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1000 lines
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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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<title>The LLVM Target-Independent Code Generator</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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The LLVM Target-Independent Code Generator
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</div>
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<ol>
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<li><a href="#introduction">Introduction</a>
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<ul>
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<li><a href="#required">Required components in the code generator</a></li>
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<li><a href="#high-level-design">The high-level design of the code generator</a></li>
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<li><a href="#tablegen">Using TableGen for target description</a></li>
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</ul>
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</li>
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<li><a href="#targetdesc">Target description classes</a>
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<ul>
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<li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
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<li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
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<li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
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<li><a href="#mregisterinfo">The <tt>MRegisterInfo</tt> class</a></li>
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<li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
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<li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
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<li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
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</ul>
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</li>
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<li><a href="#codegendesc">Machine code description classes</a>
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<ul>
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<li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
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</ul>
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</li>
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<li><a href="#codegenalgs">Target-independent code generation algorithms</a>
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<ul>
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<li><a href="#instselect">Instruction Selection</a>
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<ul>
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<li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
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<li><a href="#selectiondag_process">SelectionDAG Code Generation
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Process</a></li>
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<li><a href="#selectiondag_build">Initial SelectionDAG
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Construction</a></li>
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<li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
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<li><a href="#selectiondag_optimize">SelectionDAG Optimization
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Phase</a></li>
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<li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
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<li><a href="#selectiondag_future">Future directions for the
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SelectionDAG</a></li>
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</ul></li>
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</ul>
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</li>
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<li><a href="#targetimpls">Target description implementations</a>
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<ul>
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<li><a href="#x86">The X86 backend</a></li>
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</ul>
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</li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<div class="doc_warning">
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<p>Warning: This is a work in progress.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="introduction">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The LLVM target-independent code generator is a framework that provides a
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suite of reusable components for translating the LLVM internal representation to
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the machine code for a specified target -- either in assembly form (suitable for
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a static compiler) or in binary machine code format (usable for a JIT compiler).
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The LLVM target-independent code generator consists of five main components:</p>
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<ol>
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<li><a href="#targetdesc">Abstract target description</a> interfaces which
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capture important properties about various aspects of the machine, independently
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of how they will be used. These interfaces are defined in
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<tt>include/llvm/Target/</tt>.</li>
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<li>Classes used to represent the <a href="#codegendesc">machine code</a> being
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generated for a target. These classes are intended to be abstract enough to
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represent the machine code for <i>any</i> target machine. These classes are
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defined in <tt>include/llvm/CodeGen/</tt>.</li>
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<li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
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various phases of native code generation (register allocation, scheduling, stack
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frame representation, etc). This code lives in <tt>lib/CodeGen/</tt>.</li>
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<li><a href="#targetimpls">Implementations of the abstract target description
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interfaces</a> for particular targets. These machine descriptions make use of
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the components provided by LLVM, and can optionally provide custom
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target-specific passes, to build complete code generators for a specific target.
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Target descriptions live in <tt>lib/Target/</tt>.</li>
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<li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
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completely target independent (it uses the <tt>TargetJITInfo</tt> structure to
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interface for target-specific issues. The code for the target-independent
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JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
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</ol>
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<p>
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Depending on which part of the code generator you are interested in working on,
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different pieces of this will be useful to you. In any case, you should be
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familiar with the <a href="#targetdesc">target description</a> and <a
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href="#codegendesc">machine code representation</a> classes. If you want to add
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a backend for a new target, you will need to <a href="#targetimpls">implement the
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target description</a> classes for your new target and understand the <a
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href="LangRef.html">LLVM code representation</a>. If you are interested in
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implementing a new <a href="#codegenalgs">code generation algorithm</a>, it
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should only depend on the target-description and machine code representation
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classes, ensuring that it is portable.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="required">Required components in the code generator</a>
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</div>
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<div class="doc_text">
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<p>The two pieces of the LLVM code generator are the high-level interface to the
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code generator and the set of reusable components that can be used to build
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target-specific backends. The two most important interfaces (<a
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href="#targetmachine"><tt>TargetMachine</tt></a> and <a
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href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
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required to be defined for a backend to fit into the LLVM system, but the others
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must be defined if the reusable code generator components are going to be
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used.</p>
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<p>This design has two important implications. The first is that LLVM can
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support completely non-traditional code generation targets. For example, the C
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backend does not require register allocation, instruction selection, or any of
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the other standard components provided by the system. As such, it only
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implements these two interfaces, and does its own thing. Another example of a
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code generator like this is a (purely hypothetical) backend that converts LLVM
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to the GCC RTL form and uses GCC to emit machine code for a target.</p>
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<p>This design also implies that it is possible to design and
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implement radically different code generators in the LLVM system that do not
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make use of any of the built-in components. Doing so is not recommended at all,
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but could be required for radically different targets that do not fit into the
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LLVM machine description model: programmable FPGAs for example.</p>
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<p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code
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generator uses almost entirely different code paths than described in this
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document. For this reason, there are some deprecated interfaces (such as
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<tt>TargetRegInfo</tt> and <tt>TargetSchedInfo</tt>), which are only used by the
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V9 backend and should not be used by any other targets. Also, all code in the
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<tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered
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deprecated, and should not be used as the basis for future code generator work.
