOperationAction(ISD::FP_TO_SINT, MVT::i32, Custom);

In the LowerOperation method, for each Custom operation, a case statement should be added to indicate what function to call. In the following code, an FP_TO_SINT opcode will call the LowerFP_TO_SINT method:

SDValue SparcTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) {
  switch (Op.getOpcode()) {
  case ISD::FP_TO_SINT: return LowerFP_TO_SINT(Op, DAG);
  ...
  }
}

Finally, the LowerFP_TO_SINT method is implemented, using an FP register to convert the floating-point value to an integer.

static SDValue LowerFP_TO_SINT(SDValue Op, SelectionDAG &DAG) {
  assert(Op.getValueType() == MVT::i32);
  Op = DAG.getNode(SPISD::FTOI, MVT::f32, Op.getOperand(0));
  return DAG.getNode(ISD::BITCAST, MVT::i32, Op);
}

Calling Conventions

To support target-specific calling conventions, XXXGenCallingConv.td uses interfaces (such as CCIfType and CCAssignToReg) that are defined in lib/Target/TargetCallingConv.td. TableGen can take the target descriptor file XXXGenCallingConv.td and generate the header file XXXGenCallingConv.inc, which is typically included in XXXISelLowering.cpp. You can use the interfaces in TargetCallingConv.td to specify:

  • The order of parameter allocation.
  • Where parameters and return values are placed (that is, on the stack or in registers).
  • Which registers may be used.
  • Whether the caller or callee unwinds the stack.

The following example demonstrates the use of the CCIfType and CCAssignToReg interfaces. If the CCIfType predicate is true (that is, if the current argument is of type f32 or f64), then the action is performed. In this case, the CCAssignToReg action assigns the argument value to the first available register: either R0 or R1.

CCIfType<[f32,f64], CCAssignToReg<[R0, R1]>>

SparcCallingConv.td contains definitions for a target-specific return-value calling convention (RetCC_Sparc32) and a basic 32-bit C calling convention (CC_Sparc32). The definition of RetCC_Sparc32 (shown below) indicates which registers are used for specified scalar return types. A single-precision float is returned to register F0, and a double-precision float goes to register D0. A 32-bit integer is returned in register I0 or I1.

def RetCC_Sparc32 : CallingConv<[
  CCIfType<[i32], CCAssignToReg<[I0, I1]>>,
  CCIfType<[f32], CCAssignToReg<[F0]>>,
  CCIfType<[f64], CCAssignToReg<[D0]>>
]>;

The definition of CC_Sparc32 in SparcCallingConv.td introduces CCAssignToStack, which assigns the value to a stack slot with the specified size and alignment. In the example below, the first parameter, 4, indicates the size of the slot, and the second parameter, also 4, indicates the stack alignment along 4-byte units. (Special cases: if size is zero, then the ABI size is used; if alignment is zero, then the ABI alignment is used.)

def CC_Sparc32 : CallingConv<[
  // All arguments get passed in integer registers if there is space.
  CCIfType<[i32, f32, f64], CCAssignToReg<[I0, I1, I2, I3, I4, I5]>>,
  CCAssignToStack<4, 4>
]>;

CCDelegateTo is another commonly used interface, which tries to find a specified sub-calling convention, and, if a match is found, it is invoked. In the following example (in X86CallingConv.td), the definition of RetCC_X86_32_C ends with CCDelegateTo. After the current value is assigned to the register ST0 or ST1, the RetCC_X86Common is invoked.

def RetCC_X86_32_C : CallingConv<[
  CCIfType<[f32], CCAssignToReg<[ST0, ST1]>>,
  CCIfType<[f64], CCAssignToReg<[ST0, ST1]>>,
  CCDelegateTo<RetCC_X86Common>
]>;

CCIfCC is an interface that attempts to match the given name to the current calling convention. If the name identifies the current calling convention, then a specified action is invoked. In the following example (in X86CallingConv.td), if the Fast calling convention is in use, then RetCC_X86_32_Fast is invoked. If the SSECall calling convention is in use, then RetCC_X86_32_SSE is invoked.

