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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
                      "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
  <meta http-equiv="content-type" content="text/html; charset=utf-8">
  <title>The LLVM Target-Independent Code Generator</title>
  <link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>

<div class="doc_title">
  The LLVM Target-Independent Code Generator
</div>

<ol>
  <li><a href="#introduction">Introduction</a>
    <ul>
      <li><a href="#required">Required components in the code generator</a></li>
      <li><a href="#high-level-design">The high-level design of the code
          generator</a></li>
      <li><a href="#tablegen">Using TableGen for target description</a></li>
    </ul>
  </li>
  <li><a href="#targetdesc">Target description classes</a>
    <ul>
      <li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
      <li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
      <li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
      <li><a href="#targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a></li>
      <li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
      <li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
      <li><a href="#targetsubtarget">The <tt>TargetSubtarget</tt> class</a></li>
      <li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
    </ul>
  </li>
  <li><a href="#codegendesc">Machine code description classes</a>
    <ul>
    <li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
    <li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
                                     class</a></li>
    <li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
    </ul>
  </li>
  <li><a href="#codegenalgs">Target-independent code generation algorithms</a>
    <ul>
    <li><a href="#instselect">Instruction Selection</a>
      <ul>
      <li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
      <li><a href="#selectiondag_process">SelectionDAG Code Generation
                                          Process</a></li>
      <li><a href="#selectiondag_build">Initial SelectionDAG
                                        Construction</a></li>
      <li><a href="#selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a></li>
      <li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
      <li><a href="#selectiondag_optimize">SelectionDAG Optimization
                                           Phase: the DAG Combiner</a></li>
      <li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
      <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
                                        Phase</a></li>
      <li><a href="#selectiondag_future">Future directions for the
                                         SelectionDAG</a></li>
      </ul></li>
     <li><a href="#liveintervals">Live Intervals</a>
       <ul>
       <li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
       <li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
       </ul></li>
    <li><a href="#regalloc">Register Allocation</a>
      <ul>
      <li><a href="#regAlloc_represent">How registers are represented in
                                        LLVM</a></li>
      <li><a href="#regAlloc_howTo">Mapping virtual registers to physical
                                    registers</a></li>
      <li><a href="#regAlloc_twoAddr">Handling two address instructions</a></li>
      <li><a href="#regAlloc_ssaDecon">The SSA deconstruction phase</a></li>
      <li><a href="#regAlloc_fold">Instruction folding</a></li>
      <li><a href="#regAlloc_builtIn">Built in register allocators</a></li>
      </ul></li>
    <li><a href="#codeemit">Code Emission</a>
        <ul>
        <li><a href="#codeemit_asm">Generating Assembly Code</a></li>
        <li><a href="#codeemit_bin">Generating Binary Machine Code</a></li>
        </ul></li>
    </ul>
  </li>
  <li><a href="#targetimpls">Target-specific Implementation Notes</a>
    <ul>
    <li><a href="#tailcallopt">Tail call optimization</a></li>
    <li><a href="#sibcallopt">Sibling call optimization</a></li>
    <li><a href="#x86">The X86 backend</a></li>
    <li><a href="#ppc">The PowerPC backend</a>
      <ul>
      <li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
      <li><a href="#ppc_frame">Frame Layout</a></li>
      <li><a href="#ppc_prolog">Prolog/Epilog</a></li>
      <li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
      </ul></li>
    </ul></li>

</ol>

<div class="doc_author">
  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
                <a href="mailto:isanbard@gmail.com">Bill Wendling</a>,
                <a href="mailto:pronesto@gmail.com">Fernando Magno Quintao
                                                    Pereira</a> and
                <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
</div>

<div class="doc_warning">
  <p>Warning: This is a work in progress.</p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>The LLVM target-independent code generator is a framework that provides a
   suite of reusable components for translating the LLVM internal representation
   to the machine code for a specified target&mdash;either in assembly form
   (suitable for a static compiler) or in binary machine code format (usable for
   a JIT compiler). The LLVM target-independent code generator consists of five
   main components:</p>

<ol>
  <li><a href="#targetdesc">Abstract target description</a> interfaces which
      capture important properties about various aspects of the machine,
      independently of how they will be used.  These interfaces are defined in
      <tt>include/llvm/Target/</tt>.</li>

  <li>Classes used to represent the <a href="#codegendesc">machine code</a>
      being generated for a target.  These classes are intended to be abstract
      enough to represent the machine code for <i>any</i> target machine.  These
      classes are defined in <tt>include/llvm/CodeGen/</tt>.</li>

  <li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
      various phases of native code generation (register allocation, scheduling,
      stack frame representation, etc).  This code lives
      in <tt>lib/CodeGen/</tt>.</li>

  <li><a href="#targetimpls">Implementations of the abstract target description
      interfaces</a> for particular targets.  These machine descriptions make
      use of the components provided by LLVM, and can optionally provide custom
      target-specific passes, to build complete code generators for a specific
      target.  Target descriptions live in <tt>lib/Target/</tt>.</li>

  <li><a href="#jit">The target-independent JIT components</a>.  The LLVM JIT is
      completely target independent (it uses the <tt>TargetJITInfo</tt>
      structure to interface for target-specific issues.  The code for the
      target-independent JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
</ol>

<p>Depending on which part of the code generator you are interested in working
   on, different pieces of this will be useful to you.  In any case, you should
   be familiar with the <a href="#targetdesc">target description</a>
   and <a href="#codegendesc">machine code representation</a> classes.  If you
   want to add a backend for a new target, you will need
   to <a href="#targetimpls">implement the target description</a> classes for
   your new target and understand the <a href="LangRef.html">LLVM code
   representation</a>.  If you are interested in implementing a
   new <a href="#codegenalgs">code generation algorithm</a>, it should only
   depend on the target-description and machine code representation classes,
   ensuring that it is portable.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
 <a name="required">Required components in the code generator</a>
</div>

<div class="doc_text">

<p>The two pieces of the LLVM code generator are the high-level interface to the
   code generator and the set of reusable components that can be used to build
   target-specific backends.  The two most important interfaces
   (<a href="#targetmachine"><tt>TargetMachine</tt></a>
   and <a href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
   required to be defined for a backend to fit into the LLVM system, but the
   others must be defined if the reusable code generator components are going to
   be used.</p>

<p>This design has two important implications.  The first is that LLVM can
   support completely non-traditional code generation targets.  For example, the
   C backend does not require register allocation, instruction selection, or any
   of the other standard components provided by the system.  As such, it only
   implements these two interfaces, and does its own thing.  Another example of
   a code generator like this is a (purely hypothetical) backend that converts
   LLVM to the GCC RTL form and uses GCC to emit machine code for a target.</p>

<p>This design also implies that it is possible to design and implement
   radically different code generators in the LLVM system that do not make use
   of any of the built-in components.  Doing so is not recommended at all, but
   could be required for radically different targets that do not fit into the
   LLVM machine description model: FPGAs for example.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
 <a name="high-level-design">The high-level design of the code generator</a>
</div>

<div class="doc_text">

<p>The LLVM target-independent code generator is designed to support efficient
   and quality code generation for standard register-based microprocessors.
   Code generation in this model is divided into the following stages:</p>

<ol>
  <li><b><a href="#instselect">Instruction Selection</a></b> &mdash; This phase
      determines an efficient way to express the input LLVM code in the target
      instruction set.  This stage produces the initial code for the program in
      the target instruction set, then makes use of virtual registers in SSA
      form and physical registers that represent any required register
      assignments due to target constraints or calling conventions.  This step
      turns the LLVM code into a DAG of target instructions.</li>

  <li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> &mdash;
      This phase takes the DAG of target instructions produced by the
      instruction selection phase, determines an ordering of the instructions,
      then emits the instructions
      as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
      Note that we describe this in the <a href="#instselect">instruction
      selection section</a> because it operates on
      a <a href="#selectiondag_intro">SelectionDAG</a>.</li>

  <li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> &mdash;
      This optional stage consists of a series of machine-code optimizations
      that operate on the SSA-form produced by the instruction selector.
      Optimizations like modulo-scheduling or peephole optimization work
      here.</li>

  <li><b><a href="#regalloc">Register Allocation</a></b> &mdash; The target code
      is transformed from an infinite virtual register file in SSA form to the
      concrete register file used by the target.  This phase introduces spill
      code and eliminates all virtual register references from the program.</li>

  <li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> &mdash; Once
      the machine code has been generated for the function and the amount of
      stack space required is known (used for LLVM alloca's and spill slots),
      the prolog and epilog code for the function can be inserted and "abstract
      stack location references" can be eliminated.  This stage is responsible
      for implementing optimizations like frame-pointer elimination and stack
      packing.</li>