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The SparcV9 backend is slowly being merged into the rest of the
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target-independent code generators, but this is a low-priority process with no
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predictable completion date.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="high-level-design">The high-level design of the code generator</a>
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</div>
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<div class="doc_text">
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<p>The LLVM target-independent code generator is designed to support efficient and
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quality code generation for standard register-based microprocessors. Code
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generation in this model is divided into the following stages:</p>
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<ol>
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<li><b><a href="#instselect">Instruction Selection</a></b> - Determining an
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efficient implementation of the input LLVM code in the target instruction set.
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This stage produces the initial code for the program in the target instruction
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set, then makes use of virtual registers in SSA form and physical registers that
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represent any required register assignments due to target constraints or calling
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conventions.</li>
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<li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> - This
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optional stage consists of a series of machine-code optimizations that
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operate on the SSA-form produced by the instruction selector. Optimizations
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like modulo-scheduling, normal scheduling, or peephole optimization work here.
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</li>
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<li><b><a name="#regalloc">Register Allocation</a></b> - The
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target code is transformed from an infinite virtual register file in SSA form
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to the concrete register file used by the target. This phase introduces spill
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code and eliminates all virtual register references from the program.</li>
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<li><b><a name="#proepicode">Prolog/Epilog Code Insertion</a></b> - Once the
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machine code has been generated for the function and the amount of stack space
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required is known (used for LLVM alloca's and spill slots), the prolog and
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epilog code for the function can be inserted and "abstract stack location
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references" can be eliminated. This stage is responsible for implementing
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optimizations like frame-pointer elimination and stack packing.</li>
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<li><b><a name="latemco">Late Machine Code Optimizations</a></b> - Optimizations
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that operate on "final" machine code can go here, such as spill code scheduling
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and peephole optimizations.</li>
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<li><b><a name="codemission">Code Emission</a></b> - The final stage actually
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puts out the code for the current function, either in the target assembler
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format or in machine code.</li>
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</ol>
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<p>
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The code generator is based on the assumption that the instruction selector will
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use an optimal pattern matching selector to create high-quality sequences of
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native instructions. Alternative code generator designs based on pattern
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expansion and
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aggressive iterative peephole optimization are much slower. This design
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permits efficient compilation (important for JIT environments) and
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aggressive optimization (used when generating code offline) by allowing
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components of varying levels of sophistication to be used for any step of
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compilation.</p>
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<p>
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In addition to these stages, target implementations can insert arbitrary
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target-specific passes into the flow. For example, the X86 target uses a
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special pass to handle the 80x87 floating point stack architecture. Other
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targets with unusual requirements can be supported with custom passes as needed.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="tablegen">Using TableGen for target description</a>
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</div>
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<div class="doc_text">
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<p>The target description classes require a detailed description of the target
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architecture. These target descriptions often have a large amount of common
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information (e.g., an <tt>add</tt> instruction is almost identical to a
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<tt>sub</tt> instruction).
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In order to allow the maximum amount of commonality to be factored out, the LLVM
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code generator uses the <a href="TableGenFundamentals.html">TableGen</a> tool to
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describe big chunks of the target machine, which allows the use of
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domain-specific and target-specific abstractions to reduce the amount of
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repetition.
