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2573 lines
92 KiB
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2573 lines
92 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Writing an LLVM Compiler Backend</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">
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Writing an LLVM Compiler Backend
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</div>
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<ol>
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<li><a href="#intro">Introduction</a>
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<ul>
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<li><a href="#Audience">Audience</a></li>
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<li><a href="#Prerequisite">Prerequisite Reading</a></li>
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<li><a href="#Basic">Basic Steps</a></li>
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<li><a href="#Preliminaries">Preliminaries</a></li>
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</ul>
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<li><a href="#TargetMachine">Target Machine</a></li>
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<li><a href="#TargetRegistration">Target Registration</a></li>
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<li><a href="#RegisterSet">Register Set and Register Classes</a>
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<ul>
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<li><a href="#RegisterDef">Defining a Register</a></li>
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<li><a href="#RegisterClassDef">Defining a Register Class</a></li>
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<li><a href="#implementRegister">Implement a subclass of TargetRegisterInfo</a></li>
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</ul></li>
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<li><a href="#InstructionSet">Instruction Set</a>
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<ul>
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<li><a href="#operandMapping">Instruction Operand Mapping</a></li>
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<li><a href="#implementInstr">Implement a subclass of TargetInstrInfo</a></li>
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<li><a href="#branchFolding">Branch Folding and If Conversion</a></li>
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</ul></li>
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<li><a href="#InstructionSelector">Instruction Selector</a>
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<ul>
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<li><a href="#LegalizePhase">The SelectionDAG Legalize Phase</a>
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<ul>
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<li><a href="#promote">Promote</a></li>
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<li><a href="#expand">Expand</a></li>
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<li><a href="#custom">Custom</a></li>
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<li><a href="#legal">Legal</a></li>
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</ul></li>
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<li><a href="#callingConventions">Calling Conventions</a></li>
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</ul></li>
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<li><a href="#assemblyPrinter">Assembly Printer</a></li>
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<li><a href="#subtargetSupport">Subtarget Support</a></li>
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<li><a href="#jitSupport">JIT Support</a>
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<ul>
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<li><a href="#mce">Machine Code Emitter</a></li>
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<li><a href="#targetJITInfo">Target JIT Info</a></li>
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</ul></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="http://www.woo.com">Mason Woo</a> and
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<a href="http://misha.brukman.net">Misha Brukman</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="intro">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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This document describes techniques for writing compiler backends that convert
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the LLVM Intermediate Representation (IR) to code for a specified machine or
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other languages. Code intended for a specific machine can take the form of
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either assembly code or binary code (usable for a JIT compiler).
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</p>
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<p>
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The backend of LLVM features a target-independent code generator that may create
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output for several types of target CPUs — including X86, PowerPC, Alpha,
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and SPARC. The backend may also be used to generate code targeted at SPUs of the
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Cell processor or GPUs to support the execution of compute kernels.
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</p>
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<p>
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The document focuses on existing examples found in subdirectories
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of <tt>llvm/lib/Target</tt> in a downloaded LLVM release. In particular, this
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document focuses on the example of creating a static compiler (one that emits
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text assembly) for a SPARC target, because SPARC has fairly standard
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characteristics, such as a RISC instruction set and straightforward calling
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conventions.
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</p>
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</div>
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<div class="doc_subsection">
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<a name="Audience">Audience</a>
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</div>
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<div class="doc_text">
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<p>
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The audience for this document is anyone who needs to write an LLVM backend to
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generate code for a specific hardware or software target.
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</p>
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</div>
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<div class="doc_subsection">
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<a name="Prerequisite">Prerequisite Reading</a>
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</div>
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<div class="doc_text">
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<p>
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These essential documents must be read before reading this document:
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</p>
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<ul>
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<li><i><a href="http://www.llvm.org/docs/LangRef.html">LLVM Language Reference
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Manual</a></i> — a reference manual for the LLVM assembly language.</li>
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<li><i><a href="http://www.llvm.org/docs/CodeGenerator.html">The LLVM
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Target-Independent Code Generator</a></i> — a guide to the components
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(classes and code generation algorithms) for translating the LLVM internal
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representation into machine code for a specified target. Pay particular
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attention to the descriptions of code generation stages: Instruction
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Selection, Scheduling and Formation, SSA-based Optimization, Register
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Allocation, Prolog/Epilog Code Insertion, Late Machine Code Optimizations,
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and Code Emission.</li>
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<li><i><a href="http://www.llvm.org/docs/TableGenFundamentals.html">TableGen
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Fundamentals</a></i> —a document that describes the TableGen
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(<tt>tblgen</tt>) application that manages domain-specific information to
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support LLVM code generation. TableGen processes input from a target
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description file (<tt>.td</tt> suffix) and generates C++ code that can be
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used for code generation.</li>
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<li><i><a href="http://www.llvm.org/docs/WritingAnLLVMPass.html">Writing an LLVM
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Pass</a></i> — The assembly printer is a <tt>FunctionPass</tt>, as are
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several SelectionDAG processing steps.</li>
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</ul>
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<p>
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To follow the SPARC examples in this document, have a copy of
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<i><a href="http://www.sparc.org/standards/V8.pdf">The SPARC Architecture
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Manual, Version 8</a></i> for reference. For details about the ARM instruction
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set, refer to the <i><a href="http://infocenter.arm.com/">ARM Architecture
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Reference Manual</a></i>. For more about the GNU Assembler format
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(<tt>GAS</tt>), see
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<i><a href="http://sourceware.org/binutils/docs/as/index.html">Using As</a></i>,
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especially for the assembly printer. <i>Using As</i> contains a list of target
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machine dependent features.
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</p>
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</div>
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<div class="doc_subsection">
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<a name="Basic">Basic Steps</a>
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</div>
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<div class="doc_text">
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<p>
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To write a compiler backend for LLVM that converts the LLVM IR to code for a
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specified target (machine or other language), follow these steps:
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</p>
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<ul>
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<li>Create a subclass of the TargetMachine class that describes characteristics
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of your target machine. Copy existing examples of specific TargetMachine
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class and header files; for example, start with
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<tt>SparcTargetMachine.cpp</tt> and <tt>SparcTargetMachine.h</tt>, but
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change the file names for your target. Similarly, change code that
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references "Sparc" to reference your target. </li>
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<li>Describe the register set of the target. Use TableGen to generate code for
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register definition, register aliases, and register classes from a
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target-specific <tt>RegisterInfo.td</tt> input file. You should also write
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additional code for a subclass of the TargetRegisterInfo class that
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represents the class register file data used for register allocation and
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also describes the interactions between registers.</li>
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<li>Describe the instruction set of the target. Use TableGen to generate code
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for target-specific instructions from target-specific versions of
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<tt>TargetInstrFormats.td</tt> and <tt>TargetInstrInfo.td</tt>. You should
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write additional code for a subclass of the TargetInstrInfo class to
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represent machine instructions supported by the target machine. </li>
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<li>Describe the selection and conversion of the LLVM IR from a Directed Acyclic
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Graph (DAG) representation of instructions to native target-specific
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instructions. Use TableGen to generate code that matches patterns and
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selects instructions based on additional information in a target-specific
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version of <tt>TargetInstrInfo.td</tt>. Write code
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for <tt>XXXISelDAGToDAG.cpp</tt>, where XXX identifies the specific target,
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to perform pattern matching and DAG-to-DAG instruction selection. Also write
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code in <tt>XXXISelLowering.cpp</tt> to replace or remove operations and
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data types that are not supported natively in a SelectionDAG. </li>
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<li>Write code for an assembly printer that converts LLVM IR to a GAS format for
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your target machine. You should add assembly strings to the instructions
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defined in your target-specific version of <tt>TargetInstrInfo.td</tt>. You
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should also write code for a subclass of AsmPrinter that performs the
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LLVM-to-assembly conversion and a trivial subclass of TargetAsmInfo.</li>
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<li>Optionally, add support for subtargets (i.e., variants with different
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capabilities). You should also write code for a subclass of the
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TargetSubtarget class, which allows you to use the <tt>-mcpu=</tt>
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and <tt>-mattr=</tt> command-line options.</li>
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<li>Optionally, add JIT support and create a machine code emitter (subclass of
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TargetJITInfo) that is used to emit binary code directly into memory. </li>
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</ul>
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<p>
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In the <tt>.cpp</tt> and <tt>.h</tt>. files, initially stub up these methods and
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then implement them later. Initially, you may not know which private members
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that the class will need and which components will need to be subclassed.
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</p>
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</div>
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<div class="doc_subsection">
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<a name="Preliminaries">Preliminaries</a>
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</div>
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<div class="doc_text">
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<p>
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To actually create your compiler backend, you need to create and modify a few
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files. The absolute minimum is discussed here. But to actually use the LLVM
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target-independent code generator, you must perform the steps described in
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the <a href="http://www.llvm.org/docs/CodeGenerator.html">LLVM
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Target-Independent Code Generator</a> document.
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</p>
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<p>
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First, you should create a subdirectory under <tt>lib/Target</tt> to hold all
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the files related to your target. If your target is called "Dummy," create the
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directory <tt>lib/Target/Dummy</tt>.
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</p>
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<p>
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In this new
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directory, create a <tt>Makefile</tt>. It is easiest to copy a
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<tt>Makefile</tt> of another target and modify it. It should at least contain
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the <tt>LEVEL</tt>, <tt>LIBRARYNAME</tt> and <tt>TARGET</tt> variables, and then
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include <tt>$(LEVEL)/Makefile.common</tt>. The library can be
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named <tt>LLVMDummy</tt> (for example, see the MIPS target). Alternatively, you
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can split the library into <tt>LLVMDummyCodeGen</tt>
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and <tt>LLVMDummyAsmPrinter</tt>, the latter of which should be implemented in a
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subdirectory below <tt>lib/Target/Dummy</tt> (for example, see the PowerPC
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target).
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</p>
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<p>
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Note that these two naming schemes are hardcoded into <tt>llvm-config</tt>.
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Using any other naming scheme will confuse <tt>llvm-config</tt> and produce a
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lot of (seemingly unrelated) linker errors when linking <tt>llc</tt>.
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</p>
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<p>
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To make your target actually do something, you need to implement a subclass of
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<tt>TargetMachine</tt>. This implementation should typically be in the file
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<tt>lib/Target/DummyTargetMachine.cpp</tt>, but any file in
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the <tt>lib/Target</tt> directory will be built and should work. To use LLVM's
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target independent code generator, you should do what all current machine
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backends do: create a subclass of <tt>LLVMTargetMachine</tt>. (To create a
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target from scratch, create a subclass of <tt>TargetMachine</tt>.)
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</p>
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<p>
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To get LLVM to actually build and link your target, you need to add it to
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the <tt>TARGETS_TO_BUILD</tt> variable. To do this, you modify the configure
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script to know about your target when parsing the <tt>--enable-targets</tt>
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option. Search the configure script for <tt>TARGETS_TO_BUILD</tt>, add your
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target to the lists there (some creativity required), and then
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reconfigure. Alternatively, you can change <tt>autotools/configure.ac</tt> and
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regenerate configure by running <tt>./autoconf/AutoRegen.sh</tt>.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="TargetMachine">Target Machine</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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<tt>LLVMTargetMachine</tt> is designed as a base class for targets implemented
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with the LLVM target-independent code generator. The <tt>LLVMTargetMachine</tt>
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class should be specialized by a concrete target class that implements the
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various virtual methods. <tt>LLVMTargetMachine</tt> is defined as a subclass of
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<tt>TargetMachine</tt> in <tt>include/llvm/Target/TargetMachine.h</tt>. The
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<tt>TargetMachine</tt> class implementation (<tt>TargetMachine.cpp</tt>) also
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processes numerous command-line options.
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</p>
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<p>
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To create a concrete target-specific subclass of <tt>LLVMTargetMachine</tt>,
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start by copying an existing <tt>TargetMachine</tt> class and header. You
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should name the files that you create to reflect your specific target. For
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instance, for the SPARC target, name the files <tt>SparcTargetMachine.h</tt> and
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<tt>SparcTargetMachine.cpp</tt>.
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</p>
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<p>
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For a target machine <tt>XXX</tt>, the implementation of
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<tt>XXXTargetMachine</tt> must have access methods to obtain objects that
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represent target components. These methods are named <tt>get*Info</tt>, and are
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intended to obtain the instruction set (<tt>getInstrInfo</tt>), register set
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(<tt>getRegisterInfo</tt>), stack frame layout (<tt>getFrameInfo</tt>), and
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similar information. <tt>XXXTargetMachine</tt> must also implement the
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<tt>getTargetData</tt> method to access an object with target-specific data
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characteristics, such as data type size and alignment requirements.