  <li><b><a href="#latemco">Late Machine Code Optimizations</a></b> &mdash;
      Optimizations that operate on "final" machine code can go here, such as
      spill code scheduling and peephole optimizations.</li>

  <li><b><a href="#codeemit">Code Emission</a></b> &mdash; The final stage
      actually puts out the code for the current function, either in the target
      assembler format or in machine code.</li>
</ol>

<p>The code generator is based on the assumption that the instruction selector
   will use an optimal pattern matching selector to create high-quality
   sequences of native instructions.  Alternative code generator designs based
   on pattern expansion and aggressive iterative peephole optimization are much
   slower.  This design permits efficient compilation (important for JIT
   environments) and aggressive optimization (used when generating code offline)
   by allowing components of varying levels of sophistication to be used for any
   step of compilation.</p>

<p>In addition to these stages, target implementations can insert arbitrary
   target-specific passes into the flow.  For example, the X86 target uses a
   special pass to handle the 80x87 floating point stack architecture.  Other
   targets with unusual requirements can be supported with custom passes as
   needed.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
 <a name="tablegen">Using TableGen for target description</a>
</div>

<div class="doc_text">

<p>The target description classes require a detailed description of the target
   architecture.  These target descriptions often have a large amount of common
   information (e.g., an <tt>add</tt> instruction is almost identical to a
   <tt>sub</tt> instruction).  In order to allow the maximum amount of
   commonality to be factored out, the LLVM code generator uses
   the <a href="TableGenFundamentals.html">TableGen</a> tool to describe big
   chunks of the target machine, which allows the use of domain-specific and
   target-specific abstractions to reduce the amount of repetition.</p>

<p>As LLVM continues to be developed and refined, we plan to move more and more
   of the target description to the <tt>.td</tt> form.  Doing so gives us a
   number of advantages.  The most important is that it makes it easier to port
   LLVM because it reduces the amount of C++ code that has to be written, and
   the surface area of the code generator that needs to be understood before
   someone can get something working.  Second, it makes it easier to change
   things. In particular, if tables and other things are all emitted
   by <tt>tblgen</tt>, we only need a change in one place (<tt>tblgen</tt>) to
   update all of the targets to a new interface.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="targetdesc">Target description classes</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>The LLVM target description classes (located in the
   <tt>include/llvm/Target</tt> directory) provide an abstract description of
   the target machine independent of any particular client.  These classes are
   designed to capture the <i>abstract</i> properties of the target (such as the
   instructions and registers it has), and do not incorporate any particular
   pieces of code generation algorithms.</p>

<p>All of the target description classes (except the
   <tt><a href="#targetdata">TargetData</a></tt> class) are designed to be
   subclassed by the concrete target implementation, and have virtual methods
   implemented.  To get to these implementations, the
   <tt><a href="#targetmachine">TargetMachine</a></tt> class provides accessors
   that should be implemented by the target.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetmachine">The <tt>TargetMachine</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
   access the target-specific implementations of the various target description
   classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
   <tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.).  This class is
   designed to be specialized by a concrete target implementation
   (e.g., <tt>X86TargetMachine</tt>) which implements the various virtual
   methods.  The only required target description class is
   the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the code
   generator components are to be used, the other interfaces should be
   implemented as well.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetdata">The <tt>TargetData</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetData</tt> class is the only required target description class,
   and it is the only class that is not extensible (you cannot derived a new
   class from it).  <tt>TargetData</tt> specifies information about how the
   target lays out memory for structures, the alignment requirements for various
   data types, the size of pointers in the target, and whether the target is
   little-endian or big-endian.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetlowering">The <tt>TargetLowering</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
   selectors primarily to describe how LLVM code should be lowered to
   SelectionDAG operations.  Among other things, this class indicates:</p>

<ul>
  <li>an initial register class to use for various <tt>ValueType</tt>s,</li>

  <li>which operations are natively supported by the target machine,</li>

  <li>the return type of <tt>setcc</tt> operations,</li>

  <li>the type to use for shift amounts, and</li>

  <li>various high-level characteristics, like whether it is profitable to turn
      division by a constant into a multiplication sequence</li>
</ul>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetRegisterInfo</tt> class is used to describe the register file
   of the target and any interactions between the registers.</p>

<p>Registers in the code generator are represented in the code generator by
   unsigned integers.  Physical registers (those that actually exist in the
   target description) are unique small numbers, and virtual registers are
   generally large.  Note that register #0 is reserved as a flag value.</p>

<p>Each register in the processor description has an associated
   <tt>TargetRegisterDesc</tt> entry, which provides a textual name for the
   register (used for assembly output and debugging dumps) and a set of aliases
   (used to indicate whether one register overlaps with another).</p>

<p>In addition to the per-register description, the <tt>TargetRegisterInfo</tt>
   class exposes a set of processor specific register classes (instances of the
   <tt>TargetRegisterClass</tt> class).  Each register class contains sets of
   registers that have the same properties (for example, they are all 32-bit
   integer registers).  Each SSA virtual register created by the instruction
   selector has an associated register class.  When the register allocator runs,
   it replaces virtual registers with a physical register in the set.</p>

<p>The target-specific implementations of these classes is auto-generated from
   a <a href="TableGenFundamentals.html">TableGen</a> description of the
   register file.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
   instructions supported by the target. It is essentially an array of
   <tt>TargetInstrDescriptor</tt> objects, each of which describes one
   instruction the target supports. Descriptors define things like the mnemonic
   for the opcode, the number of operands, the list of implicit register uses
   and defs, whether the instruction has certain target-independent properties
   (accesses memory, is commutable, etc), and holds any target-specific
   flags.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
   stack frame layout of the target. It holds the direction of stack growth, the
   known stack alignment on entry to each function, and the offset to the local
   area.  The offset to the local area is the offset from the stack pointer on
   function entry to the first location where function data (local variables,
   spill locations) can be stored.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
   specific chip set being targeted.  A sub-target informs code generation of
   which instructions are supported, instruction latencies and instruction
   execution itinerary; i.e., which processing units are used, in what order,
   and for how long.</p>

</div>


<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>TargetJITInfo</tt> class exposes an abstract interface used by the
   Just-In-Time code generator to perform target-specific activities, such as
   emitting stubs.  If a <tt>TargetMachine</tt> supports JIT code generation, it
   should provide one of these objects through the <tt>getJITInfo</tt>
   method.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="codegendesc">Machine code description classes</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>At the high-level, LLVM code is translated to a machine specific
   representation formed out of
   <a href="#machinefunction"><tt>MachineFunction</tt></a>,
   <a href="#machinebasicblock"><tt>MachineBasicBlock</tt></a>,
   and <a href="#machineinstr"><tt>MachineInstr</tt></a> instances (defined
   in <tt>include/llvm/CodeGen</tt>).  This representation is completely target
   agnostic, representing instructions in their most abstract form: an opcode
   and a series of operands.  This representation is designed to support both an
   SSA representation for machine code, as well as a register allocated, non-SSA
   form.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="machineinstr">The <tt>MachineInstr</tt> class</a>
</div>

<div class="doc_text">

<p>Target machine instructions are represented as instances of the
   <tt>MachineInstr</tt> class.  This class is an extremely abstract way of
   representing machine instructions.  In particular, it only keeps track of an
   opcode number and a set of operands.</p>

<p>The opcode number is a simple unsigned integer that only has meaning to a
   specific backend.  All of the instructions for a target should be defined in
   the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values are
   auto-generated from this description.  The <tt>MachineInstr</tt> class does
   not have any information about how to interpret the instruction (i.e., what
   the semantics of the instruction are); for that you must refer to the
   <tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p> 

<p>The operands of a machine instruction can be of several different types: a
   register reference, a constant integer, a basic block reference, etc.  In
   addition, a machine operand should be marked as a def or a use of the value
   (though only registers are allowed to be defs).</p>

<p>By convention, the LLVM code generator orders instruction operands so that
   all register definitions come before the register uses, even on architectures
   that are normally printed in other orders.  For example, the SPARC add
   instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
   and stores the result into the "%i3" register.  In the LLVM code generator,
   the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the
   destination first.</p>

<p>Keeping destination (definition) operands at the beginning of the operand
   list has several advantages.  In particular, the debugging printer will print
   the instruction like this:</p>

<div class="doc_code">
<pre>
%r3 = add %i1, %i2
</pre>
</div>

<p>Also if the first operand is a def, it is easier to <a href="#buildmi">create
   instructions</a> whose only def is the first operand.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
</div>

<div class="doc_text">

<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
   located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file.  The
   <tt>BuildMI</tt> functions make it easy to build arbitrary machine
   instructions.  Usage of the <tt>BuildMI</tt> functions look like this:</p>

<div class="doc_code">
<pre>
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
// instruction.  The '1' specifies how many operands will be added.
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);