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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="targetdesc">Target description classes</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The LLVM target description classes (which are located in the
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<tt>include/llvm/Target</tt> directory) provide an abstract description of the
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target machine; independent of any particular client. These classes are
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designed to capture the <i>abstract</i> properties of the target (such as the
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instructions and registers it has), and do not incorporate any particular pieces
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of code generation algorithms. These interfaces do not take interference graphs
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as inputs or other algorithm-specific data structures.</p>
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<p>All of the target description classes (except the <tt><a
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href="#targetdata">TargetData</a></tt> class) are designed to be subclassed by
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the concrete target implementation, and have virtual methods implemented. To
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get to these implementations, the <tt><a
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href="#targetmachine">TargetMachine</a></tt> class provides accessors that
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should be implemented by the target.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetmachine">The <tt>TargetMachine</tt> class</a>
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</div>
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<div class="doc_text">
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<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
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access the target-specific implementations of the various target description
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classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
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<tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.). This class is
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designed to be specialized by
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a concrete target implementation (e.g., <tt>X86TargetMachine</tt>) which
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implements the various virtual methods. The only required target description
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class is the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the
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code generator components are to be used, the other interfaces should be
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implemented as well.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetdata">The <tt>TargetData</tt> class</a>
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</div>
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<div class="doc_text">
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<p>The <tt>TargetData</tt> class is the only required target description class,
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and it is the only class that is not extensible. You cannot derived a new
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class from it. <tt>TargetData</tt> specifies information about how the target
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lays out memory for structures, the alignment requirements for various data
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types, the size of pointers in the target, and whether the target is
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little-endian or big-endian.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetlowering">The <tt>TargetLowering</tt> class</a>
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</div>
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<div class="doc_text">
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<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
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selectors primarily to describe how LLVM code should be lowered to SelectionDAG
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operations. Among other things, this class indicates:
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<ul><li>an initial register class to use for various ValueTypes,</li>
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<li>which operations are natively supported by the target machine,</li>
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<li>the return type of setcc operations, and</li>
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<li>the type to use for shift amounts, etc</li>.
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</ol></p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="mregisterinfo">The <tt>MRegisterInfo</tt> class</a>
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</div>
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<div class="doc_text">
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<p>The <tt>MRegisterInfo</tt> class (which will eventually be renamed to
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<tt>TargetRegisterInfo</tt>) is used to describe the register file of the
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target and any interactions between the registers.</p>
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<p>Registers in the code generator are represented in the code generator by
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unsigned numbers. Physical registers (those that actually exist in the target
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description) are unique small numbers, and virtual registers are generally
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large.</p>
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<p>Each register in the processor description has an associated
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<tt>MRegisterDesc</tt> entry, which provides a textual name for the register
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(used for assembly output and debugging dumps), a set of aliases (used to
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indicate that one register overlaps with another), and some flag bits.
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</p>
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<p>In addition to the per-register description, the <tt>MRegisterInfo</tt> class
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exposes a set of processor specific register classes (instances of the
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<tt>TargetRegisterClass</tt> class). Each register class contains sets of
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registers that have the same properties (for example, they are all 32-bit
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integer registers). Each SSA virtual register created by the instruction
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selector has an associated register class. When the register allocator runs, it
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replaces virtual registers with a physical register in the set.</p>
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<p>
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The target-specific implementations of these classes is auto-generated from a <a
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href="TableGenFundamentals.html">TableGen</a> description of the register file.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="codegendesc">Machine code description classes</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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At the high-level, LLVM code is translated to a machine specific representation
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formed out of MachineFunction, MachineBasicBlock, and <a
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href="#machineinstr"><tt>MachineInstr</tt></a> instances
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(defined in include/llvm/CodeGen). This representation is completely target
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agnostic, representing instructions in their most abstract form: an opcode and a
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series of operands. This representation is designed to support both SSA
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representation for machine code, as well as a register allocated, non-SSA form.
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</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="machineinstr">The <tt>MachineInstr</tt> class</a>
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</div>
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<div class="doc_text">
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<p>Target machine instructions are represented as instances of the
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<tt>MachineInstr</tt> class. This class is an extremely abstract way of
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representing machine instructions. In particular, it only keeps track of
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an opcode number and a set of operands.</p>
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<p>The opcode number is a simple unsigned number that only has meaning to a
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specific backend. All of the instructions for a target should be defined in
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the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values
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are auto-generated from this description. The <tt>MachineInstr</tt> class does
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not have any information about how to interpret the instruction (i.e., what the
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semantics of the instruction are): for that you must refer to the
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<tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
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<p>The operands of a machine instruction can be of several different types:
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they can be a register reference, constant integer, basic block reference, etc.
