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diff --git a/docs/CodeGenerator.html b/docs/CodeGenerator.html new file mode 100644 index 0000000000..406d77cbef --- /dev/null +++ b/docs/CodeGenerator.html @@ -0,0 +1,1304 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> + <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="#mregisterinfo">The <tt>MRegisterInfo</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">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="#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="#x86">The X86 backend</a></li> + </ul> + </li> + +</ol> + +<div class="doc_author"> + <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</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 -- 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: programmable FPGAs for example.</p> + +<p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code +generator uses almost entirely different code paths than described in this +document. For this reason, there are some deprecated interfaces (such as +<tt>TargetSchedInfo</tt>), which are only used by the +V9 backend and should not be used by any other targets. Also, all code in the +<tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered +deprecated, and should not be used as the basis for future code generator work. +The SparcV9 backend is slowly being merged into the rest of the +target-independent code generators, but this is a low-priority process with no +predictable completion date.</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> - 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> - 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> - 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> - 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> - 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> - 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> - 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 be in <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 in an get something working. Second, it is also important to us because +it makes it easier to change things: in particular, if tables and other things +are all emitted by tblgen, we only need to change one place (tblgen) 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 (which are 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: +<ul><li>an initial register class to use for various ValueTypes</li> + <li>which operations are natively supported by the target machine</li> + <li>the return type of setcc operations</li> + <li>the type to use for shift amounts</li> + <li>various high-level characteristics, like whether it is profitable to turn + division by a constant into a multiplication sequence</li> +</ol></p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="mregisterinfo">The <tt>MRegisterInfo</tt> class</a> +</div> + +<div class="doc_text"> + +<p>The <tt>MRegisterInfo</tt> class (which will eventually be renamed to +<tt>TargetRegisterInfo</tt>) 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 numbers. 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 that one register overlaps with another). +</p> + +<p>In addition to the per-register description, the <tt>MRegisterInfo</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 + locals 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> + <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_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">MachineFunction</a>, +<a href="#machinebasicblock">MachineBasicBlock</a>, and <a +href="#machineinstr"><tt>MachineInstr</tt></a> instances +(defined in include/llvm/CodeGen). 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 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 number 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: +they can be a register reference, constant integer, 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> + +<pre> + %r3 = add %i1, %i2 +</pre> + +<p>If the first operand is a def, and it is also 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> + +<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 &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(&MBB); +</pre> + +<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> + +<!-- _______________________________________________________________________ --> +<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> + +<pre> + int %test(int %X, int %Y) { + %Z = div int %X, %Y + ret int %Z + } +</pre> + +<p>The X86 instruction selector produces this machine code for the div +and ret (use +"<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to get this):</p> + +<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> + +<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> + +<pre> + ;; X is in EAX, Y is in ECX + mov %EAX, %EDX + sar %EDX, 31 + idiv %ECX + ret +</pre> + +<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 and +exit of 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 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, as 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 +(<a href="#machineinstr">MachineInstr</a> 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 MachineBasicBlock 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 +(<a href="#machinebasicblock">MachineBasicBlock</a> 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 +the MachineConstantPool, MachineFrameInfo, MachineFunctionInfo, +SSARegMap, and a set of live in and live out registers for the function. See +<tt>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. In LLVM there are two main forms: +the SelectionDAG based instruction selector framework and an old-style 'simple' +instruction selector (which effectively peephole selects each LLVM instruction +into a series of machine instructions). We recommend that all targets use the +SelectionDAG infrastructure. +</p> + +<p>Portions of the DAG instruction selector are generated from the target +description files (<tt>*.td</tt>) files. Eventually, we aim for the entire +instruction selector to be generated from these <tt>.td</tt> files.</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>SDOperand</tt> class, which is +a <SDNode, unsigned> pair, indicating the node and result +value being used, respectively. Each value produced by an SDNode has an +associated MVT::ValueType, indicating what type 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 MVT::Other. These edges provide +an ordering between nodes that have side effects (such as +loads/stores/calls/return/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 ISD::EntryToken. The Root node is the +final side-effecting node in the token chain. For example, in a single basic +block function, this 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 any values of i1, +i8, i16, +or i64 type would be illegal, as would a DAG that uses a SREM or UREM operation. +The <a href="#selectiondag_legalize">legalize</a> +phase is 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> - This stage performs + a simple translation from the input LLVM code to an illegal SelectionDAG. + </li> |