// Create the same instr, but insert it at the end of a basic block.
MachineBasicBlock &amp;MBB = ...
BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);

// Create the same instr, but insert it before a specified iterator point.
MachineBasicBlock::iterator MBBI = ...
BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);

// Create a 'cmp Reg, 0' instruction, no destination reg.
MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
// Create an 'sahf' instruction which takes no operands and stores nothing.
MI = BuildMI(X86::SAHF, 0);

// Create a self looping branch instruction.
BuildMI(MBB, X86::JNE, 1).addMBB(&amp;MBB);
</pre>
</div>

<p>The key thing to remember with the <tt>BuildMI</tt> functions is that you
   have to specify the number of operands that the machine instruction will
   take.  This allows for efficient memory allocation.  You also need to specify
   if operands default to be uses of values, not definitions.  If you need to
   add a definition operand (other than the optional destination register), you
   must explicitly mark it as such:</p>

<div class="doc_code">
<pre>
MI.addReg(Reg, RegState::Define);
</pre>
</div>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="fixedregs">Fixed (preassigned) registers</a>
</div>

<div class="doc_text">

<p>One important issue that the code generator needs to be aware of is the
   presence of fixed registers.  In particular, there are often places in the
   instruction stream where the register allocator <em>must</em> arrange for a
   particular value to be in a particular register.  This can occur due to
   limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
   with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like
   calling conventions.  In any case, the instruction selector should emit code
   that copies a virtual register into or out of a physical register when
   needed.</p>

<p>For example, consider this simple LLVM example:</p>

<div class="doc_code">
<pre>
define i32 @test(i32 %X, i32 %Y) {
  %Z = udiv i32 %X, %Y
  ret i32 %Z
}
</pre>
</div>

<p>The X86 instruction selector produces this machine code for the <tt>div</tt>
   and <tt>ret</tt> (use "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to
   get this):</p>

<div class="doc_code">
<pre>
;; Start of div
%EAX = mov %reg1024           ;; Copy X (in reg1024) into EAX
%reg1027 = sar %reg1024, 31
%EDX = mov %reg1027           ;; Sign extend X into EDX
idiv %reg1025                 ;; Divide by Y (in reg1025)
%reg1026 = mov %EAX           ;; Read the result (Z) out of EAX

;; Start of ret
%EAX = mov %reg1026           ;; 32-bit return value goes in EAX
ret
</pre>
</div>

<p>By the end of code generation, the register allocator has coalesced the
   registers and deleted the resultant identity moves producing the following
   code:</p>

<div class="doc_code">
<pre>
;; X is in EAX, Y is in ECX
mov %EAX, %EDX
sar %EDX, 31
idiv %ECX
ret 
</pre>
</div>

<p>This approach is extremely general (if it can handle the X86 architecture, it
   can handle anything!) and allows all of the target specific knowledge about
   the instruction stream to be isolated in the instruction selector.  Note that
   physical registers should have a short lifetime for good code generation, and
   all physical registers are assumed dead on entry to and exit from basic
   blocks (before register allocation).  Thus, if you need a value to be live
   across basic block boundaries, it <em>must</em> live in a virtual
   register.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="ssa">Machine code in SSA form</a>
</div>

<div class="doc_text">

<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and are
   maintained in SSA-form until register allocation happens.  For the most part,
   this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes
   become machine code PHI nodes, and virtual registers are only allowed to have
   a single definition.</p>

<p>After register allocation, machine code is no longer in SSA-form because
   there are no virtual registers left in the code.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
   (<tt><a href="#machineinstr">MachineInstr</a></tt> instances).  It roughly
   corresponds to the LLVM code input to the instruction selector, but there can
   be a one-to-many mapping (i.e. one LLVM basic block can map to multiple
   machine basic blocks). The <tt>MachineBasicBlock</tt> class has a
   "<tt>getBasicBlock</tt>" method, which returns the LLVM basic block that it
   comes from.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="machinefunction">The <tt>MachineFunction</tt> class</a>
</div>

<div class="doc_text">

<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
   (<tt><a href="#machinebasicblock">MachineBasicBlock</a></tt> instances).  It
   corresponds one-to-one with the LLVM function input to the instruction
   selector.  In addition to a list of basic blocks,
   the <tt>MachineFunction</tt> contains a a <tt>MachineConstantPool</tt>,
   a <tt>MachineFrameInfo</tt>, a <tt>MachineFunctionInfo</tt>, and a
   <tt>MachineRegisterInfo</tt>.  See
   <tt>include/llvm/CodeGen/MachineFunction.h</tt> for more information.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="codegenalgs">Target-independent code generation algorithms</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>This section documents the phases described in the
   <a href="#high-level-design">high-level design of the code generator</a>.
   It explains how they work and some of the rationale behind their design.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="instselect">Instruction Selection</a>
</div>

<div class="doc_text">

<p>Instruction Selection is the process of translating LLVM code presented to
   the code generator into target-specific machine instructions.  There are
   several well-known ways to do this in the literature.  LLVM uses a
   SelectionDAG based instruction selector.</p>

<p>Portions of the DAG instruction selector are generated from the target
   description (<tt>*.td</tt>) files.  Our goal is for the entire instruction
   selector to be generated from these <tt>.td</tt> files, though currently
   there are still things that require custom C++ code.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_intro">Introduction to SelectionDAGs</a>
</div>

<div class="doc_text">

<p>The SelectionDAG provides an abstraction for code representation in a way
   that is amenable to instruction selection using automatic techniques
   (e.g. dynamic-programming based optimal pattern matching selectors). It is
   also well-suited to other phases of code generation; in particular,
   instruction scheduling (SelectionDAG's are very close to scheduling DAGs
   post-selection).  Additionally, the SelectionDAG provides a host
   representation where a large variety of very-low-level (but
   target-independent) <a href="#selectiondag_optimize">optimizations</a> may be
   performed; ones which require extensive information about the instructions
   efficiently supported by the target.</p>

<p>The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
   <tt>SDNode</tt> class.  The primary payload of the <tt>SDNode</tt> is its
   operation code (Opcode) that indicates what operation the node performs and
   the operands to the operation.  The various operation node types are
   described at the top of the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
   file.</p>

<p>Although most operations define a single value, each node in the graph may
   define multiple values.  For example, a combined div/rem operation will
   define both the dividend and the remainder. Many other situations require
   multiple values as well.  Each node also has some number of operands, which
   are edges to the node defining the used value.  Because nodes may define
   multiple values, edges are represented by instances of the <tt>SDValue</tt>
   class, which is a <tt>&lt;SDNode, unsigned&gt;</tt> pair, indicating the node
   and result value being used, respectively.  Each value produced by
   an <tt>SDNode</tt> has an associated <tt>MVT</tt> (Machine Value Type)
   indicating what the type of the value is.</p>

<p>SelectionDAGs contain two different kinds of values: those that represent
   data flow and those that represent control flow dependencies.  Data values
   are simple edges with an integer or floating point value type.  Control edges
   are represented as "chain" edges which are of type <tt>MVT::Other</tt>.
   These edges provide an ordering between nodes that have side effects (such as
   loads, stores, calls, returns, etc).  All nodes that have side effects should
   take a token chain as input and produce a new one as output.  By convention,
   token chain inputs are always operand #0, and chain results are always the
   last value produced by an operation.</p>

<p>A SelectionDAG has designated "Entry" and "Root" nodes.  The Entry node is
   always a marker node with an Opcode of <tt>ISD::EntryToken</tt>.  The Root
   node is the final side-effecting node in the token chain. For example, in a
   single basic block function it would be the return node.</p>

<p>One important concept for SelectionDAGs is the notion of a "legal" vs.
   "illegal" DAG.  A legal DAG for a target is one that only uses supported
   operations and supported types.  On a 32-bit PowerPC, for example, a DAG with
   a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that
   uses a SREM or UREM operation.  The
   <a href="#selectinodag_legalize_types">legalize types</a> and
   <a href="#selectiondag_legalize">legalize operations</a> phases are
   responsible for turning an illegal DAG into a legal DAG.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
</div>

<div class="doc_text">

<p>SelectionDAG-based instruction selection consists of the following steps:</p>

<ol>
  <li><a href="#selectiondag_build">Build initial DAG</a> &mdash; This stage
      performs a simple translation from the input LLVM code to an illegal
      SelectionDAG.</li>

  <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; This
      stage performs simple optimizations on the SelectionDAG to simplify it,
      and recognize meta instructions (like rotates
      and <tt>div</tt>/<tt>rem</tt> pairs) for targets that support these meta
      operations.  This makes the resultant code more efficient and
      the <a href="#selectiondag_select">select instructions from DAG</a> phase
      (below) simpler.</li>