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In addition, a machine operand should be marked as a def or a use of the value
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(though only registers are allowed to be defs).</p>
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<p>By convention, the LLVM code generator orders instruction operands so that
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all register definitions come before the register uses, even on architectures
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that are normally printed in other orders. For example, the SPARC add
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instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
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and stores the result into the "%i3" register. In the LLVM code generator,
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the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the destination
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first.</p>
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<p>Keeping destination (definition) operands at the beginning of the operand
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list has several advantages. In particular, the debugging printer will print
|
|
the instruction like this:</p>
|
|
|
|
<pre>
|
|
%r3 = add %i1, %i2
|
|
</pre>
|
|
|
|
<p>If the first operand is a def, and it is also easier to <a
|
|
href="#buildmi">create instructions</a> whose only def is the first
|
|
operand.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
|
|
located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
|
|
<tt>BuildMI</tt> functions make it easy to build arbitrary machine
|
|
instructions. Usage of the <tt>BuildMI</tt> functions look like this:
|
|
</p>
|
|
|
|
<pre>
|
|
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
|
|
// instruction. The '1' specifies how many operands will be added.
|
|
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create the same instr, but insert it at the end of a basic block.
|
|
MachineBasicBlock &MBB = ...
|
|
BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create the same instr, but insert it before a specified iterator point.
|
|
MachineBasicBlock::iterator MBBI = ...
|
|
BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create a 'cmp Reg, 0' instruction, no destination reg.
|
|
MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
|
|
// Create an 'sahf' instruction which takes no operands and stores nothing.
|
|
MI = BuildMI(X86::SAHF, 0);
|
|
|
|
// Create a self looping branch instruction.
|
|
BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
|
|
</pre>
|
|
|
|
<p>
|
|
The key thing to remember with the <tt>BuildMI</tt> functions is that you have
|
|
to specify the number of operands that the machine instruction will take. This
|
|
allows for efficient memory allocation. You also need to specify if operands
|
|
default to be uses of values, not definitions. If you need to add a definition
|
|
operand (other than the optional destination register), you must explicitly
|
|
mark it as such.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="fixedregs">Fixed (preassigned) registers</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>One important issue that the code generator needs to be aware of is the
|
|
presence of fixed registers. In particular, there are often places in the
|
|
instruction stream where the register allocator <em>must</em> arrange for a
|
|
particular value to be in a particular register. This can occur due to
|
|
limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
|
|
with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like calling
|
|
conventions. In any case, the instruction selector should emit code that
|
|
copies a virtual register into or out of a physical register when needed.</p>
|
|
|
|
<p>For example, consider this simple LLVM example:</p>
|
|
|
|
<pre>
|
|
int %test(int %X, int %Y) {
|
|
%Z = div int %X, %Y
|
|
ret int %Z
|
|
}
|
|
</pre>
|
|
|
|
<p>The X86 instruction selector produces this machine code for the div
|
|
and ret (use
|
|
"<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to get this):</p>
|
|
|
|
<pre>
|
|
;; Start of div
|
|
%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
|
|
%reg1027 = sar %reg1024, 31
|
|
%EDX = mov %reg1027 ;; Sign extend X into EDX
|
|
idiv %reg1025 ;; Divide by Y (in reg1025)
|
|
%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
|
|
|
|
;; Start of ret
|
|
%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
|
|
ret
|
|
</pre>
|
|
|
|
<p>By the end of code generation, the register allocator has coalesced
|
|
the registers and deleted the resultant identity moves, producing the
|
|
following code:</p>
|
|
|
|
<pre>
|
|
;; X is in EAX, Y is in ECX
|
|
mov %EAX, %EDX
|
|
sar %EDX, 31
|
|
idiv %ECX
|
|
ret
|
|
</pre>
|
|
|
|
<p>This approach is extremely general (if it can handle the X86 architecture,
|
|
it can handle anything!) and allows all of the target specific
|
|
knowledge about the instruction stream to be isolated in the instruction
|
|
selector. Note that physical registers should have a short lifetime for good
|
|
code generation, and all physical registers are assumed dead on entry and
|
|
exit of basic blocks (before register allocation). Thus if you need a value
|
|
to be live across basic block boundaries, it <em>must</em> live in a virtual
|
|
register.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ssa">Machine code SSA form</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and
|
|
are maintained in SSA-form until register allocation happens. For the most
|
|
part, this is trivially simple since LLVM is already in SSA form: LLVM PHI nodes
|
|
become machine code PHI nodes, and virtual registers are only allowed to have a
|
|
single definition.</p>
|
|
|
|
<p>After register allocation, machine code is no longer in SSA-form, as there
|
|
are no virtual registers left in the code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="codegenalgs">Target-independent code generation algorithms</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section documents the phases described in the <a
|
|
href="high-level-design">high-level design of the code generator</a>. It
|
|
explains how they work and some of the rationale behind their design.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="instselect">Instruction Selection</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
Instruction Selection is the process of translating LLVM code presented to the
|
|
code generator into target-specific machine instructions. There are several
|
|
well-known ways to do this in the literature. In LLVM there are two main forms:
|
|
the old-style 'simple' instruction selector (which effectively peephole selects
|
|
each LLVM instruction into a series of machine instructions), and the new
|
|
SelectionDAG based instruction selector.
