================================
Writing an LLVM Compiler Backend
================================

.. toctree::
   :hidden:

   HowToUseInstrMappings

.. contents::
   :local:

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 <LangRef.html>`_ --- a reference manual for
  the LLVM assembly language.

* :doc:`CodeGenerator` --- 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.

* :doc:`TableGenFundamentals` --- 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.

* :doc:`WritingAnLLVMPass` --- 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 <http://www.sparc.org/standards/V8.pdf>`_ for
reference.  For details about the ARM instruction set, refer to the `ARM
Architecture Reference Manual <http://infocenter.arm.com/>`_.  For more about
the GNU Assembler format (``GAS``), see `Using As
<http://sourceware.org/binutils/docs/as/index.html>`_, 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
:doc:`LLVM Target-Independent Code Generator <CodeGenerator>` 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 ``autotools/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.

.. code-block:: c++

  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()``

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:

.. code-block:: c++

  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:

.. code-block:: c++

  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:

.. code-block:: c++

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

For more information, see "`llvm/Target/TargetRegistry.h
</doxygen/TargetRegistry_8h-source.html>`_".

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.

.. code-block:: llvm

  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:

.. code-block:: llvm

  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:

.. code-block:: c++

  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:

.. code-block:: c++

  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:

.. code-block:: llvm

  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).

.. code-block:: llvm

  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:

.. code-block:: llvm

  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:

.. code-block:: llvm

  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``).

.. code-block:: llvm

  // 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.

.. code-block:: c++

  // 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
:ref:`TargetRegisterInfo`).  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:

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.

.. code-block:: llvm

  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 (``11``\ :sub:`2`), is the operation value for this
category of operation.  The second parameter (``000000``\ :sub:`2`) 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``).

.. code-block:: llvm

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

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

.. code-block:: llvm

  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 :doc:`CodeGenerator`.
This parameter is detailed in the next section, :ref:`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:

.. code-block:: llvm

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

.. code-block:: llvm

  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, $dst"),
                   [(set IntRegs:$dst, (OpNode IntRegs:$b, IntRegs:$c))]>;
    def ri  : F3_2 <2, Op3Val,
                   (outs IntRegs:$dst), (ins IntRegs:$b, i32imm:$c),
                   !strconcat(OpcStr, " $b, $c, $dst"),
                   [(set IntRegs:$dst, (OpNode IntRegs:$b, simm13:$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``.

.. code-block:: llvm

  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 10\ :sup:`th` bit represents the "greater than" condition for
integers, and the 22\ :sup:`nd` bit represents the "greater than" condition for
floats.

.. code-block:: llvm

  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.

.. code-block:: llvm

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

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

.. code-block:: llvm

  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.

.. code-block:: llvm

  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.

.. code-block:: llvm

  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 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 :doc:`HowToUseInstrMappings`.

Implement a subclass of ``TargetInstrInfo``
-------------------------------------------

The final step is to hand code portions of ``XXXInstrInfo``, which implements
the interface described in ``TargetInstrInfo.h`` (see :ref:`TargetInstrInfo`).
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.

.. code-block:: c++

  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.

.. code-block:: c++

    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.

.. code-block:: c++

    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.

.. code-block:: c++

    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.

.. code-block:: c++

    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:

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
:ref:`SelectionDAG-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.

.. code-block:: llvm

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

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

.. code-block:: llvm

  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:

.. code-block:: llvm

  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.

.. code-block:: c++

  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.

.. code-block:: c++

  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 IntRegs: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:

.. code-block:: c++

  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.

.. code-block:: c++

  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``:

.. code-block:: c++

  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:

.. code-block:: c++

  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:

.. code-block:: c++

  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.

.. code-block:: c++

  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);
  }

Legal
^^^^^

The ``Legal`` ``LegalizeAction`` enum value simply indicates that an operation
**is** natively supported.  ``Legal`` represents the default condition, so it
is rarely used.  In ``SparcISelLowering.cpp``, the action for ``CTPOP`` (an
operation to count the bits set in an integer) is natively supported only for
SPARC v9.  The following code enables the ``Expand`` conversion technique for
non-v9 SPARC implementations.

.. code-block:: c++

  setOperationAction(ISD::CTPOP, MVT::i32, Expand);
  ...
  if (TM.getSubtarget<SparcSubtarget>().isV9())
    setOperationAction(ISD::CTPOP, MVT::i32, Legal);

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``.

.. code-block:: llvm

  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``.

.. code-block:: llvm

  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.)

.. code-block:: llvm

  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.

.. code-block:: llvm

  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.

.. code-block:: llvm

  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
  :ref:`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``:

.. code-block:: c++

  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``.

.. code-block:: c++

  #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.)

.. code-block:: llvm

  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.

.. code-block:: llvm

  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.

.. code-block:: llvm

  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:

.. code-block:: c++

  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:

.. code-block:: c++

  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.)

.. code-block:: c++

  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.

.. code-block:: c++

  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:

.. code-block:: c++

  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.