  <li><a href="#selectiondag_legalize_types">Legalize SelectionDAG Types</a>
      &mdash; This stage transforms SelectionDAG nodes to eliminate any types
      that are unsupported on the target.</li>

  <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
      SelectionDAG optimizer is run to clean up redundancies exposed by type
      legalization.</li>

  <li><a href="#selectiondag_legalize">Legalize SelectionDAG Types</a> &mdash;
      This stage transforms SelectionDAG nodes to eliminate any types that are
      unsupported on the target.</li>

  <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
      SelectionDAG optimizer is run to eliminate inefficiencies introduced by
      operation legalization.</li>

  <li><a href="#selectiondag_select">Select instructions from DAG</a> &mdash;
      Finally, the target instruction selector matches the DAG operations to
      target instructions.  This process translates the target-independent input
      DAG into another DAG of target instructions.</li>

  <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
      &mdash; The last phase assigns a linear order to the instructions in the
      target-instruction DAG and emits them into the MachineFunction being
      compiled.  This step uses traditional prepass scheduling techniques.</li>
</ol>

<p>After all of these steps are complete, the SelectionDAG is destroyed and the
   rest of the code generation passes are run.</p>

<p>One great way to visualize what is going on here is to take advantage of a
   few LLC command line options.  The following options pop up a window
   displaying the SelectionDAG at specific times (if you only get errors printed
   to the console while using this, you probably
   <a href="ProgrammersManual.html#ViewGraph">need to configure your system</a>
   to add support for it).</p>

<ul>
  <li><tt>-view-dag-combine1-dags</tt> displays the DAG after being built,
      before the first optimization pass.</li>

  <li><tt>-view-legalize-dags</tt> displays the DAG before Legalization.</li>

  <li><tt>-view-dag-combine2-dags</tt> displays the DAG before the second
      optimization pass.</li>

  <li><tt>-view-isel-dags</tt> displays the DAG before the Select phase.</li>

  <li><tt>-view-sched-dags</tt> displays the DAG before Scheduling.</li>
</ul>

<p>The <tt>-view-sunit-dags</tt> displays the Scheduler's dependency graph.
   This graph is based on the final SelectionDAG, with nodes that must be
   scheduled together bundled into a single scheduling-unit node, and with
   immediate operands and other nodes that aren't relevant for scheduling
   omitted.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_build">Initial SelectionDAG Construction</a>
</div>

<div class="doc_text">

<p>The initial SelectionDAG is na&iuml;vely peephole expanded from the LLVM
   input by the <tt>SelectionDAGLowering</tt> class in the
   <tt>lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp</tt> file.  The intent of
   this pass is to expose as much low-level, target-specific details to the
   SelectionDAG as possible.  This pass is mostly hard-coded (e.g. an
   LLVM <tt>add</tt> turns into an <tt>SDNode add</tt> while a
   <tt>getelementptr</tt> is expanded into the obvious arithmetic). This pass
   requires target-specific hooks to lower calls, returns, varargs, etc.  For
   these features, the <tt><a href="#targetlowering">TargetLowering</a></tt>
   interface is used.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a>
</div>

<div class="doc_text">

<p>The Legalize phase is in charge of converting a DAG to only use the types
   that are natively supported by the target.</p>

<p>There are two main ways of converting values of unsupported scalar types to
   values of supported types: converting small types to larger types
   ("promoting"), and breaking up large integer types into smaller ones
   ("expanding").  For example, a target might require that all f32 values are
   promoted to f64 and that all i1/i8/i16 values are promoted to i32.  The same
   target might require that all i64 values be expanded into pairs of i32
   values.  These changes can insert sign and zero extensions as needed to make
   sure that the final code has the same behavior as the input.</p>

<p>There are two main ways of converting values of unsupported vector types to
   value of supported types: splitting vector types, multiple times if
   necessary, until a legal type is found, and extending vector types by adding
   elements to the end to round them out to legal types ("widening").  If a
   vector gets split all the way down to single-element parts with no supported
   vector type being found, the elements are converted to scalars
   ("scalarizing").</p>

<p>A target implementation tells the legalizer which types are supported (and
   which register class to use for them) by calling the
   <tt>addRegisterClass</tt> method in its TargetLowering constructor.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
</div>

<div class="doc_text">

<p>The Legalize phase is in charge of converting a DAG to only use the
   operations that are natively supported by the target.</p>

<p>Targets often have weird constraints, such as not supporting every operation
   on every supported datatype (e.g. X86 does not support byte conditional moves
   and PowerPC does not support sign-extending loads from a 16-bit memory
   location).  Legalize takes care of this by open-coding another sequence of
   operations to emulate the operation ("expansion"), by promoting one type to a
   larger type that supports the operation ("promotion"), or by using a
   target-specific hook to implement the legalization ("custom").</p>

<p>A target implementation tells the legalizer which operations are not
   supported (and which of the above three actions to take) by calling the
   <tt>setOperationAction</tt> method in its <tt>TargetLowering</tt>
   constructor.</p>

<p>Prior to the existence of the Legalize passes, we required that every target
   <a href="#selectiondag_optimize">selector</a> supported and handled every
   operator and type even if they are not natively supported.  The introduction
   of the Legalize phases allows all of the canonicalization patterns to be
   shared across targets, and makes it very easy to optimize the canonicalized
   code because it is still in the form of a DAG.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_optimize">SelectionDAG Optimization Phase: the DAG
  Combiner</a>
</div>

<div class="doc_text">

<p>The SelectionDAG optimization phase is run multiple times for code
   generation, immediately after the DAG is built and once after each
   legalization.  The first run of the pass allows the initial code to be
   cleaned up (e.g. performing optimizations that depend on knowing that the
   operators have restricted type inputs).  Subsequent runs of the pass clean up
   the messy code generated by the Legalize passes, which allows Legalize to be
   very simple (it can focus on making code legal instead of focusing on
   generating <em>good</em> and legal code).</p>

<p>One important class of optimizations performed is optimizing inserted sign
   and zero extension instructions.  We currently use ad-hoc techniques, but
   could move to more rigorous techniques in the future.  Here are some good
   papers on the subject:</p>

<p>"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
   integer arithmetic</a>"<br>
   Kevin Redwine and Norman Ramsey<br>
   International Conference on Compiler Construction (CC) 2004</p>

<p>"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
   sign extension elimination</a>"<br>
   Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
   Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
   and Implementation.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_select">SelectionDAG Select Phase</a>
</div>

<div class="doc_text">

<p>The Select phase is the bulk of the target-specific code for instruction
   selection.  This phase takes a legal SelectionDAG as input, pattern matches
   the instructions supported by the target to this DAG, and produces a new DAG
   of target code.  For example, consider the following LLVM fragment:</p>

<div class="doc_code">
<pre>
%t1 = fadd float %W, %X
%t2 = fmul float %t1, %Y
%t3 = fadd float %t2, %Z
</pre>
</div>

<p>This LLVM code corresponds to a SelectionDAG that looks basically like
   this:</p>

<div class="doc_code">
<pre>
(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
</pre>
</div>

<p>If a target supports floating point multiply-and-add (FMA) operations, one of
   the adds can be merged with the multiply.  On the PowerPC, for example, the
   output of the instruction selector might look like this DAG:</p>

<div class="doc_code">
<pre>
(FMADDS (FADDS W, X), Y, Z)
</pre>
</div>

<p>The <tt>FMADDS</tt> instruction is a ternary instruction that multiplies its
first two operands and adds the third (as single-precision floating-point
numbers).  The <tt>FADDS</tt> instruction is a simple binary single-precision
add instruction.  To perform this pattern match, the PowerPC backend includes
the following instruction definitions:</p>

<div class="doc_code">
<pre>
def FMADDS : AForm_1&lt;59, 29,
                    (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
                    "fmadds $FRT, $FRA, $FRC, $FRB",
                    [<b>(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
                                           F4RC:$FRB))</b>]&gt;;
def FADDS : AForm_2&lt;59, 21,
                    (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
                    "fadds $FRT, $FRA, $FRB",
                    [<b>(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))</b>]&gt;;
</pre>
</div>

<p>The portion of the instruction definition in bold indicates the pattern used
   to match the instruction.  The DAG operators
   (like <tt>fmul</tt>/<tt>fadd</tt>) are defined in
   the <tt>include/llvm/Target/TargetSelectionDAG.td</tt> file.  "
   <tt>F4RC</tt>" is the register class of the input and result values.</p>

<p>The TableGen DAG instruction selector generator reads the instruction
   patterns in the <tt>.td</tt> file and automatically builds parts of the
   pattern matching code for your target.  It has the following strengths:</p>

<ul>
  <li>At compiler-compiler time, it analyzes your instruction patterns and tells
      you if your patterns make sense or not.</li>

