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1305 lines
54 KiB
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1305 lines
54 KiB
HTML
<!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
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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="#targetsubtarget">The <tt>TargetSubtarget</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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<li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
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class</a></li>
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<li><a href="#machinefunction">The <tt>MachineFunction</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: the DAG Combiner</a></li>
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<li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
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<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
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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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<li><a href="#codeemit">Code Emission</a>
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<ul>
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<li><a href="#codeemit_asm">Generating Assembly Code</a></li>
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<li><a href="#codeemit_bin">Generating Binary Machine Code</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-specific Implementation Notes</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>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> - This phase
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determines an efficient way to express the input LLVM code in the target
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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. This step turns the LLVM code into a DAG of target
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instructions.</li>
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<li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> - This
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phase takes the DAG of target instructions produced by the instruction selection
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phase, determines an ordering of the instructions, then emits the instructions
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as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering. Note
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that we describe this in the <a href="#instselect">instruction selection
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section</a> because it operates on a <a
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href="#selectiondag_intro">SelectionDAG</a>.
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</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 or peephole optimization work here.
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</li>
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<li><b><a href="#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 href="#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 href="#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 href="#codeemit">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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<p>As LLVM continues to be developed and refined, we plan to move more and more
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of the target description to be in <tt>.td</tt> form. Doing so gives us a
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number of advantages. The most important is that it makes it easier to port
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LLVM, because it reduces the amount of C++ code that has to be written and the
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surface area of the code generator that needs to be understood before someone
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can get in an get something working. Second, it is also important to us because
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it makes it easier to change things: in particular, if tables and other things
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are all emitted by tblgen, we only need to change one place (tblgen) to update
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all of the targets to a new interface.</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.</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</li>
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<li>the type to use for shift amounts</li>
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<li>various high-level characteristics, like whether it is profitable to turn
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division by a constant into a multiplication sequence</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. Note that register #0 is reserved as a flag value.</p>
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<p>Each register in the processor description has an associated
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<tt>TargetRegisterDesc</tt> entry, which provides a textual name for the register
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(used for assembly output and debugging dumps) and a set of aliases (used to
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indicate that one register overlaps with another).
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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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<div class="doc_text">
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<p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
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instructions supported by the target. It is essentially an array of
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<tt>TargetInstrDescriptor</tt> objects, each of which describes one
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instruction the target supports. Descriptors define things like the mnemonic
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for the opcode, the number of operands, the list of implicit register uses
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and defs, whether the instruction has certain target-independent properties
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(accesses memory, is commutable, etc), and holds any target-specific flags.</p>
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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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<div class="doc_text">
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<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
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stack frame layout of the target. It holds the direction of stack growth,
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the known stack alignment on entry to each function, and the offset to the
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locals area. The offset to the local area is the offset from the stack
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pointer on function entry to the first location where function data (local
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variables, spill locations) can be stored.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
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</div>
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<div class="doc_text">
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<p>
|
|
<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
|
|
specific chip set being targeted. A sub-target informs code generation of
|
|
which instructions are supported, instruction latencies and instruction
|
|
execution itinerary; i.e., which processing units are used, in what order, and
|
|
for how long.
|
|
</p>
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="codegendesc">Machine code description classes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
At the high-level, LLVM code is translated to a machine specific representation
|
|
formed out of <a href="#machinefunction">MachineFunction</a>,
|
|
<a href="#machinebasicblock">MachineBasicBlock</a>, and <a
|
|
href="#machineinstr"><tt>MachineInstr</tt></a> instances
|
|
(defined in include/llvm/CodeGen). This representation is completely target
|
|
agnostic, representing instructions in their most abstract form: an opcode and a
|
|
series of operands. This representation is designed to support both SSA
|
|
representation for machine code, as well as a register allocated, non-SSA form.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machineinstr">The <tt>MachineInstr</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Target machine instructions are represented as instances of the
|
|
<tt>MachineInstr</tt> class. This class is an extremely abstract way of
|
|
representing machine instructions. In particular, it only keeps track of
|
|
an opcode number and a set of operands.</p>
|
|
|
|
<p>The opcode number is a simple unsigned number that only has meaning to a
|
|
specific backend. All of the instructions for a target should be defined in
|
|
the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values
|
|
are auto-generated from this description. The <tt>MachineInstr</tt> class does
|
|
not have any information about how to interpret the instruction (i.e., what the
|
|
semantics of the instruction are): for that you must refer to the
|
|
<tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
|
|
|
|
<p>The operands of a machine instruction can be of several different types:
|
|
they can be a register reference, constant integer, basic block reference, etc.