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</p>
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<p>
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For instance, for the SPARC target, the header file
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<tt>SparcTargetMachine.h</tt> declares prototypes for several <tt>get*Info</tt>
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and <tt>getTargetData</tt> methods that simply return a class member.
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</p>
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<div class="doc_code">
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<pre>
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namespace llvm {
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class Module;
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class SparcTargetMachine : public LLVMTargetMachine {
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const TargetData DataLayout; // Calculates type size & alignment
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SparcSubtarget Subtarget;
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SparcInstrInfo InstrInfo;
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TargetFrameInfo FrameInfo;
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protected:
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virtual const TargetAsmInfo *createTargetAsmInfo() const;
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public:
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SparcTargetMachine(const Module &M, const std::string &FS);
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virtual const SparcInstrInfo *getInstrInfo() const {return &InstrInfo; }
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virtual const TargetFrameInfo *getFrameInfo() const {return &FrameInfo; }
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virtual const TargetSubtarget *getSubtargetImpl() const{return &Subtarget; }
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virtual const TargetRegisterInfo *getRegisterInfo() const {
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return &InstrInfo.getRegisterInfo();
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}
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virtual const TargetData *getTargetData() const { return &DataLayout; }
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static unsigned getModuleMatchQuality(const Module &M);
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// Pass Pipeline Configuration
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virtual bool addInstSelector(PassManagerBase &PM, bool Fast);
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virtual bool addPreEmitPass(PassManagerBase &PM, bool Fast);
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virtual bool addAssemblyEmitter(PassManagerBase &PM, bool Fast,
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std::ostream &Out);
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};
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} // end namespace llvm
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</pre>
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</div>
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</div>
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|
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<div class="doc_text">
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<ul>
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<li><tt>getInstrInfo()</tt></li>
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<li><tt>getRegisterInfo()</tt></li>
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<li><tt>getFrameInfo()</tt></li>
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<li><tt>getTargetData()</tt></li>
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<li><tt>getSubtargetImpl()</tt></li>
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</ul>
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<p>For some targets, you also need to support the following methods:</p>
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<ul>
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<li><tt>getTargetLowering()</tt></li>
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<li><tt>getJITInfo()</tt></li>
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</ul>
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<p>
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In addition, the <tt>XXXTargetMachine</tt> constructor should specify a
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<tt>TargetDescription</tt> string that determines the data layout for the target
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machine, including characteristics such as pointer size, alignment, and
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endianness. For example, the constructor for SparcTargetMachine contains the
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following:
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</p>
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<div class="doc_code">
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<pre>
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SparcTargetMachine::SparcTargetMachine(const Module &M, const std::string &FS)
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: DataLayout("E-p:32:32-f128:128:128"),
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Subtarget(M, FS), InstrInfo(Subtarget),
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FrameInfo(TargetFrameInfo::StackGrowsDown, 8, 0) {
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}
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</pre>
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</div>
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</div>
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<div class="doc_text">
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<p>Hyphens separate portions of the <tt>TargetDescription</tt> string.</p>
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<ul>
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<li>An upper-case "<tt>E</tt>" in the string indicates a big-endian target data
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model. a lower-case "<tt>e</tt>" indicates little-endian.</li>
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<li>"<tt>p:</tt>" is followed by pointer information: size, ABI alignment, and
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preferred alignment. If only two figures follow "<tt>p:</tt>", then the
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first value is pointer size, and the second value is both ABI and preferred
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alignment.</li>
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<li>Then a letter for numeric type alignment: "<tt>i</tt>", "<tt>f</tt>",
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"<tt>v</tt>", or "<tt>a</tt>" (corresponding to integer, floating point,
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vector, or aggregate). "<tt>i</tt>", "<tt>v</tt>", or "<tt>a</tt>" are
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followed by ABI alignment and preferred alignment. "<tt>f</tt>" is followed
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by three values: the first indicates the size of a long double, then ABI
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alignment, and then ABI preferred alignment.</li>
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</ul>
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</div>
|
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|
|
<!-- *********************************************************************** -->
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|
<div class="doc_section">
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|
<a name="TargetRegistration">Target Registration</a>
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|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
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|
You must also register your target with the <tt>TargetRegistry</tt>, which is
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what other LLVM tools use to be able to lookup and use your target at
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runtime. The <tt>TargetRegistry</tt> can be used directly, but for most targets
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there are helper templates which should take care of the work for you.</p>
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<p>
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All targets should declare a global <tt>Target</tt> object which is used to
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represent the target during registration. Then, in the target's TargetInfo
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library, the target should define that object and use
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the <tt>RegisterTarget</tt> template to register the target. For example, the Sparc registration code looks like this:
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</p>
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|
|
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<div class="doc_code">
|
|
<pre>
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|
Target llvm::TheSparcTarget;
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|
|
extern "C" void LLVMInitializeSparcTargetInfo() {
|
|
RegisterTarget<Triple::sparc, /*HasJIT=*/false>
|
|
X(TheSparcTarget, "sparc", "Sparc");
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
This allows the <tt>TargetRegistry</tt> to look up the target by name or by
|
|
target triple. In addition, most targets will also register additional features
|
|
which are available in separate libraries. These registration steps are
|
|
separate, because some clients may wish to only link in some parts of the target
|
|
-- the JIT code generator does not require the use of the assembler printer, for
|
|
example. Here is an example of registering the Sparc assembly printer:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
extern "C" void LLVMInitializeSparcAsmPrinter() {
|
|
RegisterAsmPrinter<SparcAsmPrinter> X(TheSparcTarget);
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
For more information, see
|
|
"<a href="/doxygen/TargetRegistry_8h-source.html">llvm/Target/TargetRegistry.h</a>".
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="RegisterSet">Register Set and Register Classes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
You should describe a concrete target-specific class that represents the
|
|
register file of a target machine. This class is called <tt>XXXRegisterInfo</tt>
|
|
(where <tt>XXX</tt> identifies the target) and represents the class register
|
|
file data that is used for register allocation. It also describes the
|
|
interactions between registers.
|
|
</p>
|
|
|
|
<p>
|
|
You also need to define register classes to categorize related registers. A
|
|
register class should be added for groups of registers that are all treated the
|
|
same way for some instruction. Typical examples are register classes for
|
|
integer, floating-point, or vector registers. A register allocator allows an
|
|
instruction to use any register in a specified register class to perform the
|
|
instruction in a similar manner. Register classes allocate virtual registers to
|
|
instructions from these sets, and register classes let the target-independent
|
|
register allocator automatically choose the actual registers.
|
|
</p>
|
|
|
|
<p>
|
|
Much of the code for registers, including register definition, register aliases,
|
|
and register classes, is generated by TableGen from <tt>XXXRegisterInfo.td</tt>
|
|
input files and placed in <tt>XXXGenRegisterInfo.h.inc</tt> and
|
|
<tt>XXXGenRegisterInfo.inc</tt> output files. Some of the code in the
|
|
implementation of <tt>XXXRegisterInfo</tt> requires hand-coding.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="RegisterDef">Defining a Register</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The <tt>XXXRegisterInfo.td</tt> file typically starts with register definitions
|
|
for a target machine. The <tt>Register</tt> class (specified
|
|
in <tt>Target.td</tt>) is used to define an object for each register. The
|
|
specified string <tt>n</tt> becomes the <tt>Name</tt> of the register. The
|
|
basic <tt>Register</tt> object does not have any subregisters and does not
|
|
specify any aliases.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class Register<string n> {
|
|
string Namespace = "";
|
|
string AsmName = n;
|
|
string Name = n;
|
|
int SpillSize = 0;
|
|
int SpillAlignment = 0;
|
|
list<Register> Aliases = [];
|
|
list<Register> SubRegs = [];
|
|
list<int> DwarfNumbers = [];
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
For example, in the <tt>X86RegisterInfo.td</tt> file, there are register
|
|
definitions that utilize the Register class, such as:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def AL : Register<"AL">, DwarfRegNum<[0, 0, 0]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
This defines the register <tt>AL</tt> and assigns it values (with
|
|
<tt>DwarfRegNum</tt>) that are used by <tt>gcc</tt>, <tt>gdb</tt>, or a debug
|
|
information writer (such as <tt>DwarfWriter</tt>
|
|
in <tt>llvm/lib/CodeGen/AsmPrinter</tt>) to identify a register. For register
|
|
<tt>AL</tt>, <tt>DwarfRegNum</tt> takes an array of 3 values representing 3
|
|
different modes: the first element is for X86-64, the second for exception
|
|
handling (EH) on X86-32, and the third is generic. -1 is a special Dwarf number
|
|
that indicates the gcc number is undefined, and -2 indicates the register number
|
|
is invalid for this mode.
|
|
</p>
|
|
|
|
<p>
|
|
From the previously described line in the <tt>X86RegisterInfo.td</tt> file,
|
|
TableGen generates this code in the <tt>X86GenRegisterInfo.inc</tt> file:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
static const unsigned GR8[] = { X86::AL, ... };
|
|
|
|
const unsigned AL_AliasSet[] = { X86::AX, X86::EAX, X86::RAX, 0 };
|
|
|
|
const TargetRegisterDesc RegisterDescriptors[] = {
|
|
...
|
|
{ "AL", "AL", AL_AliasSet, Empty_SubRegsSet, Empty_SubRegsSet, AL_SuperRegsSet }, ...
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
From the register info file, TableGen generates a <tt>TargetRegisterDesc</tt>
|
|
object for each register. <tt>TargetRegisterDesc</tt> is defined in
|
|
<tt>include/llvm/Target/TargetRegisterInfo.h</tt> with the following fields:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
struct TargetRegisterDesc {
|
|
const char *AsmName; // Assembly language name for the register
|
|
const char *Name; // Printable name for the reg (for debugging)
|
|
const unsigned *AliasSet; // Register Alias Set
|
|
const unsigned *SubRegs; // Sub-register set
|
|
const unsigned *ImmSubRegs; // Immediate sub-register set
|
|
const unsigned *SuperRegs; // Super-register set
|
|
};</pre>
|
|
</div>
|
|
|
|
<p>
|
|
TableGen uses the entire target description file (<tt>.td</tt>) to determine
|
|
text names for the register (in the <tt>AsmName</tt> and <tt>Name</tt> fields of
|
|
<tt>TargetRegisterDesc</tt>) and the relationships of other registers to the
|
|
defined register (in the other <tt>TargetRegisterDesc</tt> fields). In this
|
|
example, other definitions establish the registers "<tt>AX</tt>",
|
|
"<tt>EAX</tt>", and "<tt>RAX</tt>" as aliases for one another, so TableGen
|
|
generates a null-terminated array (<tt>AL_AliasSet</tt>) for this register alias
|
|
set.
|
|
</p>
|
|
|
|
<p>
|
|
The <tt>Register</tt> class is commonly used as a base class for more complex
|
|
classes. In <tt>Target.td</tt>, the <tt>Register</tt> class is the base for the
|
|
<tt>RegisterWithSubRegs</tt> class that is used to define registers that need to
|
|
specify subregisters in the <tt>SubRegs</tt> list, as shown here:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class RegisterWithSubRegs<string n,
|
|
list<Register> subregs> : Register<n> {
|
|
let SubRegs = subregs;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
In <tt>SparcRegisterInfo.td</tt>, additional register classes are defined for
|
|
SPARC: a Register subclass, SparcReg, and further subclasses: <tt>Ri</tt>,
|
|
<tt>Rf</tt>, and <tt>Rd</tt>. SPARC registers are identified by 5-bit ID
|
|
numbers, which is a feature common to these subclasses. Note the use of
|
|
'<tt>let</tt>' expressions to override values that are initially defined in a
|
|
superclass (such as <tt>SubRegs</tt> field in the <tt>Rd</tt> class).