  <li>It can handle arbitrary constraints on operands for the pattern match.  In
      particular, it is straight-forward to say things like "match any immediate
      that is a 13-bit sign-extended value".  For examples, see the
      <tt>immSExt16</tt> and related <tt>tblgen</tt> classes in the PowerPC
      backend.</li>

  <li>It knows several important identities for the patterns defined.  For
      example, it knows that addition is commutative, so it allows the
      <tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
      well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
      to specially handle this case.</li>

  <li>It has a full-featured type-inferencing system.  In particular, you should
      rarely have to explicitly tell the system what type parts of your patterns
      are.  In the <tt>FMADDS</tt> case above, we didn't have to tell
      <tt>tblgen</tt> that all of the nodes in the pattern are of type 'f32'.
      It was able to infer and propagate this knowledge from the fact that
      <tt>F4RC</tt> has type 'f32'.</li>

  <li>Targets can define their own (and rely on built-in) "pattern fragments".
      Pattern fragments are chunks of reusable patterns that get inlined into
      your patterns during compiler-compiler time.  For example, the integer
      "<tt>(not x)</tt>" operation is actually defined as a pattern fragment
      that expands as "<tt>(xor x, -1)</tt>", since the SelectionDAG does not
      have a native '<tt>not</tt>' operation.  Targets can define their own
      short-hand fragments as they see fit.  See the definition of
      '<tt>not</tt>' and '<tt>ineg</tt>' for examples.</li>

  <li>In addition to instructions, targets can specify arbitrary patterns that
      map to one or more instructions using the 'Pat' class.  For example, the
      PowerPC has no way to load an arbitrary integer immediate into a register
      in one instruction. To tell tblgen how to do this, it defines:
      <br>
      <br>
<div class="doc_code">
<pre>
// Arbitrary immediate support.  Implement in terms of LIS/ORI.
def : Pat&lt;(i32 imm:$imm),
          (ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))&gt;;
</pre>
</div>
      <br>
      If none of the single-instruction patterns for loading an immediate into a
      register match, this will be used.  This rule says "match an arbitrary i32
      immediate, turning it into an <tt>ORI</tt> ('or a 16-bit immediate') and
      an <tt>LIS</tt> ('load 16-bit immediate, where the immediate is shifted to
      the left 16 bits') instruction".  To make this work, the
      <tt>LO16</tt>/<tt>HI16</tt> node transformations are used to manipulate
      the input immediate (in this case, take the high or low 16-bits of the
      immediate).</li>

  <li>While the system does automate a lot, it still allows you to write custom
      C++ code to match special cases if there is something that is hard to
      express.</li>
</ul>

<p>While it has many strengths, the system currently has some limitations,
   primarily because it is a work in progress and is not yet finished:</p>

<ul>
  <li>Overall, there is no way to define or match SelectionDAG nodes that define
      multiple values (e.g. <tt>SMUL_LOHI</tt>, <tt>LOAD</tt>, <tt>CALL</tt>,
      etc).  This is the biggest reason that you currently still <em>have
      to</em> write custom C++ code for your instruction selector.</li>

  <li>There is no great way to support matching complex addressing modes yet.
      In the future, we will extend pattern fragments to allow them to define
      multiple values (e.g. the four operands of the <a href="#x86_memory">X86
      addressing mode</a>, which are currently matched with custom C++ code).
      In addition, we'll extend fragments so that a fragment can match multiple
      different patterns.</li>

  <li>We don't automatically infer flags like isStore/isLoad yet.</li>

  <li>We don't automatically generate the set of supported registers and
      operations for the <a href="#selectiondag_legalize">Legalizer</a>
      yet.</li>

  <li>We don't have a way of tying in custom legalized nodes yet.</li>
</ul>

<p>Despite these limitations, the instruction selector generator is still quite
   useful for most of the binary and logical operations in typical instruction
   sets.  If you run into any problems or can't figure out how to do something,
   please let Chris know!</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
</div>

<div class="doc_text">

<p>The scheduling phase takes the DAG of target instructions from the selection
   phase and assigns an order.  The scheduler can pick an order depending on
   various constraints of the machines (i.e. order for minimal register pressure
   or try to cover instruction latencies).  Once an order is established, the
   DAG is converted to a list
   of <tt><a href="#machineinstr">MachineInstr</a></tt>s and the SelectionDAG is
   destroyed.</p>

<p>Note that this phase is logically separate from the instruction selection
   phase, but is tied to it closely in the code because it operates on
   SelectionDAGs.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="selectiondag_future">Future directions for the SelectionDAG</a>
</div>

<div class="doc_text">

<ol>
  <li>Optional function-at-a-time selection.</li>

  <li>Auto-generate entire selector from <tt>.td</tt> file.</li>
</ol>

</div>
 
<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="ssamco">SSA-based Machine Code Optimizations</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="liveintervals">Live Intervals</a>
</div>

<div class="doc_text">

<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
   They are used by some <a href="#regalloc">register allocator</a> passes to
   determine if two or more virtual registers which require the same physical
   register are live at the same point in the program (i.e., they conflict).
   When this situation occurs, one virtual register must be <i>spilled</i>.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="livevariable_analysis">Live Variable Analysis</a>
</div>

<div class="doc_text">

<p>The first step in determining the live intervals of variables is to calculate
   the set of registers that are immediately dead after the instruction (i.e.,
   the instruction calculates the value, but it is never used) and the set of
   registers that are used by the instruction, but are never used after the
   instruction (i.e., they are killed). Live variable information is computed
   for each <i>virtual</i> register and <i>register allocatable</i> physical
   register in the function.  This is done in a very efficient manner because it
   uses SSA to sparsely compute lifetime information for virtual registers
   (which are in SSA form) and only has to track physical registers within a
   block.  Before register allocation, LLVM can assume that physical registers
   are only live within a single basic block.  This allows it to do a single,
   local analysis to resolve physical register lifetimes within each basic
   block. If a physical register is not register allocatable (e.g., a stack
   pointer or condition codes), it is not tracked.</p>

<p>Physical registers may be live in to or out of a function. Live in values are
   typically arguments in registers. Live out values are typically return values
   in registers. Live in values are marked as such, and are given a dummy
   "defining" instruction during live intervals analysis. If the last basic
   block of a function is a <tt>return</tt>, then it's marked as using all live
   out values in the function.</p>

<p><tt>PHI</tt> nodes need to be handled specially, because the calculation of
   the live variable information from a depth first traversal of the CFG of the
   function won't guarantee that a virtual register used by the <tt>PHI</tt>
   node is defined before it's used. When a <tt>PHI</tt> node is encountered,
   only the definition is handled, because the uses will be handled in other
   basic blocks.</p>

<p>For each <tt>PHI</tt> node of the current basic block, we simulate an
   assignment at the end of the current basic block and traverse the successor
   basic blocks. If a successor basic block has a <tt>PHI</tt> node and one of
   the <tt>PHI</tt> node's operands is coming from the current basic block, then
   the variable is marked as <i>alive</i> within the current basic block and all
   of its predecessor basic blocks, until the basic block with the defining
   instruction is encountered.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="liveintervals_analysis">Live Intervals Analysis</a>
</div>

<div class="doc_text">

<p>We now have the information available to perform the live intervals analysis
   and build the live intervals themselves.  We start off by numbering the basic
   blocks and machine instructions.  We then handle the "live-in" values.  These
   are in physical registers, so the physical register is assumed to be killed
   by the end of the basic block.  Live intervals for virtual registers are
   computed for some ordering of the machine instructions <tt>[1, N]</tt>.  A
   live interval is an interval <tt>[i, j)</tt>, where <tt>1 &lt;= i &lt;= j
   &lt; N</tt>, for which a variable is live.</p>

<p><i><b>More to come...</b></i></p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="regalloc">Register Allocation</a>
</div>

<div class="doc_text">

<p>The <i>Register Allocation problem</i> consists in mapping a program
   <i>P<sub>v</sub></i>, that can use an unbounded number of virtual registers,
   to a program <i>P<sub>p</sub></i> that contains a finite (possibly small)
   number of physical registers. Each target architecture has a different number
   of physical registers. If the number of physical registers is not enough to
   accommodate all the virtual registers, some of them will have to be mapped
   into memory. These virtuals are called <i>spilled virtuals</i>.</p>

</div>

<!-- _______________________________________________________________________ -->

<div class="doc_subsubsection">
  <a name="regAlloc_represent">How registers are represented in LLVM</a>
</div>

<div class="doc_text">

<p>In LLVM, physical registers are denoted by integer numbers that normally
   range from 1 to 1023. To see how this numbering is defined for a particular
   architecture, you can read the <tt>GenRegisterNames.inc</tt> file for that
   architecture. For instance, by
   inspecting <tt>lib/Target/X86/X86GenRegisterNames.inc</tt> we see that the
   32-bit register <tt>EAX</tt> is denoted by 15, and the MMX register
   <tt>MM0</tt> is mapped to 48.</p>