|
|
In addition, a machine operand should be marked as a def or a use of the value
|
|
(though only registers are allowed to be defs).</p>
|
|
|
|
<p>By convention, the LLVM code generator orders instruction operands so that
|
|
all register definitions come before the register uses, even on architectures
|
|
that are normally printed in other orders. For example, the SPARC add
|
|
instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
|
|
and stores the result into the "%i3" register. In the LLVM code generator,
|
|
the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the destination
|
|
first.</p>
|
|
|
|
<p>Keeping destination (definition) operands at the beginning of the operand
|
|
list has several advantages. In particular, the debugging printer will print
|
|
the instruction like this:</p>
|
|
|
|
<pre>
|
|
%r3 = add %i1, %i2
|
|
</pre>
|
|
|
|
<p>If the first operand is a def, and it is also easier to <a
|
|
href="#buildmi">create instructions</a> whose only def is the first
|
|
operand.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
|
|
located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
|
|
<tt>BuildMI</tt> functions make it easy to build arbitrary machine
|
|
instructions. Usage of the <tt>BuildMI</tt> functions look like this:
|
|
</p>
|
|
|
|
<pre>
|
|
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
|
|
// instruction. The '1' specifies how many operands will be added.
|
|
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create the same instr, but insert it at the end of a basic block.
|
|
MachineBasicBlock &MBB = ...
|
|
BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create the same instr, but insert it before a specified iterator point.
|
|
MachineBasicBlock::iterator MBBI = ...
|
|
BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create a 'cmp Reg, 0' instruction, no destination reg.
|
|
MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
|
|
// Create an 'sahf' instruction which takes no operands and stores nothing.
|
|
MI = BuildMI(X86::SAHF, 0);
|
|
|
|
// Create a self looping branch instruction.
|
|
BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
|
|
</pre>
|
|
|
|
<p>
|
|
The key thing to remember with the <tt>BuildMI</tt> functions is that you have
|
|
to specify the number of operands that the machine instruction will take. This
|
|
allows for efficient memory allocation. You also need to specify if operands
|
|
default to be uses of values, not definitions. If you need to add a definition
|
|
operand (other than the optional destination register), you must explicitly
|
|
mark it as such.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="fixedregs">Fixed (preassigned) registers</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>One important issue that the code generator needs to be aware of is the
|
|
presence of fixed registers. In particular, there are often places in the
|
|
instruction stream where the register allocator <em>must</em> arrange for a
|
|
particular value to be in a particular register. This can occur due to
|
|
limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
|
|
with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like calling
|
|
conventions. In any case, the instruction selector should emit code that
|
|
copies a virtual register into or out of a physical register when needed.</p>
|
|
|
|
<p>For example, consider this simple LLVM example:</p>
|
|
|
|
<pre>
|
|
int %test(int %X, int %Y) {
|
|
%Z = div int %X, %Y
|
|
ret int %Z
|
|
}
|
|
</pre>
|
|
|
|
<p>The X86 instruction selector produces this machine code for the div
|
|
and ret (use
|
|
"<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to get this):</p>
|
|
|
|
<pre>
|
|
;; Start of div
|
|
%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
|
|
%reg1027 = sar %reg1024, 31
|
|
%EDX = mov %reg1027 ;; Sign extend X into EDX
|
|
idiv %reg1025 ;; Divide by Y (in reg1025)
|
|
%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
|
|
|
|
;; Start of ret
|
|
%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
|
|
ret
|
|
</pre>
|
|
|
|
<p>By the end of code generation, the register allocator has coalesced
|
|
the registers and deleted the resultant identity moves, producing the
|
|
following code:</p>
|
|
|
|
<pre>
|
|
;; X is in EAX, Y is in ECX
|
|
mov %EAX, %EDX