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class SparcReg<string n> : Register<n> {
|
|
field bits<5> Num;
|
|
let Namespace = "SP";
|
|
}
|
|
// Ri - 32-bit integer registers
|
|
class Ri<bits<5> num, string n> :
|
|
SparcReg<n> {
|
|
let Num = num;
|
|
}
|
|
// Rf - 32-bit floating-point registers
|
|
class Rf<bits<5> num, string n> :
|
|
SparcReg<n> {
|
|
let Num = num;
|
|
}
|
|
// Rd - Slots in the FP register file for 64-bit
|
|
floating-point values.
|
|
class Rd<bits<5> num, string n,
|
|
list<Register> subregs> : SparcReg<n> {
|
|
let Num = num;
|
|
let SubRegs = subregs;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
In the <tt>SparcRegisterInfo.td</tt> file, there are register definitions that
|
|
utilize these subclasses of <tt>Register</tt>, such as:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def G0 : Ri< 0, "G0">,
|
|
DwarfRegNum<[0]>;
|
|
def G1 : Ri< 1, "G1">, DwarfRegNum<[1]>;
|
|
...
|
|
def F0 : Rf< 0, "F0">,
|
|
DwarfRegNum<[32]>;
|
|
def F1 : Rf< 1, "F1">,
|
|
DwarfRegNum<[33]>;
|
|
...
|
|
def D0 : Rd< 0, "F0", [F0, F1]>,
|
|
DwarfRegNum<[32]>;
|
|
def D1 : Rd< 2, "F2", [F2, F3]>,
|
|
DwarfRegNum<[34]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The last two registers shown above (<tt>D0</tt> and <tt>D1</tt>) are
|
|
double-precision floating-point registers that are aliases for pairs of
|
|
single-precision floating-point sub-registers. In addition to aliases, the
|
|
sub-register and super-register relationships of the defined register are in
|
|
fields of a register's TargetRegisterDesc.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="RegisterClassDef">Defining a Register Class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The <tt>RegisterClass</tt> class (specified in <tt>Target.td</tt>) is used to
|
|
define an object that represents a group of related registers and also defines
|
|
the default allocation order of the registers. A target description file
|
|
<tt>XXXRegisterInfo.td</tt> that uses <tt>Target.td</tt> can construct register
|
|
classes using the following class:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class RegisterClass<string namespace,
|
|
list<ValueType> regTypes, int alignment,
|
|
list<Register> regList> {
|
|
string Namespace = namespace;
|
|
list<ValueType> RegTypes = regTypes;
|
|
int Size = 0; // spill size, in bits; zero lets tblgen pick the size
|
|
int Alignment = alignment;
|
|
|
|
// CopyCost is the cost of copying a value between two registers
|
|
// default value 1 means a single instruction
|
|
// A negative value means copying is extremely expensive or impossible
|
|
int CopyCost = 1;
|
|
list<Register> MemberList = regList;
|
|
|
|
// for register classes that are subregisters of this class
|
|
list<RegisterClass> SubRegClassList = [];
|
|
|
|
code MethodProtos = [{}]; // to insert arbitrary code
|
|
code MethodBodies = [{}];
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>To define a RegisterClass, use the following 4 arguments:</p>
|
|
|
|
<ul>
|
|
<li>The first argument of the definition is the name of the namespace.</li>
|
|
|
|
<li>The second argument is a list of <tt>ValueType</tt> register type values
|
|
that are defined in <tt>include/llvm/CodeGen/ValueTypes.td</tt>. Defined
|
|
values include integer types (such as <tt>i16</tt>, <tt>i32</tt>,
|
|
and <tt>i1</tt> for Boolean), floating-point types
|
|
(<tt>f32</tt>, <tt>f64</tt>), and vector types (for example, <tt>v8i16</tt>
|
|
for an <tt>8 x i16</tt> vector). All registers in a <tt>RegisterClass</tt>
|
|
must have the same <tt>ValueType</tt>, but some registers may store vector
|
|
data in different configurations. For example a register that can process a
|
|
128-bit vector may be able to handle 16 8-bit integer elements, 8 16-bit
|
|
integers, 4 32-bit integers, and so on. </li>
|
|
|
|
<li>The third argument of the <tt>RegisterClass</tt> definition specifies the
|
|
alignment required of the registers when they are stored or loaded to
|
|
memory.</li>
|
|
|
|
<li>The final argument, <tt>regList</tt>, specifies which registers are in this
|
|
class. If an <tt>allocation_order_*</tt> method is not specified,
|
|
then <tt>regList</tt> also defines the order of allocation used by the
|
|
register allocator.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
In <tt>SparcRegisterInfo.td</tt>, three RegisterClass objects are defined:
|
|
<tt>FPRegs</tt>, <tt>DFPRegs</tt>, and <tt>IntRegs</tt>. For all three register
|
|
classes, the first argument defines the namespace with the string
|
|
'<tt>SP</tt>'. <tt>FPRegs</tt> defines a group of 32 single-precision
|
|
floating-point registers (<tt>F0</tt> to <tt>F31</tt>); <tt>DFPRegs</tt> defines
|
|
a group of 16 double-precision registers
|
|
(<tt>D0-D15</tt>). For <tt>IntRegs</tt>, the <tt>MethodProtos</tt>
|
|
and <tt>MethodBodies</tt> methods are used by TableGen to insert the specified
|
|
code into generated output.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def FPRegs : RegisterClass<"SP", [f32], 32,
|
|
[F0, F1, F2, F3, F4, F5, F6, F7, F8, F9, F10, F11, F12, F13, F14, F15,
|
|
F16, F17, F18, F19, F20, F21, F22, F23, F24, F25, F26, F27, F28, F29, F30, F31]>;
|
|
|
|
def DFPRegs : RegisterClass<"SP", [f64], 64,
|
|
[D0, D1, D2, D3, D4, D5, D6, D7, D8, D9, D10, D11, D12, D13, D14, D15]>;
|
|
|
|
def IntRegs : RegisterClass<"SP", [i32], 32,
|
|
[L0, L1, L2, L3, L4, L5, L6, L7,
|
|
I0, I1, I2, I3, I4, I5,
|
|
O0, O1, O2, O3, O4, O5, O7,
|
|
G1,
|
|
// Non-allocatable regs:
|
|
G2, G3, G4,
|
|
O6, // stack ptr
|
|
I6, // frame ptr
|
|
I7, // return address
|
|
G0, // constant zero
|
|
G5, G6, G7 // reserved for kernel
|
|
]> {
|
|
let MethodProtos = [{
|
|
iterator allocation_order_end(const MachineFunction &MF) const;
|
|
}];
|
|
let MethodBodies = [{
|
|
IntRegsClass::iterator
|
|
IntRegsClass::allocation_order_end(const MachineFunction &MF) const {
|
|
return end() - 10 // Don't allocate special registers
|
|
-1;
|
|
}
|
|
}];
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
Using <tt>SparcRegisterInfo.td</tt> with TableGen generates several output files
|
|
that are intended for inclusion in other source code that you write.
|
|
<tt>SparcRegisterInfo.td</tt> generates <tt>SparcGenRegisterInfo.h.inc</tt>,
|
|
which should be included in the header file for the implementation of the SPARC
|
|
register implementation that you write (<tt>SparcRegisterInfo.h</tt>). In
|
|
<tt>SparcGenRegisterInfo.h.inc</tt> a new structure is defined called
|
|
<tt>SparcGenRegisterInfo</tt> that uses <tt>TargetRegisterInfo</tt> as its
|
|
base. It also specifies types, based upon the defined register
|
|
classes: <tt>DFPRegsClass</tt>, <tt>FPRegsClass</tt>, and <tt>IntRegsClass</tt>.
|
|
</p>
|
|
|
|
<p>
|
|
<tt>SparcRegisterInfo.td</tt> also generates <tt>SparcGenRegisterInfo.inc</tt>,
|
|
which is included at the bottom of <tt>SparcRegisterInfo.cpp</tt>, the SPARC
|
|
register implementation. The code below shows only the generated integer
|
|
registers and associated register classes. The order of registers
|
|
in <tt>IntRegs</tt> reflects the order in the definition of <tt>IntRegs</tt> in
|
|
the target description file. Take special note of the use
|
|
of <tt>MethodBodies</tt> in <tt>SparcRegisterInfo.td</tt> to create code in
|
|
<tt>SparcGenRegisterInfo.inc</tt>. <tt>MethodProtos</tt> generates similar code
|
|
in <tt>SparcGenRegisterInfo.h.inc</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre> // IntRegs Register Class...
|
|
static const unsigned IntRegs[] = {
|
|
SP::L0, SP::L1, SP::L2, SP::L3, SP::L4, SP::L5,
|
|
SP::L6, SP::L7, SP::I0, SP::I1, SP::I2, SP::I3,
|
|
SP::I4, SP::I5, SP::O0, SP::O1, SP::O2, SP::O3,
|
|
SP::O4, SP::O5, SP::O7, SP::G1, SP::G2, SP::G3,
|
|
SP::G4, SP::O6, SP::I6, SP::I7, SP::G0, SP::G5,
|
|
SP::G6, SP::G7,
|
|
};
|
|
|
|
// IntRegsVTs Register Class Value Types...
|
|
static const MVT::ValueType IntRegsVTs[] = {
|
|
MVT::i32, MVT::Other
|
|
};
|
|
|
|
namespace SP { // Register class instances
|
|
DFPRegsClass DFPRegsRegClass;
|
|
FPRegsClass FPRegsRegClass;
|
|
IntRegsClass IntRegsRegClass;
|
|
...
|
|
// IntRegs Sub-register Classess...
|
|
static const TargetRegisterClass* const IntRegsSubRegClasses [] = {
|
|
NULL
|
|
};
|
|
...
|
|
// IntRegs Super-register Classess...
|
|
static const TargetRegisterClass* const IntRegsSuperRegClasses [] = {
|
|
NULL
|
|
};
|
|
...
|
|
// IntRegs Register Class sub-classes...
|
|
static const TargetRegisterClass* const IntRegsSubclasses [] = {
|
|
NULL
|
|
};
|
|
...
|
|
// IntRegs Register Class super-classes...
|
|
static const TargetRegisterClass* const IntRegsSuperclasses [] = {
|
|
NULL
|
|
};
|
|
...
|
|
IntRegsClass::iterator
|
|
IntRegsClass::allocation_order_end(const MachineFunction &MF) const {
|
|
return end()-10 // Don't allocate special registers
|
|
-1;
|
|
}
|
|
|
|
IntRegsClass::IntRegsClass() : TargetRegisterClass(IntRegsRegClassID,
|
|
IntRegsVTs, IntRegsSubclasses, IntRegsSuperclasses, IntRegsSubRegClasses,
|
|
IntRegsSuperRegClasses, 4, 4, 1, IntRegs, IntRegs + 32) {}
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="implementRegister">Implement a subclass of</a>
|
|
<a href="http://www.llvm.org/docs/CodeGenerator.html#targetregisterinfo">TargetRegisterInfo</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The final step is to hand code portions of <tt>XXXRegisterInfo</tt>, which
|
|
implements the interface described in <tt>TargetRegisterInfo.h</tt>. These
|
|
functions return <tt>0</tt>, <tt>NULL</tt>, or <tt>false</tt>, unless
|
|
overridden. Here is a list of functions that are overridden for the SPARC
|
|
implementation in <tt>SparcRegisterInfo.cpp</tt>:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>getCalleeSavedRegs</tt> — Returns a list of callee-saved registers
|
|
in the order of the desired callee-save stack frame offset.</li>
|
|
|
|
<li><tt>getCalleeSavedRegClasses</tt> — Returns a list of preferred
|
|
register classes with which to spill each callee saved register.</li>
|
|
|
|
<li><tt>getReservedRegs</tt> — Returns a bitset indexed by physical
|
|
register numbers, indicating if a particular register is unavailable.</li>
|
|
|
|
<li><tt>hasFP</tt> — Return a Boolean indicating if a function should have
|
|
a dedicated frame pointer register.</li>
|
|
|
|
<li><tt>eliminateCallFramePseudoInstr</tt> — If call frame setup or
|
|
destroy pseudo instructions are used, this can be called to eliminate
|
|
them.</li>
|
|
|
|
<li><tt>eliminateFrameIndex</tt> — Eliminate abstract frame indices from
|
|
instructions that may use them.</li>
|
|
|
|
<li><tt>emitPrologue</tt> — Insert prologue code into the function.</li>
|
|
|
|
<li><tt>emitEpilogue</tt> — Insert epilogue code into the function.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="InstructionSet">Instruction Set</a>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
During the early stages of code generation, the LLVM IR code is converted to a
|
|
<tt>SelectionDAG</tt> with nodes that are instances of the <tt>SDNode</tt> class
|
|
containing target instructions. An <tt>SDNode</tt> has an opcode, operands, type
|
|
requirements, and operation properties. For example, is an operation
|
|
commutative, does an operation load from memory. The various operation node
|
|
types are described in the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
|
|
file (values of the <tt>NodeType</tt> enum in the <tt>ISD</tt> namespace).