<p>Some architectures contain registers that share the same physical location. A
   notable example is the X86 platform. For instance, in the X86 architecture,
   the registers <tt>EAX</tt>, <tt>AX</tt> and <tt>AL</tt> share the first eight
   bits. These physical registers are marked as <i>aliased</i> in LLVM. Given a
   particular architecture, you can check which registers are aliased by
   inspecting its <tt>RegisterInfo.td</tt> file. Moreover, the method
   <tt>TargetRegisterInfo::getAliasSet(p_reg)</tt> returns an array containing
   all the physical registers aliased to the register <tt>p_reg</tt>.</p>

<p>Physical registers, in LLVM, are grouped in <i>Register Classes</i>.
   Elements in the same register class are functionally equivalent, and can be
   interchangeably used. Each virtual register can only be mapped to physical
   registers of a particular class. For instance, in the X86 architecture, some
   virtuals can only be allocated to 8 bit registers.  A register class is
   described by <tt>TargetRegisterClass</tt> objects.  To discover if a virtual
   register is compatible with a given physical, this code can be used:</p>

<div class="doc_code">
<pre>
bool RegMapping_Fer::compatible_class(MachineFunction &amp;mf,
                                      unsigned v_reg,
                                      unsigned p_reg) {
  assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &amp;&amp;
         "Target register must be physical");
  const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
  return trc-&gt;contains(p_reg);
}
</pre>
</div>

<p>Sometimes, mostly for debugging purposes, it is useful to change the number
   of physical registers available in the target architecture. This must be done
   statically, inside the <tt>TargetRegsterInfo.td</tt> file. Just <tt>grep</tt>
   for <tt>RegisterClass</tt>, the last parameter of which is a list of
   registers. Just commenting some out is one simple way to avoid them being
   used. A more polite way is to explicitly exclude some registers from
   the <i>allocation order</i>. See the definition of the <tt>GR8</tt> register
   class in <tt>lib/Target/X86/X86RegisterInfo.td</tt> for an example of this.
   </p>

<p>Virtual registers are also denoted by integer numbers. Contrary to physical
   registers, different virtual registers never share the same number. The
   smallest virtual register is normally assigned the number 1024. This may
   change, so, in order to know which is the first virtual register, you should
   access <tt>TargetRegisterInfo::FirstVirtualRegister</tt>. Any register whose
   number is greater than or equal
   to <tt>TargetRegisterInfo::FirstVirtualRegister</tt> is considered a virtual
   register. Whereas physical registers are statically defined in
   a <tt>TargetRegisterInfo.td</tt> file and cannot be created by the
   application developer, that is not the case with virtual registers.  In order
   to create new virtual registers, use the
   method <tt>MachineRegisterInfo::createVirtualRegister()</tt>. This method
   will return a virtual register with the highest code.</p>

<p>Before register allocation, the operands of an instruction are mostly virtual
   registers, although physical registers may also be used. In order to check if
   a given machine operand is a register, use the boolean
   function <tt>MachineOperand::isRegister()</tt>. To obtain the integer code of
   a register, use <tt>MachineOperand::getReg()</tt>. An instruction may define
   or use a register. For instance, <tt>ADD reg:1026 := reg:1025 reg:1024</tt>
   defines the registers 1024, and uses registers 1025 and 1026. Given a
   register operand, the method <tt>MachineOperand::isUse()</tt> informs if that
   register is being used by the instruction. The
   method <tt>MachineOperand::isDef()</tt> informs if that registers is being
   defined.</p>

<p>We will call physical registers present in the LLVM bitcode before register
   allocation <i>pre-colored registers</i>. Pre-colored registers are used in
   many different situations, for instance, to pass parameters of functions
   calls, and to store results of particular instructions. There are two types
   of pre-colored registers: the ones <i>implicitly</i> defined, and
   those <i>explicitly</i> defined. Explicitly defined registers are normal
   operands, and can be accessed
   with <tt>MachineInstr::getOperand(int)::getReg()</tt>.  In order to check
   which registers are implicitly defined by an instruction, use
   the <tt>TargetInstrInfo::get(opcode)::ImplicitDefs</tt>,
   where <tt>opcode</tt> is the opcode of the target instruction. One important
   difference between explicit and implicit physical registers is that the
   latter are defined statically for each instruction, whereas the former may
   vary depending on the program being compiled. For example, an instruction
   that represents a function call will always implicitly define or use the same
   set of physical registers. To read the registers implicitly used by an
   instruction,
   use <tt>TargetInstrInfo::get(opcode)::ImplicitUses</tt>. Pre-colored
   registers impose constraints on any register allocation algorithm. The
   register allocator must make sure that none of them is been overwritten by
   the values of virtual registers while still alive.</p>

</div>

<!-- _______________________________________________________________________ -->

<div class="doc_subsubsection">
  <a name="regAlloc_howTo">Mapping virtual registers to physical registers</a>
</div>

<div class="doc_text">

<p>There are two ways to map virtual registers to physical registers (or to
   memory slots). The first way, that we will call <i>direct mapping</i>, is
   based on the use of methods of the classes <tt>TargetRegisterInfo</tt>,
   and <tt>MachineOperand</tt>. The second way, that we will call <i>indirect
   mapping</i>, relies on the <tt>VirtRegMap</tt> class in order to insert loads
   and stores sending and getting values to and from memory.</p>

<p>The direct mapping provides more flexibility to the developer of the register
   allocator; however, it is more error prone, and demands more implementation
   work.  Basically, the programmer will have to specify where load and store
   instructions should be inserted in the target function being compiled in
   order to get and store values in memory. To assign a physical register to a
   virtual register present in a given operand,
   use <tt>MachineOperand::setReg(p_reg)</tt>. To insert a store instruction,
   use <tt>TargetRegisterInfo::storeRegToStackSlot(...)</tt>, and to insert a
   load instruction, use <tt>TargetRegisterInfo::loadRegFromStackSlot</tt>.</p>

<p>The indirect mapping shields the application developer from the complexities
   of inserting load and store instructions. In order to map a virtual register
   to a physical one, use <tt>VirtRegMap::assignVirt2Phys(vreg, preg)</tt>.  In
   order to map a certain virtual register to memory,
   use <tt>VirtRegMap::assignVirt2StackSlot(vreg)</tt>. This method will return
   the stack slot where <tt>vreg</tt>'s value will be located.  If it is
   necessary to map another virtual register to the same stack slot,
   use <tt>VirtRegMap::assignVirt2StackSlot(vreg, stack_location)</tt>. One
   important point to consider when using the indirect mapping, is that even if
   a virtual register is mapped to memory, it still needs to be mapped to a
   physical register. This physical register is the location where the virtual
   register is supposed to be found before being stored or after being
   reloaded.</p>

<p>If the indirect strategy is used, after all the virtual registers have been
   mapped to physical registers or stack slots, it is necessary to use a spiller
   object to place load and store instructions in the code. Every virtual that
   has been mapped to a stack slot will be stored to memory after been defined
   and will be loaded before being used. The implementation of the spiller tries
   to recycle load/store instructions, avoiding unnecessary instructions. For an
   example of how to invoke the spiller,
   see <tt>RegAllocLinearScan::runOnMachineFunction</tt>
   in <tt>lib/CodeGen/RegAllocLinearScan.cpp</tt>.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="regAlloc_twoAddr">Handling two address instructions</a>
</div>

<div class="doc_text">

<p>With very rare exceptions (e.g., function calls), the LLVM machine code
   instructions are three address instructions. That is, each instruction is
   expected to define at most one register, and to use at most two registers.
   However, some architectures use two address instructions. In this case, the
   defined register is also one of the used register. For instance, an
   instruction such as <tt>ADD %EAX, %EBX</tt>, in X86 is actually equivalent
   to <tt>%EAX = %EAX + %EBX</tt>.</p>

<p>In order to produce correct code, LLVM must convert three address
   instructions that represent two address instructions into true two address
   instructions. LLVM provides the pass <tt>TwoAddressInstructionPass</tt> for
   this specific purpose. It must be run before register allocation takes
   place. After its execution, the resulting code may no longer be in SSA
   form. This happens, for instance, in situations where an instruction such
   as <tt>%a = ADD %b %c</tt> is converted to two instructions such as:</p>

<div class="doc_code">
<pre>
%a = MOVE %b
%a = ADD %a %c
</pre>
</div>

<p>Notice that, internally, the second instruction is represented as
   <tt>ADD %a[def/use] %c</tt>. I.e., the register operand <tt>%a</tt> is both
   used and defined by the instruction.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="regAlloc_ssaDecon">The SSA deconstruction phase</a>
</div>