def RetCC_X86_32 : CallingConv<[
  CCIfCC<"CallingConv::Fast", CCDelegateTo<RetCC_X86_32_Fast>>,
  CCIfCC<"CallingConv::X86_SSECall", CCDelegateTo<RetCC_X86_32_SSE>>,
  CCDelegateTo<RetCC_X86_32_C>
]>;

Other calling convention interfaces include:

  • CCIf <predicate, action> — If the predicate matches, apply the action.
  • CCIfInReg <action> — If the argument is marked with the “inreg” attribute, then apply the action.
  • CCIfNest <action> — If the argument is marked with the “nest” attribute, then apply the action.
  • CCIfNotVarArg <action> — If the current function does not take a variable number of arguments, apply the action.
  • CCAssignToRegWithShadow <registerList, shadowList> — similar to CCAssignToReg, but with a shadow list of registers.
  • CCPassByVal <size, align> — Assign value to a stack slot with the minimum specified size and alignment.
  • CCPromoteToType <type> — Promote the current value to the specified type.
  • CallingConv <[actions]> — Define each calling convention that is supported.

Assembly Printer

During the code emission stage, the code generator may utilize an LLVM pass to produce assembly output. To do this, you want to implement the code for a printer that converts LLVM IR to a GAS-format assembly language for your target machine, using the following steps:

  • Define all the assembly strings for your target, adding them to the instructions defined in the XXXInstrInfo.td file. (See Instruction Set.) TableGen will produce an output file (XXXGenAsmWriter.inc) with an implementation of the printInstruction method for the XXXAsmPrinter class.
  • Write XXXTargetAsmInfo.h, which contains the bare-bones declaration of the XXXTargetAsmInfo class (a subclass of TargetAsmInfo).
  • Write XXXTargetAsmInfo.cpp, which contains target-specific values for TargetAsmInfo properties and sometimes new implementations for methods.
  • Write XXXAsmPrinter.cpp, which implements the AsmPrinter class that performs the LLVM-to-assembly conversion.

The code in XXXTargetAsmInfo.h is usually a trivial declaration of the XXXTargetAsmInfo class for use in XXXTargetAsmInfo.cpp. Similarly, XXXTargetAsmInfo.cpp usually has a few declarations of XXXTargetAsmInfo replacement values that override the default values in TargetAsmInfo.cpp. For example in SparcTargetAsmInfo.cpp:

SparcTargetAsmInfo::SparcTargetAsmInfo(const SparcTargetMachine &TM) {
  Data16bitsDirective = "\t.half\t";
  Data32bitsDirective = "\t.word\t";
  Data64bitsDirective = 0;  // .xword is only supported by V9.
  ZeroDirective = "\t.skip\t";
  CommentString = "!";
  ConstantPoolSection = "\t.section \".rodata\",#alloc\n";
}

The X86 assembly printer implementation (X86TargetAsmInfo) is an example where the target specific TargetAsmInfo class uses an overridden methods: ExpandInlineAsm.

A target-specific implementation of AsmPrinter is written in XXXAsmPrinter.cpp, which implements the AsmPrinter class that converts the LLVM to printable assembly. The implementation must include the following headers that have declarations for the AsmPrinter and MachineFunctionPass classes. The MachineFunctionPass is a subclass of FunctionPass.

#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/MachineFunctionPass.h"

As a FunctionPass, AsmPrinter first calls doInitialization to set up the AsmPrinter. In SparcAsmPrinter, a Mangler object is instantiated to process variable names.

In XXXAsmPrinter.cpp, the runOnMachineFunction method (declared in MachineFunctionPass) must be implemented for XXXAsmPrinter. In MachineFunctionPass, the runOnFunction method invokes runOnMachineFunction. Target-specific implementations of runOnMachineFunction differ, but generally do the following to process each machine function:

  • Call SetupMachineFunction to perform initialization.
  • Call EmitConstantPool to print out (to the output stream) constants which have been spilled to memory.
  • Call EmitJumpTableInfo to print out jump tables used by the current function.
  • Print out the label for the current function.
  • Print out the code for the function, including basic block labels and the assembly for the instruction (using printInstruction)

The XXXAsmPrinter implementation must also include the code generated by TableGen that is output in the XXXGenAsmWriter.inc file. The code in XXXGenAsmWriter.inc contains an implementation of the printInstruction method that may call these methods:

  • printOperand
  • printMemOperand
  • printCCOperand (for conditional statements)
  • printDataDirective
  • printDeclare
  • printImplicitDef
  • printInlineAsm

The implementations of printDeclare, printImplicitDef, printInlineAsm, and printLabel in AsmPrinter.cpp are generally adequate for printing assembly and do not need to be overridden.