|
|
</p>
|
|
|
|
<p>The 'simple' instruction selectors are tedious to write, require a lot of
|
|
boiler plate code, and are difficult to get correct. Additionally, any
|
|
optimizations written for a simple instruction selector cannot be used by other
|
|
targets. For this reason, LLVM is moving to a new SelectionDAG based
|
|
instruction selector, which is described in this section. If you are starting a
|
|
new port, we recommend that you write the instruction selector using the
|
|
SelectionDAG infrastructure.</p>
|
|
|
|
<p>In time, most of the target-specific code for instruction selection will be
|
|
auto-generated from the target description (<tt>*.td</tt>) files. For now,
|
|
however, the <a href="#selectiondag_select">Select Phase</a> must still be
|
|
written by hand.</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_intro">Introduction to SelectionDAGs</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The SelectionDAG provides an abstraction for code representation in a way that
|
|
is amenable to instruction selection using automatic techniques
|
|
(e.g. dynamic-programming based optimal pattern matching selectors), It is also
|
|
well suited to other phases of code generation; in particular, instruction scheduling. Additionally, the SelectionDAG provides a host representation where a
|
|
large variety of very-low-level (but target-independent)
|
|
<a href="#selectiondag_optimize">optimizations</a> may be
|
|
performed: ones which require extensive information about the instructions
|
|
efficiently supported by the target.
|
|
</p>
|
|
|
|
<p>
|
|
The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
|
|
<tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
|
|
operation code (Opcode) that indicates what operation the node performs.
|
|
The various operation node types are described at the top of the
|
|
<tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt> file. Depending on the
|
|
operation, nodes may contain additional information (e.g. the condition code
|
|
for a SETCC node) contained in a derived class.</p>
|
|
|
|
<p>Although most operations define a single value, each node in the graph may
|
|
define multiple values. For example, a combined div/rem operation will define
|
|
both the dividend and the remainder. Many other situations require multiple
|
|
values as well. Each node also has some number of operands, which are edges
|
|
to the node defining the used value. Because nodes may define multiple values,
|
|
edges are represented by instances of the <tt>SDOperand</tt> class, which is
|
|
a <SDNode, unsigned> pair, indicating the node and result
|
|
value being used, respectively. Each value produced by an SDNode has an
|
|
associated MVT::ValueType, indicating what type the value is.
|
|
</p>
|
|
|
|
<p>
|
|
SelectionDAGs contain two different kinds of values: those that represent data
|
|
flow and those that represent control flow dependencies. Data values are simple
|
|
edges with an integer or floating point value type. Control edges are
|
|
represented as "chain" edges which are of type MVT::Other. These edges provide
|
|
an ordering between nodes that have side effects (such as
|
|
loads/stores/calls/return/etc). All nodes that have side effects should take a
|
|
token chain as input and produce a new one as output. By convention, token
|
|
chain inputs are always operand #0, and chain results are always the last
|
|
value produced by an operation.</p>
|
|
|
|
<p>
|
|
A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
|
|
always a marker node with an Opcode of ISD::TokenFactor. The Root node is the
|
|
final side-effecting node in the token chain. For example, in a single basic
|
|
block function, this would be the return node.