<div class="doc_text">

<p>An important transformation that happens during register allocation is called
   the <i>SSA Deconstruction Phase</i>. The SSA form simplifies many analyses
   that are performed on the control flow graph of programs. However,
   traditional instruction sets do not implement PHI instructions. Thus, in
   order to generate executable code, compilers must replace PHI instructions
   with other instructions that preserve their semantics.</p>

<p>There are many ways in which PHI instructions can safely be removed from the
   target code. The most traditional PHI deconstruction algorithm replaces PHI
   instructions with copy instructions. That is the strategy adopted by
   LLVM. The SSA deconstruction algorithm is implemented
   in <tt>lib/CodeGen/PHIElimination.cpp</tt>. In order to invoke this pass, the
   identifier <tt>PHIEliminationID</tt> must be marked as required in the code
   of the register allocator.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="regAlloc_fold">Instruction folding</a>
</div>

<div class="doc_text">

<p><i>Instruction folding</i> is an optimization performed during register
   allocation that removes unnecessary copy instructions. For instance, a
   sequence of instructions such as:</p>

<div class="doc_code">
<pre>
%EBX = LOAD %mem_address
%EAX = COPY %EBX
</pre>
</div>

<p>can be safely substituted by the single instruction:</p>

<div class="doc_code">
<pre>
%EAX = LOAD %mem_address
</pre>
</div>

<p>Instructions can be folded with
   the <tt>TargetRegisterInfo::foldMemoryOperand(...)</tt> method. Care must be
   taken when folding instructions; a folded instruction can be quite different
   from the original
   instruction. See <tt>LiveIntervals::addIntervalsForSpills</tt>
   in <tt>lib/CodeGen/LiveIntervalAnalysis.cpp</tt> for an example of its
   use.</p>

</div>

<!-- _______________________________________________________________________ -->

<div class="doc_subsubsection">
  <a name="regAlloc_builtIn">Built in register allocators</a>
</div>

<div class="doc_text">

<p>The LLVM infrastructure provides the application developer with three
   different register allocators:</p>

<ul>
  <li><i>Simple</i> &mdash; This is a very simple implementation that does not
      keep values in registers across instructions. This register allocator
      immediately spills every value right after it is computed, and reloads all
      used operands from memory to temporary registers before each
      instruction.</li>

  <li><i>Local</i> &mdash; This register allocator is an improvement on the
      <i>Simple</i> implementation. It allocates registers on a basic block
      level, attempting to keep values in registers and reusing registers as
      appropriate.</li>

  <li><i>Linear Scan</i> &mdash; <i>The default allocator</i>. This is the
      well-know linear scan register allocator. Whereas the
      <i>Simple</i> and <i>Local</i> algorithms use a direct mapping
      implementation technique, the <i>Linear Scan</i> implementation
      uses a spiller in order to place load and stores.</li>
</ul>

<p>The type of register allocator used in <tt>llc</tt> can be chosen with the
   command line option <tt>-regalloc=...</tt>:</p>

<div class="doc_code">
<pre>
$ llc -regalloc=simple file.bc -o sp.s;
$ llc -regalloc=local file.bc -o lc.s;
$ llc -regalloc=linearscan file.bc -o ln.s;
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="proepicode">Prolog/Epilog Code Insertion</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="latemco">Late Machine Code Optimizations</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="codeemit">Code Emission</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="codeemit_asm">Generating Assembly Code</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="codeemit_bin">Generating Binary Machine Code</a>
</div>

<div class="doc_text">
   <p>For the JIT or <tt>.o</tt> file writer</p>
</div>


<!-- *********************************************************************** -->
<div class="doc_section">
  <a name="targetimpls">Target-specific Implementation Notes</a>
</div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>This section of the document explains features or design decisions that are
   specific to the code generator for a particular target.</p>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="tailcallopt">Tail call optimization</a>
</div>

<div class="doc_text">

<p>Tail call optimization, callee reusing the stack of the caller, is currently
   supported on x86/x86-64 and PowerPC. It is performed if:</p>

<ul>
  <li>Caller and callee have the calling convention <tt>fastcc</tt> or
       <tt>cc 10</tt> (GHC call convention).</li>

  <li>The call is a tail call - in tail position (ret immediately follows call
      and ret uses value of call or is void).</li>

  <li>Option <tt>-tailcallopt</tt> is enabled.</li>

  <li>Platform specific constraints are met.</li>
</ul>

<p>x86/x86-64 constraints:</p>

<ul>
  <li>No variable argument lists are used.</li>

  <li>On x86-64 when generating GOT/PIC code only module-local calls (visibility
  = hidden or protected) are supported.</li>
</ul>

<p>PowerPC constraints:</p>

<ul>
  <li>No variable argument lists are used.</li>

  <li>No byval parameters are used.</li>

  <li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
</ul>

<p>Example:</p>

<p>Call as <tt>llc -tailcallopt test.ll</tt>.</p>

<div class="doc_code">
<pre>
declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)

define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
  %l1 = add i32 %in1, %in2
  %tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
  ret i32 %tmp
}
</pre>
</div>

<p>Implications of <tt>-tailcallopt</tt>:</p>

<p>To support tail call optimization in situations where the callee has more
   arguments than the caller a 'callee pops arguments' convention is used. This
   currently causes each <tt>fastcc</tt> call that is not tail call optimized
   (because one or more of above constraints are not met) to be followed by a
   readjustment of the stack. So performance might be worse in such cases.</p>

</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="sibcallopt">Sibling call optimization</a>
</div>

<div class="doc_text">

<p>Sibling call optimization is a restricted form of tail call optimization.
   Unlike tail call optimization described in the previous section, it can be
   performed automatically on any tail calls when <tt>-tailcallopt</tt> option
   is not specified.</p>

<p>Sibling call optimization is currently performed on x86/x86-64 when the
   following constraints are met:</p>

<ul>
  <li>Caller and callee have the same calling convention. It can be either
      <tt>c</tt> or <tt>fastcc</tt>.

  <li>The call is a tail call - in tail position (ret immediately follows call
      and ret uses value of call or is void).</li>

  <li>Caller and callee have matching return type or the callee result is not
      used.

  <li>If any of the callee arguments are being passed in stack, they must be
      available in caller's own incoming argument stack and the frame offsets
      must be the same.
</ul>

<p>Example:</p>
<div class="doc_code">
<pre>
declare i32 @bar(i32, i32)

define i32 @foo(i32 %a, i32 %b, i32 %c) {
entry:
  %0 = tail call i32 @bar(i32 %a, i32 %b)
  ret i32 %0
}
</pre>
</div>

</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="x86">The X86 backend</a>
</div>

<div class="doc_text">

<p>The X86 code generator lives in the <tt>lib/Target/X86</tt> directory.  This
   code generator is capable of targeting a variety of x86-32 and x86-64
   processors, and includes support for ISA extensions such as MMX and SSE.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="x86_tt">X86 Target Triples supported</a>
</div>

<div class="doc_text">

<p>The following are the known target triples that are supported by the X86
   backend.  This is not an exhaustive list, and it would be useful to add those
   that people test.</p>

<ul>
  <li><b>i686-pc-linux-gnu</b> &mdash; Linux</li>

  <li><b>i386-unknown-freebsd5.3</b> &mdash; FreeBSD 5.3</li>

  <li><b>i686-pc-cygwin</b> &mdash; Cygwin on Win32</li>

  <li><b>i686-pc-mingw32</b> &mdash; MingW on Win32</li>

  <li><b>i386-pc-mingw32msvc</b> &mdash; MingW crosscompiler on Linux</li>

  <li><b>i686-apple-darwin*</b> &mdash; Apple Darwin on X86</li>

  <li><b>x86_64-unknown-linux-gnu</b> &mdash; Linux</li>
</ul>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="x86_cc">X86 Calling Conventions supported</a>
</div>


<div class="doc_text">

<p>The following target-specific calling conventions are known to backend:</p>

<ul>
  <li><b>x86_StdCall</b> &mdash; stdcall calling convention seen on Microsoft
      Windows platform (CC ID = 64).</li>

  <li><b>x86_FastCall</b> &mdash; fastcall calling convention seen on Microsoft
      Windows platform (CC ID = 65).</li>
</ul>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
</div>

<div class="doc_text">

<p>The x86 has a very flexible way of accessing memory.  It is capable of
   forming memory addresses of the following expression directly in integer
   instructions (which use ModR/M addressing):</p>

<div class="doc_code">
<pre>
SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32
</pre>
</div>

<p>In order to represent this, LLVM tracks no less than 5 operands for each
   memory operand of this form.  This means that the "load" form of
   '<tt>mov</tt>' has the following <tt>MachineOperand</tt>s in this order:</p>