|
|
sar %EDX, 31
|
|
idiv %ECX
|
|
ret
|
|
</pre>
|
|
|
|
<p>This approach is extremely general (if it can handle the X86 architecture,
|
|
it can handle anything!) and allows all of the target specific
|
|
knowledge about the instruction stream to be isolated in the instruction
|
|
selector. Note that physical registers should have a short lifetime for good
|
|
code generation, and all physical registers are assumed dead on entry and
|
|
exit of basic blocks (before register allocation). Thus if you need a value
|
|
to be live across basic block boundaries, it <em>must</em> live in a virtual
|
|
register.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ssa">Machine code SSA form</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and
|
|
are maintained in SSA-form until register allocation happens. For the most
|
|
part, this is trivially simple since LLVM is already in SSA form: LLVM PHI nodes
|
|
become machine code PHI nodes, and virtual registers are only allowed to have a
|
|
single definition.</p>
|
|
|
|
<p>After register allocation, machine code is no longer in SSA-form, as there
|
|
are no virtual registers left in the code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
|
|
(<a href="#machineinstr">MachineInstr</a> instances). It roughly corresponds to
|
|
the LLVM code input to the instruction selector, but there can be a one-to-many
|
|
mapping (i.e. one LLVM basic block can map to multiple machine basic blocks).
|
|
The MachineBasicBlock class has a "<tt>getBasicBlock</tt>" method, which returns
|
|
the LLVM basic block that it comes from.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machinefunction">The <tt>MachineFunction</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
|
|
(<a href="#machinebasicblock">MachineBasicBlock</a> instances). It corresponds
|
|
one-to-one with the LLVM function input to the instruction selector. In
|
|
addition to a list of basic blocks, the <tt>MachineFunction</tt> contains a
|
|
the MachineConstantPool, MachineFrameInfo, MachineFunctionInfo,
|
|
SSARegMap, and a set of live in and live out registers for the function. See
|
|
<tt>MachineFunction.h</tt> for more information.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="codegenalgs">Target-independent code generation algorithms</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section documents the phases described in the <a
|
|
href="#high-level-design">high-level design of the code generator</a>. It
|
|
explains how they work and some of the rationale behind their design.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="instselect">Instruction Selection</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
Instruction Selection is the process of translating LLVM code presented to the
|
|
code generator into target-specific machine instructions. There are several
|
|
well-known ways to do this in the literature. In LLVM there are two main forms:
|
|
the SelectionDAG based instruction selector framework and an old-style 'simple'
|
|
instruction selector (which effectively peephole selects each LLVM instruction
|
|
into a series of machine instructions). We recommend that all targets use the
|
|
SelectionDAG infrastructure.
|
|
</p>
|
|
|
|
<p>Portions of the DAG instruction selector are generated from the target
|
|
description files (<tt>*.td</tt>) files. Eventually, we aim for the entire
|
|
instruction selector to be generated from these <tt>.td</tt> files.</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_intro">Introduction to SelectionDAGs</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
The SelectionDAG provides an abstraction for code representation in a way that
|
|
is amenable to instruction selection using automatic techniques
|
|
(e.g. dynamic-programming based optimal pattern matching selectors), It is also
|
|
well suited to other phases of code generation; in particular,
|
|
instruction scheduling (SelectionDAG's are very close to scheduling DAGs
|
|
post-selection). Additionally, the SelectionDAG provides a host representation
|
|
where a large variety of very-low-level (but target-independent)
|
|
<a href="#selectiondag_optimize">optimizations</a> may be
|
|
performed: ones which require extensive information about the instructions
|
|
efficiently supported by the target.