The printOperand method is implemented with a long switch/case statement for the type of operand: register, immediate, basic block, external symbol, global address, constant pool index, or jump table index. For an instruction with a memory address operand, the printMemOperand method should be implemented to generate the proper output. Similarly, printCCOperand should be used to print a conditional operand.

doFinalization should be overridden in XXXAsmPrinter, and it should be called to shut down the assembly printer. During doFinalization, global variables and constants are printed to output.

Subtarget Support

Subtarget support is used to inform the code generation process of instruction set variations for a given chip set. For example, the LLVM SPARC implementation provided covers three major versions of the SPARC microprocessor architecture: Version 8 (V8, which is a 32-bit architecture), Version 9 (V9, a 64-bit architecture), and the UltraSPARC architecture. V8 has 16 double-precision floating-point registers that are also usable as either 32 single-precision or 8 quad-precision registers. V8 is also purely big-endian. V9 has 32 double-precision floating-point registers that are also usable as 16 quad-precision registers, but cannot be used as single-precision registers. The UltraSPARC architecture combines V9 with UltraSPARC Visual Instruction Set extensions.

If subtarget support is needed, you should implement a target-specific XXXSubtarget class for your architecture. This class should process the command-line options -mcpu= and -mattr=.

TableGen uses definitions in the Target.td and Sparc.td files to generate code in SparcGenSubtarget.inc. In Target.td, shown below, the SubtargetFeature interface is defined. The first 4 string parameters of the SubtargetFeature interface are a feature name, an attribute set by the feature, the value of the attribute, and a description of the feature. (The fifth parameter is a list of features whose presence is implied, and its default value is an empty array.)

class SubtargetFeature<string n, string a, string v, string d,
                       list<SubtargetFeature> i = []> {
  string Name = n;
  string Attribute = a;
  string Value = v;
  string Desc = d;
  list<SubtargetFeature> Implies = i;
}

In the Sparc.td file, the SubtargetFeature is used to define the following features.

def FeatureV9 : SubtargetFeature<"v9", "IsV9", "true",
                     "Enable SPARC-V9 instructions">;
def FeatureV8Deprecated : SubtargetFeature<"deprecated-v8",
                     "V8DeprecatedInsts", "true",
                     "Enable deprecated V8 instructions in V9 mode">;
def FeatureVIS : SubtargetFeature<"vis", "IsVIS", "true",
                     "Enable UltraSPARC Visual Instruction Set extensions">;

Elsewhere in Sparc.td, the Proc class is defined and then is used to define particular SPARC processor subtypes that may have the previously described features.

class Proc<string Name, list<SubtargetFeature> Features>
  : Processor<Name, NoItineraries, Features>;

def : Proc<"generic",         []>;
def : Proc<"v8",              []>;
def : Proc<"supersparc",      []>;
def : Proc<"sparclite",       []>;
def : Proc<"f934",            []>;
def : Proc<"hypersparc",      []>;
def : Proc<"sparclite86x",    []>;
def : Proc<"sparclet",        []>;
def : Proc<"tsc701",          []>;
def : Proc<"v9",              [FeatureV9]>;
def : Proc<"ultrasparc",      [FeatureV9, FeatureV8Deprecated]>;
def : Proc<"ultrasparc3",     [FeatureV9, FeatureV8Deprecated]>;
def : Proc<"ultrasparc3-vis", [FeatureV9, FeatureV8Deprecated, FeatureVIS]>;