|
|
</p>
|
|
|
|
<p>
|
|
TableGen uses the following target description (<tt>.td</tt>) input files to
|
|
generate much of the code for instruction definition:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>Target.td</tt> — Where the <tt>Instruction</tt>, <tt>Operand</tt>,
|
|
<tt>InstrInfo</tt>, and other fundamental classes are defined.</li>
|
|
|
|
<li><tt>TargetSelectionDAG.td</tt>— Used by <tt>SelectionDAG</tt>
|
|
instruction selection generators, contains <tt>SDTC*</tt> classes (selection
|
|
DAG type constraint), definitions of <tt>SelectionDAG</tt> nodes (such as
|
|
<tt>imm</tt>, <tt>cond</tt>, <tt>bb</tt>, <tt>add</tt>, <tt>fadd</tt>,
|
|
<tt>sub</tt>), and pattern support (<tt>Pattern</tt>, <tt>Pat</tt>,
|
|
<tt>PatFrag</tt>, <tt>PatLeaf</tt>, <tt>ComplexPattern</tt>.</li>
|
|
|
|
<li><tt>XXXInstrFormats.td</tt> — Patterns for definitions of
|
|
target-specific instructions.</li>
|
|
|
|
<li><tt>XXXInstrInfo.td</tt> — Target-specific definitions of instruction
|
|
templates, condition codes, and instructions of an instruction set. For
|
|
architecture modifications, a different file name may be used. For example,
|
|
for Pentium with SSE instruction, this file is <tt>X86InstrSSE.td</tt>, and
|
|
for Pentium with MMX, this file is <tt>X86InstrMMX.td</tt>.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
There is also a target-specific <tt>XXX.td</tt> file, where <tt>XXX</tt> is the
|
|
name of the target. The <tt>XXX.td</tt> file includes the other <tt>.td</tt>
|
|
input files, but its contents are only directly important for subtargets.
|
|
</p>
|
|
|
|
<p>
|
|
You should describe a concrete target-specific class <tt>XXXInstrInfo</tt> that
|
|
represents machine instructions supported by a target machine.
|
|
<tt>XXXInstrInfo</tt> contains an array of <tt>XXXInstrDescriptor</tt> objects,
|
|
each of which describes one instruction. An instruction descriptor defines:</p>
|
|
|
|
<ul>
|
|
<li>Opcode mnemonic</li>
|
|
|
|
<li>Number of operands</li>
|
|
|
|
<li>List of implicit register definitions and uses</li>
|
|
|
|
<li>Target-independent properties (such as memory access, is commutable)</li>
|
|
|
|
<li>Target-specific flags </li>
|
|
</ul>
|
|
|
|
<p>
|
|
The Instruction class (defined in <tt>Target.td</tt>) is mostly used as a base
|
|
for more complex instruction classes.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>class Instruction {
|
|
string Namespace = "";
|
|
dag OutOperandList; // An dag containing the MI def operand list.
|
|
dag InOperandList; // An dag containing the MI use operand list.
|
|
string AsmString = ""; // The .s format to print the instruction with.
|
|
list<dag> Pattern; // Set to the DAG pattern for this instruction
|
|
list<Register> Uses = [];
|
|
list<Register> Defs = [];
|
|
list<Predicate> Predicates = []; // predicates turned into isel match code
|
|
... remainder not shown for space ...
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
A <tt>SelectionDAG</tt> node (<tt>SDNode</tt>) should contain an object
|
|
representing a target-specific instruction that is defined
|
|
in <tt>XXXInstrInfo.td</tt>. The instruction objects should represent
|
|
instructions from the architecture manual of the target machine (such as the
|
|
SPARC Architecture Manual for the SPARC target).
|
|
</p>
|
|
|
|
<p>
|
|
A single instruction from the architecture manual is often modeled as multiple
|
|
target instructions, depending upon its operands. For example, a manual might
|
|
describe an add instruction that takes a register or an immediate operand. An
|
|
LLVM target could model this with two instructions named <tt>ADDri</tt> and
|
|
<tt>ADDrr</tt>.
|
|
</p>
|
|
|
|
<p>
|
|
You should define a class for each instruction category and define each opcode
|
|
as a subclass of the category with appropriate parameters such as the fixed
|
|
binary encoding of opcodes and extended opcodes. You should map the register
|
|
bits to the bits of the instruction in which they are encoded (for the
|
|
JIT). Also you should specify how the instruction should be printed when the
|
|
automatic assembly printer is used.
|
|
</p>
|
|
|
|
<p>
|
|
As is described in the SPARC Architecture Manual, Version 8, there are three
|
|
major 32-bit formats for instructions. Format 1 is only for the <tt>CALL</tt>
|
|
instruction. Format 2 is for branch on condition codes and <tt>SETHI</tt> (set
|
|
high bits of a register) instructions. Format 3 is for other instructions.
|
|
</p>
|
|
|
|
<p>
|
|
Each of these formats has corresponding classes in <tt>SparcInstrFormat.td</tt>.
|
|
<tt>InstSP</tt> is a base class for other instruction classes. Additional base
|
|
classes are specified for more precise formats: for example
|
|
in <tt>SparcInstrFormat.td</tt>, <tt>F2_1</tt> is for <tt>SETHI</tt>,
|
|
and <tt>F2_2</tt> is for branches. There are three other base
|
|
classes: <tt>F3_1</tt> for register/register operations, <tt>F3_2</tt> for
|
|
register/immediate operations, and <tt>F3_3</tt> for floating-point
|
|
operations. <tt>SparcInstrInfo.td</tt> also adds the base class Pseudo for
|
|
synthetic SPARC instructions.
|
|
</p>
|
|
|
|
<p>
|
|
<tt>SparcInstrInfo.td</tt> largely consists of operand and instruction
|
|
definitions for the SPARC target. In <tt>SparcInstrInfo.td</tt>, the following
|
|
target description file entry, <tt>LDrr</tt>, defines the Load Integer
|
|
instruction for a Word (the <tt>LD</tt> SPARC opcode) from a memory address to a
|
|
register. The first parameter, the value 3 (<tt>11<sub>2</sub></tt>), is the
|
|
operation value for this category of operation. The second parameter
|
|
(<tt>000000<sub>2</sub></tt>) is the specific operation value
|
|
for <tt>LD</tt>/Load Word. The third parameter is the output destination, which
|
|
is a register operand and defined in the <tt>Register</tt> target description
|
|
file (<tt>IntRegs</tt>).
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>def LDrr : F3_1 <3, 0b000000, (outs IntRegs:$dst), (ins MEMrr:$addr),
|
|
"ld [$addr], $dst",
|
|
[(set IntRegs:$dst, (load ADDRrr:$addr))]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The fourth parameter is the input source, which uses the address
|
|
operand <tt>MEMrr</tt> that is defined earlier in <tt>SparcInstrInfo.td</tt>:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>def MEMrr : Operand<i32> {
|
|
let PrintMethod = "printMemOperand";
|
|
let MIOperandInfo = (ops IntRegs, IntRegs);
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The fifth parameter is a string that is used by the assembly printer and can be
|
|
left as an empty string until the assembly printer interface is implemented. The
|
|
sixth and final parameter is the pattern used to match the instruction during
|
|
the SelectionDAG Select Phase described in
|
|
(<a href="http://www.llvm.org/docs/CodeGenerator.html">The LLVM
|
|
Target-Independent Code Generator</a>). This parameter is detailed in the next
|
|
section, <a href="#InstructionSelector">Instruction Selector</a>.
|
|
</p>
|
|
|
|
<p>
|
|
Instruction class definitions are not overloaded for different operand types, so
|
|
separate versions of instructions are needed for register, memory, or immediate
|
|
value operands. For example, to perform a Load Integer instruction for a Word
|
|
from an immediate operand to a register, the following instruction class is
|
|
defined:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>def LDri : F3_2 <3, 0b000000, (outs IntRegs:$dst), (ins MEMri:$addr),
|
|
"ld [$addr], $dst",
|
|
[(set IntRegs:$dst, (load ADDRri:$addr))]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
Writing these definitions for so many similar instructions can involve a lot of
|
|
cut and paste. In td files, the <tt>multiclass</tt> directive enables the
|
|
creation of templates to define several instruction classes at once (using
|
|
the <tt>defm</tt> directive). For example in <tt>SparcInstrInfo.td</tt>, the
|
|
<tt>multiclass</tt> pattern <tt>F3_12</tt> is defined to create 2 instruction
|
|
classes each time <tt>F3_12</tt> is invoked:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>multiclass F3_12 <string OpcStr, bits<6> Op3Val, SDNode OpNode> {
|
|
def rr : F3_1 <2, Op3Val,
|
|
(outs IntRegs:$dst), (ins IntRegs:$b, IntRegs:$c),
|
|
!strconcat(OpcStr, " $b, $c, $dst"),
|
|
[(set IntRegs:$dst, (OpNode IntRegs:$b, IntRegs:$c))]>;
|
|
def ri : F3_2 <2, Op3Val,
|
|
(outs IntRegs:$dst), (ins IntRegs:$b, i32imm:$c),
|
|
!strconcat(OpcStr, " $b, $c, $dst"),
|
|
[(set IntRegs:$dst, (OpNode IntRegs:$b, simm13:$c))]>;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
So when the <tt>defm</tt> directive is used for the <tt>XOR</tt>
|
|
and <tt>ADD</tt> instructions, as seen below, it creates four instruction
|
|
objects: <tt>XORrr</tt>, <tt>XORri</tt>, <tt>ADDrr</tt>, and <tt>ADDri</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
defm XOR : F3_12<"xor", 0b000011, xor>;
|
|
defm ADD : F3_12<"add", 0b000000, add>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>SparcInstrInfo.td</tt> also includes definitions for condition codes that
|
|
are referenced by branch instructions. The following definitions
|
|
in <tt>SparcInstrInfo.td</tt> indicate the bit location of the SPARC condition
|
|
code. For example, the 10<sup>th</sup> bit represents the 'greater than'
|
|
condition for integers, and the 22<sup>nd</sup> bit represents the 'greater
|
|
than' condition for floats.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def ICC_NE : ICC_VAL< 9>; // Not Equal
|
|
def ICC_E : ICC_VAL< 1>; // Equal
|
|
def ICC_G : ICC_VAL<10>; // Greater
|
|
...
|
|
def FCC_U : FCC_VAL<23>; // Unordered
|
|
def FCC_G : FCC_VAL<22>; // Greater
|
|
def FCC_UG : FCC_VAL<21>; // Unordered or Greater
|
|
...
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
(Note that <tt>Sparc.h</tt> also defines enums that correspond to the same SPARC
|
|
condition codes. Care must be taken to ensure the values in <tt>Sparc.h</tt>
|
|
correspond to the values in <tt>SparcInstrInfo.td</tt>. I.e.,
|
|
<tt>SPCC::ICC_NE = 9</tt>, <tt>SPCC::FCC_U = 23</tt> and so on.)