|
|
</p>
|
|
|
|
<p>
|
|
The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
|
|
<tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
|
|
operation code (Opcode) that indicates what operation the node performs and
|
|
the operands to the operation.
|
|
The various operation node types are described at the top of the
|
|
<tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt> file.</p>
|
|
|
|
<p>Although most operations define a single value, each node in the graph may
|
|
define multiple values. For example, a combined div/rem operation will define
|
|
both the dividend and the remainder. Many other situations require multiple
|
|
values as well. Each node also has some number of operands, which are edges
|
|
to the node defining the used value. Because nodes may define multiple values,
|
|
edges are represented by instances of the <tt>SDOperand</tt> class, which is
|
|
a <SDNode, unsigned> pair, indicating the node and result
|
|
value being used, respectively. Each value produced by an SDNode has an
|
|
associated MVT::ValueType, indicating what type the value is.
|
|
</p>
|
|
|
|
<p>
|
|
SelectionDAGs contain two different kinds of values: those that represent data
|
|
flow and those that represent control flow dependencies. Data values are simple
|
|
edges with an integer or floating point value type. Control edges are
|
|
represented as "chain" edges which are of type MVT::Other. These edges provide
|
|
an ordering between nodes that have side effects (such as
|
|
loads/stores/calls/return/etc). All nodes that have side effects should take a
|
|
token chain as input and produce a new one as output. By convention, token
|
|
chain inputs are always operand #0, and chain results are always the last
|
|
value produced by an operation.</p>
|
|
|
|
<p>
|
|
A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
|
|
always a marker node with an Opcode of ISD::EntryToken. The Root node is the
|
|
final side-effecting node in the token chain. For example, in a single basic
|
|
block function, this would be the return node.
|
|
</p>
|
|
|
|
<p>
|
|
One important concept for SelectionDAGs is the notion of a "legal" vs. "illegal"
|
|
DAG. A legal DAG for a target is one that only uses supported operations and
|
|
supported types. On a 32-bit PowerPC, for example, a DAG with any values of i1,
|
|
i8, i16,
|
|
or i64 type would be illegal, as would a DAG that uses a SREM or UREM operation.
|
|
The <a href="#selectiondag_legalize">legalize</a>
|
|
phase is responsible for turning an illegal DAG into a legal DAG.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
SelectionDAG-based instruction selection consists of the following steps:
|
|
</p>
|
|
|
|
<ol>
|
|
<li><a href="#selectiondag_build">Build initial DAG</a> - This stage performs
|
|
a simple translation from the input LLVM code to an illegal SelectionDAG.
|
|
</li>
|
|
<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. This process translates the target-independent input DAG into
|
|
another DAG of target instructions.</li>
|
|
<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
|
|
- The last phase assigns a linear order to the instructions in the
|
|
target-instruction DAG and emits them into the MachineFunction being
|
|
compiled. This step uses traditional prepass scheduling techniques.</li>
|
|
</ol>
|
|
|
|
<p>After all of these steps are complete, the SelectionDAG is destroyed and the
|
|
rest of the code generation passes are run.</p>
|
|
|
|
<p>One great way to visualize what is going on here is to take advantage of a
|
|
few LLC command line options. In particular, the <tt>-view-isel-dags</tt>
|
|
option pops up a window with the SelectionDAG input to the Select phase for all
|
|
of the code compiled (if you only get errors printed to the console while using
|
|
this, you probably <a href="ProgrammersManual.html#ViewGraph">need to configure
|
|
your system</a> to add support for it). The <tt>-view-sched-dags</tt> option
|
|
views the SelectionDAG output from the Select phase and input to the Scheduler
|
|
phase.