<div class="doc_code">
<pre>
Index:        0     |    1        2       3           4          5
Meaning:   DestReg, | BaseReg,  Scale, IndexReg, Displacement Segment
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg,   SignExtImm  PhysReg
</pre>
</div>

<p>Stores, and all other instructions, treat the four memory operands in the
   same way and in the same order.  If the segment register is unspecified
   (regno = 0), then no segment override is generated.  "Lea" operations do not
   have a segment register specified, so they only have 4 operands for their
   memory reference.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="x86_memory">X86 address spaces supported</a>
</div>

<div class="doc_text">

<p>x86 has an experimental feature which provides
   the ability to perform loads and stores to different address spaces
   via the x86 segment registers.  A segment override prefix byte on an
   instruction causes the instruction's memory access to go to the specified
   segment.  LLVM address space 0 is the default address space, which includes
   the stack, and any unqualified memory accesses in a program.  Address spaces
   1-255 are currently reserved for user-defined code.  The GS-segment is
   represented by address space 256, while the FS-segment is represented by 
   address space 257. Other x86 segments have yet to be allocated address space
   numbers.</p>

<p>While these address spaces may seem similar to TLS via the
   <tt>thread_local</tt> keyword, and often use the same underlying hardware,
   there are some fundamental differences.</p>

<p>The <tt>thread_local</tt> keyword applies to global variables and
   specifies that they are to be allocated in thread-local memory. There are
   no type qualifiers involved, and these variables can be pointed to with
   normal pointers and accessed with normal loads and stores.
   The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
   level (though LLVM doesn't yet have implementations of it for some
   configurations).<p>

<p>Special address spaces, in contrast, apply to static types. Every
   load and store has a particular address space in its address operand type,
   and this is what determines which address space is accessed.
   LLVM ignores these special address space qualifiers on global variables,
   and does not provide a way to directly allocate storage in them.
   At the LLVM IR level, the behavior of these special address spaces depends
   in part on the underlying OS or runtime environment, and they are specific
   to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>

<p>Some operating systems and runtime environments use (or may in the future
   use) the FS/GS-segment registers for various low-level purposes, so care
   should be taken when considering them.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="x86_names">Instruction naming</a>
</div>

<div class="doc_text">

<p>An instruction name consists of the base name, a default operand size, and a
   a character per operand with an optional special size. For example:</p>

<div class="doc_code">
<pre>
ADD8rr      -&gt; add, 8-bit register, 8-bit register
IMUL16rmi   -&gt; imul, 16-bit register, 16-bit memory, 16-bit immediate
IMUL16rmi8  -&gt; imul, 16-bit register, 16-bit memory, 8-bit immediate
MOVSX32rm16 -&gt; movsx, 32-bit register, 16-bit memory
</pre>
</div>

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="ppc">The PowerPC backend</a>
</div>

<div class="doc_text">

<p>The PowerPC code generator lives in the lib/Target/PowerPC directory.  The
   code generation is retargetable to several variations or <i>subtargets</i> of
   the PowerPC ISA; including ppc32, ppc64 and altivec.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="ppc_abi">LLVM PowerPC ABI</a>
</div>

<div class="doc_text">

<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
   relative (PIC) or static addressing for accessing global values, so no TOC
   (r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth
   of a stack frame.  LLVM takes advantage of having no TOC to provide space to
   save the frame pointer in the PowerPC linkage area of the caller frame.
   Other details of PowerPC ABI can be found at <a href=
   "http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html"
   >PowerPC ABI.</a> Note: This link describes the 32 bit ABI.  The 64 bit ABI
   is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved
   for system use.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="ppc_frame">Frame Layout</a>
</div>

<div class="doc_text">

<p>The size of a PowerPC frame is usually fixed for the duration of a
   function's invocation.  Since the frame is fixed size, all references
   into the frame can be accessed via fixed offsets from the stack pointer.  The
   exception to this is when dynamic alloca or variable sized arrays are
   present, then a base pointer (r31) is used as a proxy for the stack pointer
   and stack pointer is free to grow or shrink.  A base pointer is also used if
   llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is
   always aligned to 16 bytes, so that space allocated for altivec vectors will
   be properly aligned.</p>

<p>An invocation frame is laid out as follows (low memory at top);</p>

<table class="layout">
  <tr>
    <td>Linkage<br><br></td>
  </tr>
  <tr>
    <td>Parameter area<br><br></td>
  </tr>
  <tr>
    <td>Dynamic area<br><br></td>
  </tr>
  <tr>
    <td>Locals area<br><br></td>
  </tr>
  <tr>
    <td>Saved registers area<br><br></td>
  </tr>
  <tr style="border-style: none hidden none hidden;">
    <td><br></td>
  </tr>
  <tr>
    <td>Previous Frame<br><br></td>
  </tr>
</table>

<p>The <i>linkage</i> area is used by a callee to save special registers prior
   to allocating its own frame.  Only three entries are relevant to LLVM. The
   first entry is the previous stack pointer (sp), aka link.  This allows
   probing tools like gdb or exception handlers to quickly scan the frames in
   the stack.  A function epilog can also use the link to pop the frame from the
   stack.  The third entry in the linkage area is used to save the return
   address from the lr register. Finally, as mentioned above, the last entry is
   used to save the previous frame pointer (r31.)  The entries in the linkage
   area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit
   mode and 48 bytes in 64 bit mode.</p>

<p>32 bit linkage area</p>

<table class="layout">
  <tr>
    <td>0</td>
    <td>Saved SP (r1)</td>
  </tr>
  <tr>
    <td>4</td>
    <td>Saved CR</td>
  </tr>
  <tr>
    <td>8</td>
    <td>Saved LR</td>
  </tr>
  <tr>
    <td>12</td>
    <td>Reserved</td>
  </tr>
  <tr>
    <td>16</td>
    <td>Reserved</td>
  </tr>
  <tr>
    <td>20</td>
    <td>Saved FP (r31)</td>
  </tr>
</table>

<p>64 bit linkage area</p>

<table class="layout">
  <tr>
    <td>0</td>
    <td>Saved SP (r1)</td>
  </tr>
  <tr>
    <td>8</td>
    <td>Saved CR</td>
  </tr>
  <tr>
    <td>16</td>
    <td>Saved LR</td>
  </tr>
  <tr>
    <td>24</td>
    <td>Reserved</td>
  </tr>
  <tr>
    <td>32</td>
    <td>Reserved</td>
  </tr>
  <tr>
    <td>40</td>
    <td>Saved FP (r31)</td>
  </tr>
</table>

<p>The <i>parameter area</i> is used to store arguments being passed to a callee
   function.  Following the PowerPC ABI, the first few arguments are actually
   passed in registers, with the space in the parameter area unused.  However,
   if there are not enough registers or the callee is a thunk or vararg
   function, these register arguments can be spilled into the parameter area.
   Thus, the parameter area must be large enough to store all the parameters for
   the largest call sequence made by the caller.  The size must also be
   minimally large enough to spill registers r3-r10.  This allows callees blind
   to the call signature, such as thunks and vararg functions, enough space to
   cache the argument registers.  Therefore, the parameter area is minimally 32
   bytes (64 bytes in 64 bit mode.)  Also note that since the parameter area is
   a fixed offset from the top of the frame, that a callee can access its spilt
   arguments using fixed offsets from the stack pointer (or base pointer.)</p>

<p>Combining the information about the linkage, parameter areas and alignment. A
   stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
   mode.</p>

<p>The <i>dynamic area</i> starts out as size zero.  If a function uses dynamic
   alloca then space is added to the stack, the linkage and parameter areas are
   shifted to top of stack, and the new space is available immediately below the
   linkage and parameter areas.  The cost of shifting the linkage and parameter
   areas is minor since only the link value needs to be copied.  The link value
   can be easily fetched by adding the original frame size to the base pointer.
   Note that allocations in the dynamic space need to observe 16 byte
   alignment.</p>

<p>The <i>locals area</i> is where the llvm compiler reserves space for local
   variables.</p>

<p>The <i>saved registers area</i> is where the llvm compiler spills callee
   saved registers on entry to the callee.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="ppc_prolog">Prolog/Epilog</a>
</div>

<div class="doc_text">

<p>The llvm prolog and epilog are the same as described in the PowerPC ABI, with
   the following exceptions.  Callee saved registers are spilled after the frame
   is created.  This allows the llvm epilog/prolog support to be common with
   other targets.  The base pointer callee saved register r31 is saved in the
   TOC slot of linkage area.  This simplifies allocation of space for the base
   pointer and makes it convenient to locate programatically and during
   debugging.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
  <a name="ppc_dynamic">Dynamic Allocation</a>
</div>

<div class="doc_text">

<p><i>TODO - More to come.</i></p>

</div>


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