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="operandMapping">Instruction Operand Mapping</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The code generator backend maps instruction operands to fields in the
|
|
instruction. Operands are assigned to unbound fields in the instruction in the
|
|
order they are defined. Fields are bound when they are assigned a value. For
|
|
example, the Sparc target defines the <tt>XNORrr</tt> instruction as
|
|
a <tt>F3_1</tt> format instruction having three operands.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def XNORrr : F3_1<2, 0b000111,
|
|
(outs IntRegs:$dst), (ins IntRegs:$b, IntRegs:$c),
|
|
"xnor $b, $c, $dst",
|
|
[(set IntRegs:$dst, (not (xor IntRegs:$b, IntRegs:$c)))]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The instruction templates in <tt>SparcInstrFormats.td</tt> show the base class
|
|
for <tt>F3_1</tt> is <tt>InstSP</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class InstSP<dag outs, dag ins, string asmstr, list<dag> pattern> : Instruction {
|
|
field bits<32> Inst;
|
|
let Namespace = "SP";
|
|
bits<2> op;
|
|
let Inst{31-30} = op;
|
|
dag OutOperandList = outs;
|
|
dag InOperandList = ins;
|
|
let AsmString = asmstr;
|
|
let Pattern = pattern;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p><tt>InstSP</tt> leaves the <tt>op</tt> field unbound.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class F3<dag outs, dag ins, string asmstr, list<dag> pattern>
|
|
: InstSP<outs, ins, asmstr, pattern> {
|
|
bits<5> rd;
|
|
bits<6> op3;
|
|
bits<5> rs1;
|
|
let op{1} = 1; // Op = 2 or 3
|
|
let Inst{29-25} = rd;
|
|
let Inst{24-19} = op3;
|
|
let Inst{18-14} = rs1;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>F3</tt> binds the <tt>op</tt> field and defines the <tt>rd</tt>,
|
|
<tt>op3</tt>, and <tt>rs1</tt> fields. <tt>F3</tt> format instructions will
|
|
bind the operands <tt>rd</tt>, <tt>op3</tt>, and <tt>rs1</tt> fields.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class F3_1<bits<2> opVal, bits<6> op3val, dag outs, dag ins,
|
|
string asmstr, list<dag> pattern> : F3<outs, ins, asmstr, pattern> {
|
|
bits<8> asi = 0; // asi not currently used
|
|
bits<5> rs2;
|
|
let op = opVal;
|
|
let op3 = op3val;
|
|
let Inst{13} = 0; // i field = 0
|
|
let Inst{12-5} = asi; // address space identifier
|
|
let Inst{4-0} = rs2;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>F3_1</tt> binds the <tt>op3</tt> field and defines the <tt>rs2</tt>
|
|
fields. <tt>F3_1</tt> format instructions will bind the operands to the <tt>rd</tt>,
|
|
<tt>rs1</tt>, and <tt>rs2</tt> fields. This results in the <tt>XNORrr</tt>
|
|
instruction binding <tt>$dst</tt>, <tt>$b</tt>, and <tt>$c</tt> operands to
|
|
the <tt>rd</tt>, <tt>rs1</tt>, and <tt>rs2</tt> fields respectively.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="implementInstr">Implement a subclass of </a>
|
|
<a href="http://www.llvm.org/docs/CodeGenerator.html#targetinstrinfo">TargetInstrInfo</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The final step is to hand code portions of <tt>XXXInstrInfo</tt>, which
|
|
implements the interface described in <tt>TargetInstrInfo.h</tt>. These
|
|
functions return <tt>0</tt> or a Boolean or they assert, unless
|
|
overridden. Here's a list of functions that are overridden for the SPARC
|
|
implementation in <tt>SparcInstrInfo.cpp</tt>:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>isMoveInstr</tt> — Return true if the instruction is a register to
|
|
register move; false, otherwise.</li>
|
|
|
|
<li><tt>isLoadFromStackSlot</tt> — If the specified machine instruction is
|
|
a direct load from a stack slot, return the register number of the
|
|
destination and the <tt>FrameIndex</tt> of the stack slot.</li>
|
|
|
|
<li><tt>isStoreToStackSlot</tt> — If the specified machine instruction is
|
|
a direct store to a stack slot, return the register number of the
|
|
destination and the <tt>FrameIndex</tt> of the stack slot.</li>
|
|
|
|
<li><tt>copyRegToReg</tt> — Copy values between a pair of registers.</li>
|
|
|
|
<li><tt>storeRegToStackSlot</tt> — Store a register value to a stack
|
|
slot.</li>
|
|
|
|
<li><tt>loadRegFromStackSlot</tt> — Load a register value from a stack
|
|
slot.</li>
|
|
|
|
<li><tt>storeRegToAddr</tt> — Store a register value to memory.</li>
|
|
|
|
<li><tt>loadRegFromAddr</tt> — Load a register value from memory.</li>
|
|
|
|
<li><tt>foldMemoryOperand</tt> — Attempt to combine instructions of any
|
|
load or store instruction for the specified operand(s).</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="branchFolding">Branch Folding and If Conversion</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
Performance can be improved by combining instructions or by eliminating
|
|
instructions that are never reached. The <tt>AnalyzeBranch</tt> method
|
|
in <tt>XXXInstrInfo</tt> may be implemented to examine conditional instructions
|
|
and remove unnecessary instructions. <tt>AnalyzeBranch</tt> looks at the end of
|
|
a machine basic block (MBB) for opportunities for improvement, such as branch
|
|
folding and if conversion. The <tt>BranchFolder</tt> and <tt>IfConverter</tt>
|
|
machine function passes (see the source files <tt>BranchFolding.cpp</tt> and
|
|
<tt>IfConversion.cpp</tt> in the <tt>lib/CodeGen</tt> directory) call
|
|
<tt>AnalyzeBranch</tt> to improve the control flow graph that represents the
|
|
instructions.
|
|
</p>
|
|
|
|
<p>
|
|
Several implementations of <tt>AnalyzeBranch</tt> (for ARM, Alpha, and X86) can
|
|
be examined as models for your own <tt>AnalyzeBranch</tt> implementation. Since
|
|
SPARC does not implement a useful <tt>AnalyzeBranch</tt>, the ARM target
|
|
implementation is shown below.
|
|
</p>
|
|
|
|
<p><tt>AnalyzeBranch</tt> returns a Boolean value and takes four parameters:</p>
|
|
|
|
<ul>
|
|
<li><tt>MachineBasicBlock &MBB</tt> — The incoming block to be
|
|
examined.</li>
|
|
|
|
<li><tt>MachineBasicBlock *&TBB</tt> — A destination block that is
|
|
returned. For a conditional branch that evaluates to true, <tt>TBB</tt> is
|
|
the destination.</li>
|
|
|
|
<li><tt>MachineBasicBlock *&FBB</tt> — For a conditional branch that
|
|
evaluates to false, <tt>FBB</tt> is returned as the destination.</li>
|
|
|
|
<li><tt>std::vector<MachineOperand> &Cond</tt> — List of
|
|
operands to evaluate a condition for a conditional branch.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
In the simplest case, if a block ends without a branch, then it falls through to
|
|
the successor block. No destination blocks are specified for either <tt>TBB</tt>
|
|
or <tt>FBB</tt>, so both parameters return <tt>NULL</tt>. The start of
|
|
the <tt>AnalyzeBranch</tt> (see code below for the ARM target) shows the
|
|
function parameters and the code for the simplest case.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>bool ARMInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
|
|
MachineBasicBlock *&TBB, MachineBasicBlock *&FBB,
|
|
std::vector<MachineOperand> &Cond) const
|
|
{
|
|
MachineBasicBlock::iterator I = MBB.end();
|
|
if (I == MBB.begin() || !isUnpredicatedTerminator(--I))
|
|
return false;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
If a block ends with a single unconditional branch instruction, then
|
|
<tt>AnalyzeBranch</tt> (shown below) should return the destination of that
|
|
branch in the <tt>TBB</tt> parameter.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (LastOpc == ARM::B || LastOpc == ARM::tB) {
|
|
TBB = LastInst->getOperand(0).getMBB();
|
|
return false;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
If a block ends with two unconditional branches, then the second branch is never
|
|
reached. In that situation, as shown below, remove the last branch instruction
|
|
and return the penultimate branch in the <tt>TBB</tt> parameter.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if ((SecondLastOpc == ARM::B || SecondLastOpc==ARM::tB) &&
|
|
(LastOpc == ARM::B || LastOpc == ARM::tB)) {
|
|
TBB = SecondLastInst->getOperand(0).getMBB();
|
|
I = LastInst;
|
|
I->eraseFromParent();
|
|
return false;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
A block may end with a single conditional branch instruction that falls through
|
|
to successor block if the condition evaluates to false. In that case,
|
|
<tt>AnalyzeBranch</tt> (shown below) should return the destination of that
|
|
conditional branch in the <tt>TBB</tt> parameter and a list of operands in
|
|
the <tt>Cond</tt> parameter to evaluate the condition.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (LastOpc == ARM::Bcc || LastOpc == ARM::tBcc) {
|
|
// Block ends with fall-through condbranch.
|
|
TBB = LastInst->getOperand(0).getMBB();
|
|
Cond.push_back(LastInst->getOperand(1));
|
|
Cond.push_back(LastInst->getOperand(2));
|
|
return false;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
If a block ends with both a conditional branch and an ensuing unconditional
|
|
branch, then <tt>AnalyzeBranch</tt> (shown below) should return the conditional
|
|
branch destination (assuming it corresponds to a conditional evaluation of
|
|
'<tt>true</tt>') in the <tt>TBB</tt> parameter and the unconditional branch
|
|
destination in the <tt>FBB</tt> (corresponding to a conditional evaluation of
|
|
'<tt>false</tt>'). A list of operands to evaluate the condition should be
|
|
returned in the <tt>Cond</tt> parameter.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
unsigned SecondLastOpc = SecondLastInst->getOpcode();
|
|
|
|
if ((SecondLastOpc == ARM::Bcc && LastOpc == ARM::B) ||
|
|
(SecondLastOpc == ARM::tBcc && LastOpc == ARM::tB)) {
|
|
TBB = SecondLastInst->getOperand(0).getMBB();
|
|
Cond.push_back(SecondLastInst->getOperand(1));
|
|
Cond.push_back(SecondLastInst->getOperand(2));
|
|
FBB = LastInst->getOperand(0).getMBB();
|
|
return false;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
For the last two cases (ending with a single conditional branch or ending with
|
|
one conditional and one unconditional branch), the operands returned in
|
|
the <tt>Cond</tt> parameter can be passed to methods of other instructions to
|
|
create new branches or perform other operations. An implementation
|
|
of <tt>AnalyzeBranch</tt> requires the helper methods <tt>RemoveBranch</tt>
|
|
and <tt>InsertBranch</tt> to manage subsequent operations.
|
|
</p>
|
|
|
|
<p>
|
|
<tt>AnalyzeBranch</tt> should return false indicating success in most circumstances.
|
|
<tt>AnalyzeBranch</tt> should only return true when the method is stumped about what to
|
|
do, for example, if a block has three terminating branches. <tt>AnalyzeBranch</tt> may
|
|
return true if it encounters a terminator it cannot handle, such as an indirect
|
|
branch.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="InstructionSelector">Instruction Selector</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
LLVM uses a <tt>SelectionDAG</tt> to represent LLVM IR instructions, and nodes
|
|
of the <tt>SelectionDAG</tt> ideally represent native target
|
|
instructions. During code generation, instruction selection passes are performed
|
|
to convert non-native DAG instructions into native target-specific
|
|
instructions. The pass described in <tt>XXXISelDAGToDAG.cpp</tt> is used to
|
|
match patterns and perform DAG-to-DAG instruction selection. Optionally, a pass
|
|
may be defined (in <tt>XXXBranchSelector.cpp</tt>) to perform similar DAG-to-DAG
|
|
operations for branch instructions. Later, the code in
|
|
<tt>XXXISelLowering.cpp</tt> replaces or removes operations and data types not
|
|
supported natively (legalizes) in a <tt>SelectionDAG</tt>.
|
|
</p>
|
|
|
|
<p>
|
|
TableGen generates code for instruction selection using the following target
|
|
description input files:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>XXXInstrInfo.td</tt> — Contains definitions of instructions in a
|
|
target-specific instruction set, generates <tt>XXXGenDAGISel.inc</tt>, which
|
|
is included in <tt>XXXISelDAGToDAG.cpp</tt>.</li>
|
|
|
|
<li><tt>XXXCallingConv.td</tt> — Contains the calling and return value
|
|
conventions for the target architecture, and it generates
|
|
<tt>XXXGenCallingConv.inc</tt>, which is included in
|
|
<tt>XXXISelLowering.cpp</tt>.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
The implementation of an instruction selection pass must include a header that
|
|
declares the <tt>FunctionPass</tt> class or a subclass of <tt>FunctionPass</tt>. In
|
|
<tt>XXXTargetMachine.cpp</tt>, a Pass Manager (PM) should add each instruction
|
|
selection pass into the queue of passes to run.
|
|
</p>
|
|
|
|
<p>
|
|
The LLVM static compiler (<tt>llc</tt>) is an excellent tool for visualizing the
|
|
contents of DAGs. To display the <tt>SelectionDAG</tt> before or after specific
|
|
processing phases, use the command line options for <tt>llc</tt>, described
|
|
at <a href="http://llvm.org/docs/CodeGenerator.html#selectiondag_process">
|
|
SelectionDAG Instruction Selection Process</a>.
|
|
</p>
|
|
|
|
<p>
|
|
To describe instruction selector behavior, you should add patterns for lowering
|
|
LLVM code into a <tt>SelectionDAG</tt> as the last parameter of the instruction
|
|
definitions in <tt>XXXInstrInfo.td</tt>. For example, in
|
|
<tt>SparcInstrInfo.td</tt>, this entry defines a register store operation, and
|
|
the last parameter describes a pattern with the store DAG operator.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def STrr : F3_1< 3, 0b000100, (outs), (ins MEMrr:$addr, IntRegs:$src),
|
|
"st $src, [$addr]", [(store IntRegs:$src, ADDRrr:$addr)]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>ADDRrr</tt> is a memory mode that is also defined in
|
|
<tt>SparcInstrInfo.td</tt>:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def ADDRrr : ComplexPattern<i32, 2, "SelectADDRrr", [], []>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The definition of <tt>ADDRrr</tt> refers to <tt>SelectADDRrr</tt>, which is a
|
|
function defined in an implementation of the Instructor Selector (such
|
|
as <tt>SparcISelDAGToDAG.cpp</tt>).
|
|
</p>
|
|
|
|
<p>
|
|
In <tt>lib/Target/TargetSelectionDAG.td</tt>, the DAG operator for store is
|
|
defined below:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def store : PatFrag<(ops node:$val, node:$ptr),
|
|
(st node:$val, node:$ptr), [{
|
|
if (StoreSDNode *ST = dyn_cast<StoreSDNode>(N))
|
|
return !ST->isTruncatingStore() &&
|
|
ST->getAddressingMode() == ISD::UNINDEXED;
|
|
return false;
|
|
}]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>XXXInstrInfo.td</tt> also generates (in <tt>XXXGenDAGISel.inc</tt>) the
|
|
<tt>SelectCode</tt> method that is used to call the appropriate processing
|
|
method for an instruction. In this example, <tt>SelectCode</tt>
|
|
calls <tt>Select_ISD_STORE</tt> for the <tt>ISD::STORE</tt> opcode.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
SDNode *SelectCode(SDValue N) {
|
|
...
|
|
MVT::ValueType NVT = N.getNode()->getValueType(0);
|
|
switch (N.getOpcode()) {
|
|
case ISD::STORE: {
|
|
switch (NVT) {
|
|
default:
|
|
return Select_ISD_STORE(N);
|
|
break;
|
|
}
|
|
break;
|
|
}
|
|
...