|
|
</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 <a
|
|
href="#targetlowering">TargetLowering</a> 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: converting small types to
|
|
larger types ("promoting"), and breaking up large integer types
|
|
into smaller ones ("expanding"). For example, a target might require
|
|
that all f32 values are promoted to f64 and that all i1/i8/i16 values
|
|
are promoted to i32. The same target might require that all i64 values
|
|
be expanded into i32 values. These changes can insert sign and zero
|
|
extensions as
|
|
needed to make sure that the final code has the same behavior as the
|
|
input.</p>
|
|
<p>A target implementation tells the legalizer which types are supported
|
|
(and which register class to use for them) by calling the
|
|
"addRegisterClass" method in its TargetLowering constructor.</p>
|
|
</li>
|
|
|
|
<li><p>Eliminate operations that are not supported by the target.</p>
|
|
<p>Targets often have weird constraints, such as not supporting every
|
|
operation on every supported datatype (e.g. X86 does not support byte
|
|
conditional moves and PowerPC does not support sign-extending loads from
|
|
a 16-bit memory location). Legalize takes care by open-coding
|
|
another sequence of operations to emulate the operation ("expansion"), by
|
|
promoting to a larger type that supports the operation
|
|
(promotion), or using a target-specific hook to implement the
|
|
legalization (custom).</p>
|
|
<p>A target implementation tells the legalizer which operations are not
|
|
supported (and which of the above three actions to take) by calling the
|
|
"setOperationAction" method in its TargetLowering constructor.</p>
|
|
</li>
|
|
</ol>
|
|
|
|
<p>
|
|
Prior to the existance of the Legalize pass, we required that every
|
|
target <a href="#selectiondag_optimize">selector</a> supported and handled every
|
|
operator and type even if they are not natively supported. The introduction of
|
|
the Legalize phase allows all of the
|
|
cannonicalization patterns to be shared across targets, and 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: the DAG
|
|
Combiner</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, which allows Legalize to be very simple (it can focus on making
|
|
code legal instead of focusing on generating <i>good</i> and legal code).
|
|
</p>
|
|
|
|
<p>
|
|
One important class of optimizations performed is optimizing inserted sign and
|
|
zero extension instructions. We currently use ad-hoc techniques, but could move
|
|
to more rigorous techniques in the future. Here are some good
|
|
papers on the subject:</p>
|
|
|
|
<p>
|
|
"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
|
|
integer arithmetic</a>"<br>
|
|
Kevin Redwine and Norman Ramsey<br>
|
|
International Conference on Compiler Construction (CC) 2004
|
|
</p>
|
|
|
|
|
|
<p>
|
|
"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
|
|
sign extension elimination</a>"<br>
|
|
Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
|
|
Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
|
|
and Implementation.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_select">SelectionDAG Select Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Select phase is the bulk of the target-specific code for instruction
|
|
selection. This phase takes a legal SelectionDAG as input,
|
|
pattern matches the instructions supported by the target to this DAG, and
|
|
produces a new DAG of target code. For example, consider the following LLVM
|
|
fragment:</p>
|
|
|
|
<pre>
|
|
%t1 = add float %W, %X
|
|
%t2 = mul float %t1, %Y
|
|
%t3 = add float %t2, %Z
|
|
</pre>
|
|
|
|
<p>This LLVM code corresponds to a SelectionDAG that looks basically like this:
|
|
</p>
|
|
|
|
<pre>
|
|
(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
|
|
</pre>
|
|
|
|
<p>If a target supports floating point multiply-and-add (FMA) operations, one
|
|
of the adds can be merged with the multiply. On the PowerPC, for example, the
|
|
output of the instruction selector might look like this DAG:</p>
|
|
|
|
<pre>
|
|
(FMADDS (FADDS W, X), Y, Z)
|
|
</pre>
|
|
|
|
<p>
|
|
The FMADDS instruction is a ternary instruction that multiplies its first two
|
|
operands and adds the third (as single-precision floating-point numbers). The
|
|
FADDS instruction is a simple binary single-precision add instruction. To
|
|
perform this pattern match, the PowerPC backend includes the following
|
|
instruction definitions:
|
|
</p>
|
|
|
|
<pre>
|
|
def FMADDS : AForm_1<59, 29,
|
|
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
|
|
"fmadds $FRT, $FRA, $FRC, $FRB",
|
|
[<b>(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
|
|
F4RC:$FRB))</b>]>;
|
|
def FADDS : AForm_2<59, 21,
|
|
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
|
|
"fadds $FRT, $FRA, $FRB",
|
|
[<b>(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))</b>]>;
|
|
</pre>
|
|
|
|
<p>The portion of the instruction definition in bold indicates the pattern used
|
|
to match the instruction. The DAG operators (like <tt>fmul</tt>/<tt>fadd</tt>)
|
|
are defined in the <tt>lib/Target/TargetSelectionDAG.td</tt> file.