|
|
</p>
|
|
|
|
<p>
|
|
One important concept for SelectionDAGs is the notion of a "legal" vs. "illegal"
|
|
DAG. A legal DAG for a target is one that only uses supported operations and
|
|
supported types. On PowerPC, for example, a DAG with any values of i1, i8, i16,
|
|
or i64 type would be illegal. The <a href="#selectiondag_legalize">legalize</a>
|
|
phase is responsible for turning an illegal DAG into a legal DAG.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
SelectionDAG-based instruction selection consists of the following steps:
|
|
</p>
|
|
|
|
<ol>
|
|
<li><a href="#selectiondag_build">Build initial DAG</a> - This stage performs
|
|
a simple translation from the input LLVM code to an illegal SelectionDAG.
|
|
</li>
|
|
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> - This stage
|
|
performs simple optimizations on the SelectionDAG to simplify it and
|
|
recognize meta instructions (like rotates and div/rem pairs) for
|
|
targets that support these meta operations. This makes the resultant code
|
|
more efficient and the 'select instructions from DAG' phase (below) simpler.
|
|
</li>
|
|
<li><a href="#selectiondag_legalize">Legalize SelectionDAG</a> - This stage
|
|
converts the illegal SelectionDAG to a legal SelectionDAG, by eliminating
|
|
unsupported operations and data types.</li>
|
|
<li><a href="#selectiondag_optimize">Optimize SelectionDAG (#2)</a> - This
|
|
second run of the SelectionDAG optimized the newly legalized DAG, to
|
|
eliminate inefficiencies introduced by legalization.</li>
|
|
<li><a href="#selectiondag_select">Select instructions from DAG</a> - Finally,
|
|
the target instruction selector matches the DAG operations to target
|
|
instructions, emitting them and building the MachineFunction being
|
|
compiled.</li>
|
|
</ol>
|
|
|
|
<p>After all of these steps are complete, the SelectionDAG is destroyed and the
|
|
rest of the code generation passes are run.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_build">Initial SelectionDAG Construction</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The initial SelectionDAG is naively peephole expanded from the LLVM input by
|
|
the <tt>SelectionDAGLowering</tt> class in the SelectionDAGISel.cpp file. The
|
|
intent of this pass is to expose as much low-level, target-specific details
|
|
to the SelectionDAG as possible. This pass is mostly hard-coded (e.g. an LLVM
|
|
add turns into an SDNode add while a geteelementptr is expanded into the obvious
|
|
arithmetic). This pass requires target-specific hooks to lower calls and
|
|
returns, varargs, etc. For these features, the TargetLowering interface is
|
|
used.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Legalize phase is in charge of converting a DAG to only use the types and
|
|
operations that are natively supported by the target. This involves two major
|
|
tasks:</p>
|
|
|
|
<ol>
|
|
<li><p>Convert values of unsupported types to values of supported types.</p>
|
|
<p>There are two main ways of doing this: promoting a small type to a larger
|
|
type (e.g. f32 -> f64, or i16 -> i32), and breaking up large
|
|
integer types
|
|
to smaller ones (e.g. implementing i64 with i32 operations where
|
|
possible). Type conversions can insert sign and zero extensions as
|
|
needed to make sure that the final code has the same behavior as the
|
|
input.</p>
|
|
</li>
|
|
|
|
<li><p>Eliminate operations that are not supported by the target in a supported
|
|
type.</p>
|
|
<p>Targets often have wierd constraints, such as not supporting every
|
|
operation on every supported datatype (e.g. X86 does not support byte
|
|
conditional moves). Legalize takes care of either open-coding another
|
|
sequence of operations to emulate the operation (this is known as
|
|
expansion), promoting to a larger type that supports the operation
|
|
(promotion), or using a target-specific hook to implement the
|
|
legalization.</p>
|
|
</li>
|
|
</ol>
|
|
|
|
<p>
|
|
Instead of using a Legalize pass, we could require that every target-specific
|
|
<a href="#selectiondag_optimize">selector</a> supports and expands every
|
|
operator and type even if they are not supported and may require many
|
|
instructions to implement (in fact, this is the approach taken by the
|
|
"simple" selectors). However, using a Legalize pass allows all of the
|
|
cannonicalization patterns to be shared across targets which makes it very
|
|
easy to optimize the cannonicalized code because it is still in the form of
|
|
a DAG.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_optimize">SelectionDAG Optimization Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The SelectionDAG optimization phase is run twice for code generation: once
|
|
immediately after the DAG is built and once after legalization. The first run
|
|
of the pass allows the initial code to be cleaned up (e.g. performing
|
|
optimizations that depend on knowing that the operators have restricted type
|
|
inputs). The second run of the pass cleans up the messy code generated by the
|
|
Legalize pass, allowing Legalize to be very simple since it can ignore many
|
|
special cases.