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The pattern for <tt>STrr</tt> is matched, so elsewhere in
|
|
<tt>XXXGenDAGISel.inc</tt>, code for <tt>STrr</tt> is created for
|
|
<tt>Select_ISD_STORE</tt>. The <tt>Emit_22</tt> method is also generated
|
|
in <tt>XXXGenDAGISel.inc</tt> to complete the processing of this
|
|
instruction.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
SDNode *Select_ISD_STORE(const SDValue &N) {
|
|
SDValue Chain = N.getOperand(0);
|
|
if (Predicate_store(N.getNode())) {
|
|
SDValue N1 = N.getOperand(1);
|
|
SDValue N2 = N.getOperand(2);
|
|
SDValue CPTmp0;
|
|
SDValue CPTmp1;
|
|
|
|
// Pattern: (st:void IntRegs:i32:$src,
|
|
// ADDRrr:i32:$addr)<<P:Predicate_store>>
|
|
// Emits: (STrr:void ADDRrr:i32:$addr, IntRegs:i32:$src)
|
|
// Pattern complexity = 13 cost = 1 size = 0
|
|
if (SelectADDRrr(N, N2, CPTmp0, CPTmp1) &&
|
|
N1.getNode()->getValueType(0) == MVT::i32 &&
|
|
N2.getNode()->getValueType(0) == MVT::i32) {
|
|
return Emit_22(N, SP::STrr, CPTmp0, CPTmp1);
|
|
}
|
|
...
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="LegalizePhase">The SelectionDAG Legalize Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The Legalize phase converts a DAG to use types and operations that are natively
|
|
supported by the target. For natively unsupported types and operations, you need
|
|
to add code to the target-specific XXXTargetLowering implementation to convert
|
|
unsupported types and operations to supported ones.
|
|
</p>
|
|
|
|
<p>
|
|
In the constructor for the <tt>XXXTargetLowering</tt> class, first use the
|
|
<tt>addRegisterClass</tt> method to specify which types are supports and which
|
|
register classes are associated with them. The code for the register classes are
|
|
generated by TableGen from <tt>XXXRegisterInfo.td</tt> and placed
|
|
in <tt>XXXGenRegisterInfo.h.inc</tt>. For example, the implementation of the
|
|
constructor for the SparcTargetLowering class (in
|
|
<tt>SparcISelLowering.cpp</tt>) starts with the following code:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
addRegisterClass(MVT::i32, SP::IntRegsRegisterClass);
|
|
addRegisterClass(MVT::f32, SP::FPRegsRegisterClass);
|
|
addRegisterClass(MVT::f64, SP::DFPRegsRegisterClass);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
You should examine the node types in the <tt>ISD</tt> namespace
|
|
(<tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>) and determine which
|
|
operations the target natively supports. For operations that do <b>not</b> have
|
|
native support, add a callback to the constructor for the XXXTargetLowering
|
|
class, so the instruction selection process knows what to do. The TargetLowering
|
|
class callback methods (declared in <tt>llvm/Target/TargetLowering.h</tt>) are:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>setOperationAction</tt> — General operation.</li>
|
|
|
|
<li><tt>setLoadExtAction</tt> — Load with extension.</li>
|
|
|
|
<li><tt>setTruncStoreAction</tt> — Truncating store.</li>
|
|
|
|
<li><tt>setIndexedLoadAction</tt> — Indexed load.</li>
|
|
|
|
<li><tt>setIndexedStoreAction</tt> — Indexed store.</li>
|
|
|
|
<li><tt>setConvertAction</tt> — Type conversion.</li>
|
|
|
|
<li><tt>setCondCodeAction</tt> — Support for a given condition code.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
Note: on older releases, <tt>setLoadXAction</tt> is used instead
|
|
of <tt>setLoadExtAction</tt>. Also, on older releases,
|
|
<tt>setCondCodeAction</tt> may not be supported. Examine your release
|
|
to see what methods are specifically supported.
|
|
</p>
|
|
|
|
<p>
|
|
These callbacks are used to determine that an operation does or does not work
|
|
with a specified type (or types). And in all cases, the third parameter is
|
|
a <tt>LegalAction</tt> type enum value: <tt>Promote</tt>, <tt>Expand</tt>,
|
|
<tt>Custom</tt>, or <tt>Legal</tt>. <tt>SparcISelLowering.cpp</tt>
|
|
contains examples of all four <tt>LegalAction</tt> values.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="promote">Promote</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
For an operation without native support for a given type, the specified type may
|
|
be promoted to a larger type that is supported. For example, SPARC does not
|
|
support a sign-extending load for Boolean values (<tt>i1</tt> type), so
|
|
in <tt>SparcISelLowering.cpp</tt> the third parameter below, <tt>Promote</tt>,
|
|
changes <tt>i1</tt> type values to a large type before loading.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
setLoadExtAction(ISD::SEXTLOAD, MVT::i1, Promote);
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="expand">Expand</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
For a type without native support, a value may need to be broken down further,
|
|
rather than promoted. For an operation without native support, a combination of
|
|
other operations may be used to similar effect. In SPARC, the floating-point
|
|
sine and cosine trig operations are supported by expansion to other operations,
|
|
as indicated by the third parameter, <tt>Expand</tt>, to
|
|
<tt>setOperationAction</tt>:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
setOperationAction(ISD::FSIN, MVT::f32, Expand);
|
|
setOperationAction(ISD::FCOS, MVT::f32, Expand);
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="custom">Custom</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
For some operations, simple type promotion or operation expansion may be
|
|
insufficient. In some cases, a special intrinsic function must be implemented.
|
|
</p>
|
|
|
|
<p>
|
|
For example, a constant value may require special treatment, or an operation may
|
|
require spilling and restoring registers in the stack and working with register
|
|
allocators.
|
|
</p>
|
|
|
|
<p>
|
|
As seen in <tt>SparcISelLowering.cpp</tt> code below, to perform a type
|
|
conversion from a floating point value to a signed integer, first the
|
|
<tt>setOperationAction</tt> should be called with <tt>Custom</tt> as the third
|
|
parameter:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
setOperationAction(ISD::FP_TO_SINT, MVT::i32, Custom);
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
In the <tt>LowerOperation</tt> method, for each <tt>Custom</tt> operation, a
|
|
case statement should be added to indicate what function to call. In the
|
|
following code, an <tt>FP_TO_SINT</tt> opcode will call
|
|
the <tt>LowerFP_TO_SINT</tt> method:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
SDValue SparcTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) {
|
|
switch (Op.getOpcode()) {
|
|
case ISD::FP_TO_SINT: return LowerFP_TO_SINT(Op, DAG);
|
|
...
|
|
}
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
Finally, the <tt>LowerFP_TO_SINT</tt> method is implemented, using an FP
|
|
register to convert the floating-point value to an integer.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
static SDValue LowerFP_TO_SINT(SDValue Op, SelectionDAG &DAG) {
|
|
assert(Op.getValueType() == MVT::i32);
|
|
Op = DAG.getNode(SPISD::FTOI, MVT::f32, Op.getOperand(0));
|
|
return DAG.getNode(ISD::BIT_CONVERT, MVT::i32, Op);
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="legal">Legal</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The <tt>Legal</tt> LegalizeAction enum value simply indicates that an
|
|
operation <b>is</b> natively supported. <tt>Legal</tt> represents the default
|
|
condition, so it is rarely used. In <tt>SparcISelLowering.cpp</tt>, the action
|
|
for <tt>CTPOP</tt> (an operation to count the bits set in an integer) is
|
|
natively supported only for SPARC v9. The following code enables
|
|
the <tt>Expand</tt> conversion technique for non-v9 SPARC implementations.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
setOperationAction(ISD::CTPOP, MVT::i32, Expand);
|
|
...
|
|
if (TM.getSubtarget<SparcSubtarget>().isV9())
|
|
setOperationAction(ISD::CTPOP, MVT::i32, Legal);
|
|
case ISD::SETULT: return SPCC::ICC_CS;
|
|
case ISD::SETULE: return SPCC::ICC_LEU;
|
|
case ISD::SETUGT: return SPCC::ICC_GU;
|
|
case ISD::SETUGE: return SPCC::ICC_CC;
|
|
}
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="callingConventions">Calling Conventions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
To support target-specific calling conventions, <tt>XXXGenCallingConv.td</tt>
|
|
uses interfaces (such as CCIfType and CCAssignToReg) that are defined in
|
|
<tt>lib/Target/TargetCallingConv.td</tt>. TableGen can take the target
|
|
descriptor file <tt>XXXGenCallingConv.td</tt> and generate the header
|
|
file <tt>XXXGenCallingConv.inc</tt>, which is typically included
|
|
in <tt>XXXISelLowering.cpp</tt>. You can use the interfaces in
|
|
<tt>TargetCallingConv.td</tt> to specify:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>The order of parameter allocation.</li>
|
|
|
|
<li>Where parameters and return values are placed (that is, on the stack or in
|
|
registers).</li>
|
|
|
|
<li>Which registers may be used.</li>
|
|
|
|
<li>Whether the caller or callee unwinds the stack.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
The following example demonstrates the use of the <tt>CCIfType</tt> and
|
|
<tt>CCAssignToReg</tt> interfaces. If the <tt>CCIfType</tt> predicate is true
|
|
(that is, if the current argument is of type <tt>f32</tt> or <tt>f64</tt>), then
|
|
the action is performed. In this case, the <tt>CCAssignToReg</tt> action assigns
|
|
the argument value to the first available register: either <tt>R0</tt>
|
|
or <tt>R1</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
CCIfType<[f32,f64], CCAssignToReg<[R0, R1]>>
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>SparcCallingConv.td</tt> contains definitions for a target-specific
|
|
return-value calling convention (RetCC_Sparc32) and a basic 32-bit C calling
|
|
convention (<tt>CC_Sparc32</tt>). The definition of <tt>RetCC_Sparc32</tt>
|
|
(shown below) indicates which registers are used for specified scalar return
|
|
types. A single-precision float is returned to register <tt>F0</tt>, and a
|
|
double-precision float goes to register <tt>D0</tt>. A 32-bit integer is
|
|
returned in register <tt>I0</tt> or <tt>I1</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def RetCC_Sparc32 : CallingConv<[
|
|
CCIfType<[i32], CCAssignToReg<[I0, I1]>>,
|
|
CCIfType<[f32], CCAssignToReg<[F0]>>,
|
|
CCIfType<[f64], CCAssignToReg<[D0]>>
|
|
]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The definition of <tt>CC_Sparc32</tt> in <tt>SparcCallingConv.td</tt> introduces
|
|
<tt>CCAssignToStack</tt>, which assigns the value to a stack slot with the
|
|
specified size and alignment. In the example below, the first parameter, 4,
|
|
indicates the size of the slot, and the second parameter, also 4, indicates the
|
|
stack alignment along 4-byte units. (Special cases: if size is zero, then the
|
|
ABI size is used; if alignment is zero, then the ABI alignment is used.)