|
|
"<tt>F4RC</tt>" is the register class of the input and result values.<p>
|
|
|
|
<p>The TableGen DAG instruction selector generator reads the instruction
|
|
patterns in the .td and automatically builds parts of the pattern matching code
|
|
for your target. It has the following strengths:</p>
|
|
|
|
<ul>
|
|
<li>At compiler-compiler time, it analyzes your instruction patterns and tells
|
|
you if your patterns make sense or not.</li>
|
|
<li>It can handle arbitrary constraints on operands for the pattern match. In
|
|
particular, it is straight-forward to say things like "match any immediate
|
|
that is a 13-bit sign-extended value". For examples, see the
|
|
<tt>immSExt16</tt> and related tblgen classes in the PowerPC backend.</li>
|
|
<li>It knows several important identities for the patterns defined. For
|
|
example, it knows that addition is commutative, so it allows the
|
|
<tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
|
|
well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
|
|
to specially handle this case.</li>
|
|
<li>It has a full-featured type-inferencing system. In particular, you should
|
|
rarely have to explicitly tell the system what type parts of your patterns
|
|
are. In the FMADDS case above, we didn't have to tell tblgen that all of
|
|
the nodes in the pattern are of type 'f32'. It was able to infer and
|
|
propagate this knowledge from the fact that F4RC has type 'f32'.</li>
|
|
<li>Targets can define their own (and rely on built-in) "pattern fragments".
|
|
Pattern fragments are chunks of reusable patterns that get inlined into your
|
|
patterns during compiler-compiler time. For example, the integer "(not x)"
|
|
operation is actually defined as a pattern fragment that expands as
|
|
"(xor x, -1)", since the SelectionDAG does not have a native 'not'
|
|
operation. Targets can define their own short-hand fragments as they see
|
|
fit. See the definition of 'not' and 'ineg' for examples.</li>
|
|
<li>In addition to instructions, targets can specify arbitrary patterns that
|
|
map to one or more instructions, using the 'Pat' class. For example,
|
|
the PowerPC has no way to load an arbitrary integer immediate into a
|
|
register in one instruction. To tell tblgen how to do this, it defines:
|
|
|
|
<pre>
|
|
// Arbitrary immediate support. Implement in terms of LIS/ORI.
|
|
def : Pat<(i32 imm:$imm),
|
|
(ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))>;
|
|
</pre>
|
|
|
|
If none of the single-instruction patterns for loading an immediate into a
|
|
register match, this will be used. This rule says "match an arbitrary i32
|
|
immediate, turning it into an ORI ('or a 16-bit immediate') and an LIS
|
|
('load 16-bit immediate, where the immediate is shifted to the left 16
|
|
bits') instruction". To make this work, the LO16/HI16 node transformations
|
|
are used to manipulate the input immediate (in this case, take the high or
|
|
low 16-bits of the immediate).