|
|
</p>
|
|
|
|
<p>
|
|
One important class of optimizations that this pass will do in the future is
|
|
optimizing inserted sign and zero extension instructions. Here are some good
|
|
papers on the subject:</p>
|
|
|
|
<p>
|
|
"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
|
|
integer arithmetic</a>"<br>
|
|
Kevin Redwine and Norman Ramsey<br>
|
|
International Conference on Compiler Construction (CC) 2004
|
|
</p>
|
|
|
|
|
|
<p>
|
|
"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
|
|
sign extension elimination</a>"<br>
|
|
Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
|
|
Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
|
|
and Implementation.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_select">SelectionDAG Select Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Select phase is the bulk of the target-specific code for instruction
|
|
selection. This phase takes a legal SelectionDAG as input, and does simple
|
|
pattern matching on the DAG to generate code. In time, the Select phase will
|
|
be automatically generated from the target's InstrInfo.td file, which is why we
|
|
want to make the Select phase as simple and mechanical as possible.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_future">Future directions for the SelectionDAG</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<ol>
|
|
<li>Optional whole-function selection.</li>
|
|
<li>Select is a graph translation phase.</li>
|
|
<li>Place the machine instructions resulting from Select according to register
|
|
pressure or a schedule.</li>
|
|
<li>DAG Scheduling.</li>
|
|
<li>Auto-generate the Select phase from the target description (*.td) files.
|
|
</li>
|
|
</ol>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ssamco">SSA-based Machine Code Optimizations</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="regalloc">Register Allocation</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="proepicode">Prolog/Epilog Code Insertion</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="latemco">Late Machine Code Optimizations</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="codemission">Code Emission</a>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="targetimpls">Target description implementations</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section of the document explains features or design decisions that
|
|
are specific to the code generator for a particular target.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="x86">The X86 backend</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
|
|
code generator currently targets a generic P6-like processor. As such, it
|
|
produces a few P6-and-above instructions (like conditional moves), but it does
|
|
not make use of newer features like MMX or SSE. In the future, the X86 backend
|
|
will have sub-target support added for specific processor families and
|
|
implementations.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
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</div>
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<div class="doc_text">
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<p>The x86 has a very flexible way of accessing memory. It is capable of
|
|
forming memory addresses of the following expression directly in integer
|
|
instructions (which use ModR/M addressing):</p>
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|
|
|
<pre>
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|
Base+[1,2,4,8]*IndexReg+Disp32
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</pre>
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|
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|
<p>In order to represent this, LLVM tracks no less than 4 operands for each
|
|
memory operand of this form. This means that the "load" form of 'mov' has the
|
|
following <tt>MachineOperand</tt>s in this order:</p>
|
|
|
|
<pre>
|
|
Index: 0 | 1 2 3 4
|
|
Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement
|
|
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm
|
|
</pre>
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|
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|
<p>Stores, and all other instructions, treat the four memory operands in the
|
|
same way, in the same order.</p>
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|
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|
</div>
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|
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|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_names">Instruction naming</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
An instruction name consists of the base name, a default operand size, and a
|
|
a character per operand with an optional special size. For example:</p>
|
|
|
|
<p>
|
|
<tt>ADD8rr</tt> -> add, 8-bit register, 8-bit register<br>
|
|
<tt>IMUL16rmi</tt> -> imul, 16-bit register, 16-bit memory, 16-bit immediate<br>
|
|
<tt>IMUL16rmi8</tt> -> imul, 16-bit register, 16-bit memory, 8-bit immediate<br>
|
|
<tt>MOVSX32rm16</tt> -> movsx, 32-bit register, 16-bit memory
|
|
</p>
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|
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|
</div>
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<!-- *********************************************************************** -->
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<address>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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