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def CC_Sparc32 : CallingConv<[
|
|
// All arguments get passed in integer registers if there is space.
|
|
CCIfType<[i32, f32, f64], CCAssignToReg<[I0, I1, I2, I3, I4, I5]>>,
|
|
CCAssignToStack<4, 4>
|
|
]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>CCDelegateTo</tt> is another commonly used interface, which tries to find a
|
|
specified sub-calling convention, and, if a match is found, it is invoked. In
|
|
the following example (in <tt>X86CallingConv.td</tt>), the definition of
|
|
<tt>RetCC_X86_32_C</tt> ends with <tt>CCDelegateTo</tt>. After the current value
|
|
is assigned to the register <tt>ST0</tt> or <tt>ST1</tt>,
|
|
the <tt>RetCC_X86Common</tt> is invoked.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def RetCC_X86_32_C : CallingConv<[
|
|
CCIfType<[f32], CCAssignToReg<[ST0, ST1]>>,
|
|
CCIfType<[f64], CCAssignToReg<[ST0, ST1]>>,
|
|
CCDelegateTo<RetCC_X86Common>
|
|
]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>CCIfCC</tt> is an interface that attempts to match the given name to the
|
|
current calling convention. If the name identifies the current calling
|
|
convention, then a specified action is invoked. In the following example (in
|
|
<tt>X86CallingConv.td</tt>), if the <tt>Fast</tt> calling convention is in use,
|
|
then <tt>RetCC_X86_32_Fast</tt> is invoked. If the <tt>SSECall</tt> calling
|
|
convention is in use, then <tt>RetCC_X86_32_SSE</tt> is invoked.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def RetCC_X86_32 : CallingConv<[
|
|
CCIfCC<"CallingConv::Fast", CCDelegateTo<RetCC_X86_32_Fast>>,
|
|
CCIfCC<"CallingConv::X86_SSECall", CCDelegateTo<RetCC_X86_32_SSE>>,
|
|
CCDelegateTo<RetCC_X86_32_C>
|
|
]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Other calling convention interfaces include:</p>
|
|
|
|
<ul>
|
|
<li><tt>CCIf <predicate, action></tt> — If the predicate matches,
|
|
apply the action.</li>
|
|
|
|
<li><tt>CCIfInReg <action></tt> — If the argument is marked with the
|
|
'<tt>inreg</tt>' attribute, then apply the action.</li>
|
|
|
|
<li><tt>CCIfNest <action></tt> — Inf the argument is marked with the
|
|
'<tt>nest</tt>' attribute, then apply the action.</li>
|
|
|
|
<li><tt>CCIfNotVarArg <action></tt> — If the current function does
|
|
not take a variable number of arguments, apply the action.</li>
|
|
|
|
<li><tt>CCAssignToRegWithShadow <registerList, shadowList></tt> —
|
|
similar to <tt>CCAssignToReg</tt>, but with a shadow list of registers.</li>
|
|
|
|
<li><tt>CCPassByVal <size, align></tt> — Assign value to a stack
|
|
slot with the minimum specified size and alignment.</li>
|
|
|
|
<li><tt>CCPromoteToType <type></tt> — Promote the current value to
|
|
the specified type.</li>
|
|
|
|
<li><tt>CallingConv <[actions]></tt> — Define each calling
|
|
convention that is supported.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="assemblyPrinter">Assembly Printer</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
During the code emission stage, the code generator may utilize an LLVM pass to
|
|
produce assembly output. To do this, you want to implement the code for a
|
|
printer that converts LLVM IR to a GAS-format assembly language for your target
|
|
machine, using the following steps:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Define all the assembly strings for your target, adding them to the
|
|
instructions defined in the <tt>XXXInstrInfo.td</tt> file.
|
|
(See <a href="#InstructionSet">Instruction Set</a>.) TableGen will produce
|
|
an output file (<tt>XXXGenAsmWriter.inc</tt>) with an implementation of
|
|
the <tt>printInstruction</tt> method for the XXXAsmPrinter class.</li>
|
|
|
|
<li>Write <tt>XXXTargetAsmInfo.h</tt>, which contains the bare-bones declaration
|
|
of the <tt>XXXTargetAsmInfo</tt> class (a subclass
|
|
of <tt>TargetAsmInfo</tt>).</li>
|
|
|
|
<li>Write <tt>XXXTargetAsmInfo.cpp</tt>, which contains target-specific values
|
|
for <tt>TargetAsmInfo</tt> properties and sometimes new implementations for
|
|
methods.</li>
|
|
|
|
<li>Write <tt>XXXAsmPrinter.cpp</tt>, which implements the <tt>AsmPrinter</tt>
|
|
class that performs the LLVM-to-assembly conversion.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
The code in <tt>XXXTargetAsmInfo.h</tt> is usually a trivial declaration of the
|
|
<tt>XXXTargetAsmInfo</tt> class for use in <tt>XXXTargetAsmInfo.cpp</tt>.
|
|
Similarly, <tt>XXXTargetAsmInfo.cpp</tt> usually has a few declarations of
|
|
<tt>XXXTargetAsmInfo</tt> replacement values that override the default values
|
|
in <tt>TargetAsmInfo.cpp</tt>. For example in <tt>SparcTargetAsmInfo.cpp</tt>:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
SparcTargetAsmInfo::SparcTargetAsmInfo(const SparcTargetMachine &TM) {
|
|
Data16bitsDirective = "\t.half\t";
|
|
Data32bitsDirective = "\t.word\t";
|
|
Data64bitsDirective = 0; // .xword is only supported by V9.
|
|
ZeroDirective = "\t.skip\t";
|
|
CommentString = "!";
|
|
ConstantPoolSection = "\t.section \".rodata\",#alloc\n";
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The X86 assembly printer implementation (<tt>X86TargetAsmInfo</tt>) is an
|
|
example where the target specific <tt>TargetAsmInfo</tt> class uses overridden
|
|
methods: <tt>ExpandInlineAsm</tt> and <tt>PreferredEHDataFormat</tt>.
|
|
</p>
|
|
|
|
<p>
|
|
A target-specific implementation of AsmPrinter is written in
|
|
<tt>XXXAsmPrinter.cpp</tt>, which implements the <tt>AsmPrinter</tt> class that
|
|
converts the LLVM to printable assembly. The implementation must include the
|
|
following headers that have declarations for the <tt>AsmPrinter</tt> and
|
|
<tt>MachineFunctionPass</tt> classes. The <tt>MachineFunctionPass</tt> is a
|
|
subclass of <tt>FunctionPass</tt>.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
#include "llvm/CodeGen/AsmPrinter.h"
|
|
#include "llvm/CodeGen/MachineFunctionPass.h"
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
As a <tt>FunctionPass</tt>, <tt>AsmPrinter</tt> first
|
|
calls <tt>doInitialization</tt> to set up the <tt>AsmPrinter</tt>. In
|
|
<tt>SparcAsmPrinter</tt>, a <tt>Mangler</tt> object is instantiated to process
|
|
variable names.
|
|
</p>
|
|
|
|
<p>
|
|
In <tt>XXXAsmPrinter.cpp</tt>, the <tt>runOnMachineFunction</tt> method
|
|
(declared in <tt>MachineFunctionPass</tt>) must be implemented
|
|
for <tt>XXXAsmPrinter</tt>. In <tt>MachineFunctionPass</tt>,
|
|
the <tt>runOnFunction</tt> method invokes <tt>runOnMachineFunction</tt>.
|
|
Target-specific implementations of <tt>runOnMachineFunction</tt> differ, but
|
|
generally do the following to process each machine function:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Call <tt>SetupMachineFunction</tt> to perform initialization.</li>
|
|
|
|
<li>Call <tt>EmitConstantPool</tt> to print out (to the output stream) constants
|
|
which have been spilled to memory.</li>
|
|
|
|
<li>Call <tt>EmitJumpTableInfo</tt> to print out jump tables used by the current
|
|
function.</li>
|
|
|
|
<li>Print out the label for the current function.</li>
|
|
|
|
<li>Print out the code for the function, including basic block labels and the
|
|
assembly for the instruction (using <tt>printInstruction</tt>)</li>
|
|
</ul>
|
|
|
|
<p>
|
|
The <tt>XXXAsmPrinter</tt> implementation must also include the code generated
|
|
by TableGen that is output in the <tt>XXXGenAsmWriter.inc</tt> file. The code
|
|
in <tt>XXXGenAsmWriter.inc</tt> contains an implementation of the
|
|
<tt>printInstruction</tt> method that may call these methods:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>printOperand</tt></li>
|
|
|
|
<li><tt>printMemOperand</tt></li>
|
|
|
|
<li><tt>printCCOperand (for conditional statements)</tt></li>
|
|
|
|
<li><tt>printDataDirective</tt></li>
|
|
|
|
<li><tt>printDeclare</tt></li>
|
|
|
|
<li><tt>printImplicitDef</tt></li>
|
|
|
|
<li><tt>printInlineAsm</tt></li>
|
|
|
|
<li><tt>printLabel</tt></li>
|
|
|
|
<li><tt>printPICJumpTableEntry</tt></li>
|
|
|
|
<li><tt>printPICJumpTableSetLabel</tt></li>
|
|
</ul>
|
|
|
|
<p>
|
|
The implementations of <tt>printDeclare</tt>, <tt>printImplicitDef</tt>,
|
|
<tt>printInlineAsm</tt>, and <tt>printLabel</tt> in <tt>AsmPrinter.cpp</tt> are
|
|
generally adequate for printing assembly and do not need to be
|
|
overridden. (<tt>printBasicBlockLabel</tt> is another method that is implemented
|
|
in <tt>AsmPrinter.cpp</tt> that may be directly used in an implementation of
|
|
<tt>XXXAsmPrinter</tt>.)
|
|
</p>
|
|
|
|
<p>
|
|
The <tt>printOperand</tt> method is implemented with a long switch/case
|
|
statement for the type of operand: register, immediate, basic block, external
|
|
symbol, global address, constant pool index, or jump table index. For an
|
|
instruction with a memory address operand, the <tt>printMemOperand</tt> method
|
|
should be implemented to generate the proper output. Similarly,
|
|
<tt>printCCOperand</tt> should be used to print a conditional operand.
|
|
</p>
|
|
|
|
<p><tt>doFinalization</tt> should be overridden in <tt>XXXAsmPrinter</tt>, and
|
|
it should be called to shut down the assembly printer. During
|
|
<tt>doFinalization</tt>, global variables and constants are printed to
|
|
output.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="subtargetSupport">Subtarget Support</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
Subtarget support is used to inform the code generation process of instruction
|
|
set variations for a given chip set. For example, the LLVM SPARC implementation
|
|
provided covers three major versions of the SPARC microprocessor architecture:
|
|
Version 8 (V8, which is a 32-bit architecture), Version 9 (V9, a 64-bit
|
|
architecture), and the UltraSPARC architecture. V8 has 16 double-precision
|
|
floating-point registers that are also usable as either 32 single-precision or 8
|
|
quad-precision registers. V8 is also purely big-endian. V9 has 32
|
|
double-precision floating-point registers that are also usable as 16
|
|
quad-precision registers, but cannot be used as single-precision registers. The
|
|
UltraSPARC architecture combines V9 with UltraSPARC Visual Instruction Set
|
|
extensions.
|
|
</p>
|
|
|
|
<p>
|
|
If subtarget support is needed, you should implement a target-specific
|
|
XXXSubtarget class for your architecture. This class should process the
|
|
command-line options <tt>-mcpu=</tt> and <tt>-mattr=</tt>.
|
|
</p>
|
|
|
|
<p>
|
|
TableGen uses definitions in the <tt>Target.td</tt> and <tt>Sparc.td</tt> files
|
|
to generate code in <tt>SparcGenSubtarget.inc</tt>. In <tt>Target.td</tt>, shown
|
|
below, the <tt>SubtargetFeature</tt> interface is defined. The first 4 string
|
|
parameters of the <tt>SubtargetFeature</tt> interface are a feature name, an
|
|
attribute set by the feature, the value of the attribute, and a description of
|
|
the feature. (The fifth parameter is a list of features whose presence is
|
|
implied, and its default value is an empty array.)