|
|
</li>
|
|
<li>While the system does automate a lot, it still allows you to write custom
|
|
C++ code to match special cases, in case there is something that is hard
|
|
to express.</li>
|
|
</ul>
|
|
|
|
<p>
|
|
While it has many strengths, the system currently has some limitations,
|
|
primarily because it is a work in progress and is not yet finished:
|
|
</p>
|
|
|
|
<ul>
|
|
<li>Overall, there is no way to define or match SelectionDAG nodes that define
|
|
multiple values (e.g. ADD_PARTS, LOAD, CALL, etc). This is the biggest
|
|
reason that you currently still <i>have to</i> write custom C++ code for
|
|
your instruction selector.</li>
|
|
<li>There is no great way to support match complex addressing modes yet. In the
|
|
future, we will extend pattern fragments to allow them to define multiple
|
|
values (e.g. the four operands of the <a href="#x86_memory">X86 addressing
|
|
mode</a>). In addition, we'll extend fragments so that a fragment can match
|
|
multiple different patterns.</li>
|
|
<li>We don't automatically infer flags like isStore/isLoad yet.</li>
|
|
<li>We don't automatically generate the set of supported registers and
|
|
operations for the <a href="#"selectiondag_legalize>Legalizer</a> yet.</li>
|
|
<li>We don't have a way of tying in custom legalized nodes yet.</li>
|
|
</ul>
|
|
|
|
<p>Despite these limitations, the instruction selector generator is still quite
|
|
useful for most of the binary and logical operations in typical instruction
|
|
sets. If you run into any problems or can't figure out how to do something,
|
|
please let Chris know!</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The scheduling phase takes the DAG of target instructions from the selection
|
|
phase and assigns an order. The scheduler can pick an order depending on
|
|
various constraints of the machines (i.e. order for minimal register pressure or
|
|
try to cover instruction latencies). Once an order is established, the DAG is
|
|
converted to a list of <a href="#machineinstr">MachineInstr</a>s and the
|
|
Selection DAG is destroyed.
|
|
</p>
|
|
|
|
<p>Note that this phase is logically seperate from the instruction selection
|
|
phase, but is tied to it closely in the code because it operates on
|
|
SelectionDAGs.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_future">Future directions for the SelectionDAG</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<ol>
|
|
<li>Optional function-at-a-time selection.</li>
|
|
<li>Auto-generate entire selector from .td file.</li>
|
|
</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="codeemit">Code Emission</a>
|
|
</div>
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="codeemit_asm">Generating Assembly Code</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
</div>
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="codeemit_bin">Generating Binary Machine Code</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>For the JIT or .o file writer</p>
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="targetimpls">Target-specific Implementation Notes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section of the document explains features or design decisions that
|
|
are specific to the code generator for a particular target.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="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>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_tt">X86 Target Triples Supported</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
The following are the known target triples that are supported by the X86
|
|
backend. This is not an exhaustive list, but it would be useful to add those
|
|
that people test.
|
|
</p>
|
|
|
|
<ul>
|
|
<li><b>i686-pc-linux-gnu</b> - Linux</li>
|
|
<li><b>i386-unknown-freebsd5.3</b> - FreeBSD 5.3</li>
|
|
<li><b>i686-pc-cygwin</b> - Cygwin on Win32</li>
|
|
<li><b>i686-pc-mingw32</b> - MingW on Win32</li>
|
|
<li><b>i686-apple-darwin*</b> - Apple Darwin on X86</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The x86 has a very flexible way of accessing memory. It is capable of
|
|
forming memory addresses of the following expression directly in integer
|
|
instructions (which use ModR/M addressing):</p>
|
|
|
|
<pre>
|
|
Base+[1,2,4,8]*IndexReg+Disp32
|
|
</pre>
|
|
|
|
<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>
|
|
|
|
<p>Stores, and all other instructions, treat the four memory operands in the
|
|
same way, in the same order.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_names">Instruction naming</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>
|
|
An instruction name consists of the base name, a default operand size, and a
|
|
a character per operand with an optional special size. For example:</p>
|
|
|
|
<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>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|