From Target.td and Sparc.td files, the resulting SparcGenSubtarget.inc specifies enum values to identify the features, arrays of constants to represent the CPU features and CPU subtypes, and the ParseSubtargetFeatures method that parses the features string that sets specified subtarget options. The generated SparcGenSubtarget.inc file should be included in the SparcSubtarget.cpp. The target-specific implementation of the XXXSubtarget method should follow this pseudocode:

XXXSubtarget::XXXSubtarget(const Module &M, const std::string &FS) {
  // Set the default features
  // Determine default and user specified characteristics of the CPU
  // Call ParseSubtargetFeatures(FS, CPU) to parse the features string
  // Perform any additional operations
}

JIT Support

The implementation of a target machine optionally includes a Just-In-Time (JIT) code generator that emits machine code and auxiliary structures as binary output that can be written directly to memory. To do this, implement JIT code generation by performing the following steps:

  • Write an XXXCodeEmitter.cpp file that contains a machine function pass that transforms target-machine instructions into relocatable machine code.
  • Write an XXXJITInfo.cpp file that implements the JIT interfaces for target-specific code-generation activities, such as emitting machine code and stubs.
  • Modify XXXTargetMachine so that it provides a TargetJITInfo object through its getJITInfo method.

There are several different approaches to writing the JIT support code. For instance, TableGen and target descriptor files may be used for creating a JIT code generator, but are not mandatory. For the Alpha and PowerPC target machines, TableGen is used to generate XXXGenCodeEmitter.inc, which contains the binary coding of machine instructions and the getBinaryCodeForInstr method to access those codes. Other JIT implementations do not.

Both XXXJITInfo.cpp and XXXCodeEmitter.cpp must include the llvm/CodeGen/MachineCodeEmitter.h header file that defines the MachineCodeEmitter class containing code for several callback functions that write data (in bytes, words, strings, etc.) to the output stream.

Machine Code Emitter

In XXXCodeEmitter.cpp, a target-specific of the Emitter class is implemented as a function pass (subclass of MachineFunctionPass). The target-specific implementation of runOnMachineFunction (invoked by runOnFunction in MachineFunctionPass) iterates through the MachineBasicBlock calls emitInstruction to process each instruction and emit binary code. emitInstruction is largely implemented with case statements on the instruction types defined in XXXInstrInfo.h. For example, in X86CodeEmitter.cpp, the emitInstruction method is built around the following switch/case statements:

switch (Desc->TSFlags & X86::FormMask) {
case X86II::Pseudo:  // for not yet implemented instructions
   ...               // or pseudo-instructions
   break;
case X86II::RawFrm:  // for instructions with a fixed opcode value
   ...
   break;
case X86II::AddRegFrm: // for instructions that have one register operand
   ...                 // added to their opcode
   break;
case X86II::MRMDestReg:// for instructions that use the Mod/RM byte
   ...                 // to specify a destination (register)
   break;
case X86II::MRMDestMem:// for instructions that use the Mod/RM byte
   ...                 // to specify a destination (memory)
   break;
case X86II::MRMSrcReg: // for instructions that use the Mod/RM byte
   ...                 // to specify a source (register)
   break;
case X86II::MRMSrcMem: // for instructions that use the Mod/RM byte
   ...                 // to specify a source (memory)
   break;
case X86II::MRM0r: case X86II::MRM1r:  // for instructions that operate on
case X86II::MRM2r: case X86II::MRM3r:  // a REGISTER r/m operand and
case X86II::MRM4r: case X86II::MRM5r:  // use the Mod/RM byte and a field
case X86II::MRM6r: case X86II::MRM7r:  // to hold extended opcode data
   ...
   break;
case X86II::MRM0m: case X86II::MRM1m:  // for instructions that operate on
case X86II::MRM2m: case X86II::MRM3m:  // a MEMORY r/m operand and
case X86II::MRM4m: case X86II::MRM5m:  // use the Mod/RM byte and a field
case X86II::MRM6m: case X86II::MRM7m:  // to hold extended opcode data
   ...
   break;
case X86II::MRMInitReg: // for instructions whose source and
   ...                  // destination are the same register
   break;
}