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class SubtargetFeature<string n, string a, string v, string d,
|
|
list<SubtargetFeature> i = []> {
|
|
string Name = n;
|
|
string Attribute = a;
|
|
string Value = v;
|
|
string Desc = d;
|
|
list<SubtargetFeature> Implies = i;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
In the <tt>Sparc.td</tt> file, the SubtargetFeature is used to define the
|
|
following features.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def FeatureV9 : SubtargetFeature<"v9", "IsV9", "true",
|
|
"Enable SPARC-V9 instructions">;
|
|
def FeatureV8Deprecated : SubtargetFeature<"deprecated-v8",
|
|
"V8DeprecatedInsts", "true",
|
|
"Enable deprecated V8 instructions in V9 mode">;
|
|
def FeatureVIS : SubtargetFeature<"vis", "IsVIS", "true",
|
|
"Enable UltraSPARC Visual Instruction Set extensions">;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
Elsewhere in <tt>Sparc.td</tt>, the Proc class is defined and then is used to
|
|
define particular SPARC processor subtypes that may have the previously
|
|
described features.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
class Proc<string Name, list<SubtargetFeature> Features>
|
|
: Processor<Name, NoItineraries, Features>;
|
|
|
|
def : Proc<"generic", []>;
|
|
def : Proc<"v8", []>;
|
|
def : Proc<"supersparc", []>;
|
|
def : Proc<"sparclite", []>;
|
|
def : Proc<"f934", []>;
|
|
def : Proc<"hypersparc", []>;
|
|
def : Proc<"sparclite86x", []>;
|
|
def : Proc<"sparclet", []>;
|
|
def : Proc<"tsc701", []>;
|
|
def : Proc<"v9", [FeatureV9]>;
|
|
def : Proc<"ultrasparc", [FeatureV9, FeatureV8Deprecated]>;
|
|
def : Proc<"ultrasparc3", [FeatureV9, FeatureV8Deprecated]>;
|
|
def : Proc<"ultrasparc3-vis", [FeatureV9, FeatureV8Deprecated, FeatureVIS]>;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
From <tt>Target.td</tt> and <tt>Sparc.td</tt> files, the resulting
|
|
SparcGenSubtarget.inc specifies enum values to identify the features, arrays of
|
|
constants to represent the CPU features and CPU subtypes, and the
|
|
ParseSubtargetFeatures method that parses the features string that sets
|
|
specified subtarget options. The generated <tt>SparcGenSubtarget.inc</tt> file
|
|
should be included in the <tt>SparcSubtarget.cpp</tt>. The target-specific
|
|
implementation of the XXXSubtarget method should follow this pseudocode:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
XXXSubtarget::XXXSubtarget(const Module &M, const std::string &FS) {
|
|
// Set the default features
|
|
// Determine default and user specified characteristics of the CPU
|
|
// Call ParseSubtargetFeatures(FS, CPU) to parse the features string
|
|
// Perform any additional operations
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="jitSupport">JIT Support</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The implementation of a target machine optionally includes a Just-In-Time (JIT)
|
|
code generator that emits machine code and auxiliary structures as binary output
|
|
that can be written directly to memory. To do this, implement JIT code
|
|
generation by performing the following steps:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Write an <tt>XXXCodeEmitter.cpp</tt> file that contains a machine function
|
|
pass that transforms target-machine instructions into relocatable machine
|
|
code.</li>
|
|
|
|
<li>Write an <tt>XXXJITInfo.cpp</tt> file that implements the JIT interfaces for
|
|
target-specific code-generation activities, such as emitting machine code
|
|
and stubs.</li>
|
|
|
|
<li>Modify <tt>XXXTargetMachine</tt> so that it provides a
|
|
<tt>TargetJITInfo</tt> object through its <tt>getJITInfo</tt> method.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
There are several different approaches to writing the JIT support code. For
|
|
instance, TableGen and target descriptor files may be used for creating a JIT
|
|
code generator, but are not mandatory. For the Alpha and PowerPC target
|
|
machines, TableGen is used to generate <tt>XXXGenCodeEmitter.inc</tt>, which
|
|
contains the binary coding of machine instructions and the
|
|
<tt>getBinaryCodeForInstr</tt> method to access those codes. Other JIT
|
|
implementations do not.
|
|
</p>
|
|
|
|
<p>
|
|
Both <tt>XXXJITInfo.cpp</tt> and <tt>XXXCodeEmitter.cpp</tt> must include the
|
|
<tt>llvm/CodeGen/MachineCodeEmitter.h</tt> header file that defines the
|
|
<tt>MachineCodeEmitter</tt> class containing code for several callback functions
|
|
that write data (in bytes, words, strings, etc.) to the output stream.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="mce">Machine Code Emitter</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
In <tt>XXXCodeEmitter.cpp</tt>, a target-specific of the <tt>Emitter</tt> class
|
|
is implemented as a function pass (subclass
|
|
of <tt>MachineFunctionPass</tt>). The target-specific implementation
|
|
of <tt>runOnMachineFunction</tt> (invoked by
|
|
<tt>runOnFunction</tt> in <tt>MachineFunctionPass</tt>) iterates through the
|
|
<tt>MachineBasicBlock</tt> calls <tt>emitInstruction</tt> to process each
|
|
instruction and emit binary code. <tt>emitInstruction</tt> is largely
|
|
implemented with case statements on the instruction types defined in
|
|
<tt>XXXInstrInfo.h</tt>. For example, in <tt>X86CodeEmitter.cpp</tt>,
|
|
the <tt>emitInstruction</tt> method is built around the following switch/case
|
|
statements:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
switch (Desc->TSFlags & X86::FormMask) {
|
|
case X86II::Pseudo: // for not yet implemented instructions
|
|
... // or pseudo-instructions
|
|
break;
|
|
case X86II::RawFrm: // for instructions with a fixed opcode value
|
|
...
|
|
break;
|
|
case X86II::AddRegFrm: // for instructions that have one register operand
|
|
... // added to their opcode
|
|
break;
|
|
case X86II::MRMDestReg:// for instructions that use the Mod/RM byte
|
|
... // to specify a destination (register)
|
|
break;
|
|
case X86II::MRMDestMem:// for instructions that use the Mod/RM byte
|
|
... // to specify a destination (memory)
|
|
break;
|
|
case X86II::MRMSrcReg: // for instructions that use the Mod/RM byte
|
|
... // to specify a source (register)
|
|
break;
|
|
case X86II::MRMSrcMem: // for instructions that use the Mod/RM byte
|
|
... // to specify a source (memory)
|
|
break;
|
|
case X86II::MRM0r: case X86II::MRM1r: // for instructions that operate on
|
|
case X86II::MRM2r: case X86II::MRM3r: // a REGISTER r/m operand and
|
|
case X86II::MRM4r: case X86II::MRM5r: // use the Mod/RM byte and a field
|
|
case X86II::MRM6r: case X86II::MRM7r: // to hold extended opcode data
|
|
...
|
|
break;
|
|
case X86II::MRM0m: case X86II::MRM1m: // for instructions that operate on
|
|
case X86II::MRM2m: case X86II::MRM3m: // a MEMORY r/m operand and
|
|
case X86II::MRM4m: case X86II::MRM5m: // use the Mod/RM byte and a field
|
|
case X86II::MRM6m: case X86II::MRM7m: // to hold extended opcode data
|
|
...
|
|
break;
|
|
case X86II::MRMInitReg: // for instructions whose source and
|
|
... // destination are the same register
|
|
break;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
The implementations of these case statements often first emit the opcode and
|
|
then get the operand(s). Then depending upon the operand, helper methods may be
|
|
called to process the operand(s). For example, in <tt>X86CodeEmitter.cpp</tt>,
|
|
for the <tt>X86II::AddRegFrm</tt> case, the first data emitted
|
|
(by <tt>emitByte</tt>) is the opcode added to the register operand. Then an
|
|
object representing the machine operand, <tt>MO1</tt>, is extracted. The helper
|
|
methods such as <tt>isImmediate</tt>,
|
|
<tt>isGlobalAddress</tt>, <tt>isExternalSymbol</tt>, <tt>isConstantPoolIndex</tt>, and
|
|
<tt>isJumpTableIndex</tt> determine the operand
|
|
type. (<tt>X86CodeEmitter.cpp</tt> also has private methods such
|
|
as <tt>emitConstant</tt>, <tt>emitGlobalAddress</tt>,
|
|
<tt>emitExternalSymbolAddress</tt>, <tt>emitConstPoolAddress</tt>,
|
|
and <tt>emitJumpTableAddress</tt> that emit the data into the output stream.)
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
case X86II::AddRegFrm:
|
|
MCE.emitByte(BaseOpcode + getX86RegNum(MI.getOperand(CurOp++).getReg()));
|
|
|
|
if (CurOp != NumOps) {
|
|
const MachineOperand &MO1 = MI.getOperand(CurOp++);
|
|
unsigned Size = X86InstrInfo::sizeOfImm(Desc);
|
|
if (MO1.isImmediate())
|
|
emitConstant(MO1.getImm(), Size);
|
|
else {
|
|
unsigned rt = Is64BitMode ? X86::reloc_pcrel_word
|
|
: (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
|
|
if (Opcode == X86::MOV64ri)
|
|
rt = X86::reloc_absolute_dword; // FIXME: add X86II flag?
|
|
if (MO1.isGlobalAddress()) {
|
|
bool NeedStub = isa<Function>(MO1.getGlobal());
|
|
bool isLazy = gvNeedsLazyPtr(MO1.getGlobal());
|
|
emitGlobalAddress(MO1.getGlobal(), rt, MO1.getOffset(), 0,
|
|
NeedStub, isLazy);
|
|
} else if (MO1.isExternalSymbol())
|
|
emitExternalSymbolAddress(MO1.getSymbolName(), rt);
|
|
else if (MO1.isConstantPoolIndex())
|
|
emitConstPoolAddress(MO1.getIndex(), rt);
|
|
else if (MO1.isJumpTableIndex())
|
|
emitJumpTableAddress(MO1.getIndex(), rt);
|
|
}
|
|
}
|
|
break;
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
In the previous example, <tt>XXXCodeEmitter.cpp</tt> uses the
|
|
variable <tt>rt</tt>, which is a RelocationType enum that may be used to
|
|
relocate addresses (for example, a global address with a PIC base offset). The
|
|
<tt>RelocationType</tt> enum for that target is defined in the short
|
|
target-specific <tt>XXXRelocations.h</tt> file. The <tt>RelocationType</tt> is used by
|
|
the <tt>relocate</tt> method defined in <tt>XXXJITInfo.cpp</tt> to rewrite
|
|
addresses for referenced global symbols.
|
|
</p>
|
|
|
|
<p>
|
|
For example, <tt>X86Relocations.h</tt> specifies the following relocation types
|
|
for the X86 addresses. In all four cases, the relocated value is added to the
|
|
value already in memory. For <tt>reloc_pcrel_word</tt>
|
|
and <tt>reloc_picrel_word</tt>, there is an additional initial adjustment.
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
enum RelocationType {
|
|
reloc_pcrel_word = 0, // add reloc value after adjusting for the PC loc
|
|
reloc_picrel_word = 1, // add reloc value after adjusting for the PIC base
|
|
reloc_absolute_word = 2, // absolute relocation; no additional adjustment
|
|
reloc_absolute_dword = 3 // absolute relocation; no additional adjustment
|
|
};
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="targetJITInfo">Target JIT Info</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
<tt>XXXJITInfo.cpp</tt> implements the JIT interfaces for target-specific
|
|
code-generation activities, such as emitting machine code and stubs. At minimum,
|
|
a target-specific version of <tt>XXXJITInfo</tt> implements the following:
|
|
</p>
|
|
|
|
<ul>
|
|
<li><tt>getLazyResolverFunction</tt> — Initializes the JIT, gives the
|
|
target a function that is used for compilation.</li>
|
|
|
|
<li><tt>emitFunctionStub</tt> — Returns a native function with a specified
|
|
address for a callback function.</li>
|
|
|
|
<li><tt>relocate</tt> — Changes the addresses of referenced globals, based
|
|
on relocation types.</li>
|
|
|
|
<li>Callback function that are wrappers to a function stub that is used when the
|
|
real target is not initially known.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
<tt>getLazyResolverFunction</tt> is generally trivial to implement. It makes the
|
|
incoming parameter as the global <tt>JITCompilerFunction</tt> and returns the
|
|
callback function that will be used a function wrapper. For the Alpha target
|
|
(in <tt>AlphaJITInfo.cpp</tt>), the <tt>getLazyResolverFunction</tt>
|
|
implementation is simply:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
TargetJITInfo::LazyResolverFn AlphaJITInfo::getLazyResolverFunction(
|
|
JITCompilerFn F) {
|
|
JITCompilerFunction = F;
|
|
return AlphaCompilationCallback;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
For the X86 target, the <tt>getLazyResolverFunction</tt> implementation is a
|
|
little more complication, because it returns a different callback function for
|
|
processors with SSE instructions and XMM registers.
|
|
</p>
|
|
|
|
<p>
|
|
The callback function initially saves and later restores the callee register
|
|
values, incoming arguments, and frame and return address. The callback function
|
|
needs low-level access to the registers or stack, so it is typically implemented
|
|
with assembler.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
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src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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<a href="http://www.woo.com">Mason Woo</a> and <a href="http://misha.brukman.net">Misha Brukman</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a>
|
|
<br>
|
|
Last modified: $Date$
|
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</address>
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</html>
|