The implementations of these case statements often first emit the opcode and then get the operand(s). Then depending upon the operand, helper methods may be called to process the operand(s). For example, in X86CodeEmitter.cpp, for the X86II::AddRegFrm case, the first data emitted (by emitByte) is the opcode added to the register operand. Then an object representing the machine operand, MO1, is extracted. The helper methods such as isImmediate, isGlobalAddress, isExternalSymbol, isConstantPoolIndex, and isJumpTableIndex determine the operand type. (X86CodeEmitter.cpp also has private methods such as emitConstant, emitGlobalAddress, emitExternalSymbolAddress, emitConstPoolAddress, and emitJumpTableAddress that emit the data into the output stream.)

case X86II::AddRegFrm:
  MCE.emitByte(BaseOpcode + getX86RegNum(MI.getOperand(CurOp++).getReg()));

  if (CurOp != NumOps) {
    const MachineOperand &MO1 = MI.getOperand(CurOp++);
    unsigned Size = X86InstrInfo::sizeOfImm(Desc);
    if (MO1.isImmediate())
      emitConstant(MO1.getImm(), Size);
    else {
      unsigned rt = Is64BitMode ? X86::reloc_pcrel_word
        : (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
      if (Opcode == X86::MOV64ri)
        rt = X86::reloc_absolute_dword;  // FIXME: add X86II flag?
      if (MO1.isGlobalAddress()) {
        bool NeedStub = isa<Function>(MO1.getGlobal());
        bool isLazy = gvNeedsLazyPtr(MO1.getGlobal());
        emitGlobalAddress(MO1.getGlobal(), rt, MO1.getOffset(), 0,
                          NeedStub, isLazy);
      } else if (MO1.isExternalSymbol())
        emitExternalSymbolAddress(MO1.getSymbolName(), rt);
      else if (MO1.isConstantPoolIndex())
        emitConstPoolAddress(MO1.getIndex(), rt);
      else if (MO1.isJumpTableIndex())
        emitJumpTableAddress(MO1.getIndex(), rt);
    }
  }
  break;

In the previous example, XXXCodeEmitter.cpp uses the variable rt, which is a RelocationType enum that may be used to relocate addresses (for example, a global address with a PIC base offset). The RelocationType enum for that target is defined in the short target-specific XXXRelocations.h file. The RelocationType is used by the relocate method defined in XXXJITInfo.cpp to rewrite addresses for referenced global symbols.

For example, X86Relocations.h specifies the following relocation types for the X86 addresses. In all four cases, the relocated value is added to the value already in memory. For reloc_pcrel_word and reloc_picrel_word, there is an additional initial adjustment.

enum RelocationType {
  reloc_pcrel_word = 0,    // add reloc value after adjusting for the PC loc
  reloc_picrel_word = 1,   // add reloc value after adjusting for the PIC base
  reloc_absolute_word = 2, // absolute relocation; no additional adjustment
  reloc_absolute_dword = 3 // absolute relocation; no additional adjustment
};

Target JIT Info

XXXJITInfo.cpp implements the JIT interfaces for target-specific code-generation activities, such as emitting machine code and stubs. At minimum, a target-specific version of XXXJITInfo implements the following:

  • getLazyResolverFunction — Initializes the JIT, gives the target a function that is used for compilation.
  • emitFunctionStub — Returns a native function with a specified address for a callback function.
  • relocate — Changes the addresses of referenced globals, based on relocation types.
  • Callback function that are wrappers to a function stub that is used when the real target is not initially known.

getLazyResolverFunction is generally trivial to implement. It makes the incoming parameter as the global JITCompilerFunction and returns the callback function that will be used a function wrapper. For the Alpha target (in AlphaJITInfo.cpp), the getLazyResolverFunction implementation is simply:

TargetJITInfo::LazyResolverFn AlphaJITInfo::getLazyResolverFunction(
                                            JITCompilerFn F) {
  JITCompilerFunction = F;
  return AlphaCompilationCallback;
}

For the X86 target, the getLazyResolverFunction implementation is a little more complicated, because it returns a different callback function for processors with SSE instructions and XMM registers.

The callback function initially saves and later restores the callee register values, incoming arguments, and frame and return address. The callback function needs low-level access to the registers or stack, so it is typically implemented with assembler.