forked from OSchip/llvm-project
2170 lines
91 KiB
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2170 lines
91 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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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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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="#targetregisterinfo">The <tt>TargetRegisterInfo</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_types">SelectionDAG LegalizeTypes Phase</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="#liveintervals">Live Intervals</a>
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<ul>
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<li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
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<li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
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</ul></li>
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<li><a href="#regalloc">Register Allocation</a>
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<ul>
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<li><a href="#regAlloc_represent">How registers are represented in
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LLVM</a></li>
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<li><a href="#regAlloc_howTo">Mapping virtual registers to physical
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registers</a></li>
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<li><a href="#regAlloc_twoAddr">Handling two address instructions</a></li>
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<li><a href="#regAlloc_ssaDecon">The SSA deconstruction phase</a></li>
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<li><a href="#regAlloc_fold">Instruction folding</a></li>
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<li><a href="#regAlloc_builtIn">Built in register allocators</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="#tailcallopt">Tail call optimization</a></li>
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<li><a href="#sibcallopt">Sibling call optimization</a></li>
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<li><a href="#x86">The X86 backend</a></li>
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<li><a href="#ppc">The PowerPC backend</a>
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<ul>
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<li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
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<li><a href="#ppc_frame">Frame Layout</a></li>
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<li><a href="#ppc_prolog">Prolog/Epilog</a></li>
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<li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
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</ul></li>
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</ul></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
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<a href="mailto:isanbard@gmail.com">Bill Wendling</a>,
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<a href="mailto:pronesto@gmail.com">Fernando Magno Quintao
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Pereira</a> and
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<a href="mailto:jlaskey@mac.com">Jim Laskey</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
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to the machine code for a specified target—either in assembly form
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(suitable for a static compiler) or in binary machine code format (usable for
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a JIT compiler). The LLVM target-independent code generator consists of five
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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,
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independently 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>
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being generated for a target. These classes are intended to be abstract
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enough to represent the machine code for <i>any</i> target machine. These
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classes are 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,
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stack frame representation, etc). This code lives
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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
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use of 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
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target. 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>
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structure to interface for target-specific issues. The code for the
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target-independent JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
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</ol>
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<p>Depending on which part of the code generator you are interested in working
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on, different pieces of this will be useful to you. In any case, you should
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be familiar with the <a href="#targetdesc">target description</a>
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and <a href="#codegendesc">machine code representation</a> classes. If you
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want to add a backend for a new target, you will need
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to <a href="#targetimpls">implement the target description</a> classes for
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your new target and understand the <a href="LangRef.html">LLVM code
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representation</a>. If you are interested in implementing a
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new <a href="#codegenalgs">code generation algorithm</a>, it should only
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depend on the target-description and machine code representation classes,
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ensuring that it is portable.</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
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(<a href="#targetmachine"><tt>TargetMachine</tt></a>
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and <a 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
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others must be defined if the reusable code generator components are going to
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be 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
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C backend does not require register allocation, instruction selection, or any
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of 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
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a code generator like this is a (purely hypothetical) backend that converts
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LLVM 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 implement
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radically different code generators in the LLVM system that do not make use
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of any of the built-in components. Doing so is not recommended at all, but
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could be required for radically different targets that do not fit into the
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LLVM machine description model: FPGAs for example.</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
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and quality code generation for standard register-based microprocessors.
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Code 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. This stage produces the initial code for the program in
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the target instruction set, then makes use of virtual registers in SSA
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form and physical registers that represent any required register
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assignments due to target constraints or calling conventions. This step
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turns the LLVM code into a DAG of target instructions.</li>
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<li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> —
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This phase takes the DAG of target instructions produced by the
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instruction selection phase, determines an ordering of the instructions,
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then emits the instructions
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as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
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Note that we describe this in the <a href="#instselect">instruction
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selection section</a> because it operates on
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a <a href="#selectiondag_intro">SelectionDAG</a>.</li>
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<li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> —
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This optional stage consists of a series of machine-code optimizations
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that operate on the SSA-form produced by the instruction selector.
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Optimizations like modulo-scheduling or peephole optimization work
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here.</li>
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<li><b><a href="#regalloc">Register Allocation</a></b> — The target code
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is transformed from an infinite virtual register file in SSA form to the
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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
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the machine code has been generated for the function and the amount of
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stack space required is known (used for LLVM alloca's and spill slots),
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the prolog and epilog code for the function can be inserted and "abstract
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stack location references" can be eliminated. This stage is responsible
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for implementing optimizations like frame-pointer elimination and stack
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packing.</li>
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<li><b><a href="#latemco">Late Machine Code Optimizations</a></b> —
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Optimizations that operate on "final" machine code can go here, such as
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spill code scheduling and peephole optimizations.</li>
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<li><b><a href="#codeemit">Code Emission</a></b> — The final stage
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actually puts out the code for the current function, either in the target
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assembler format or in machine code.</li>
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</ol>
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<p>The code generator is based on the assumption that the instruction selector
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will use an optimal pattern matching selector to create high-quality
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sequences of native instructions. Alternative code generator designs based
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on pattern expansion and aggressive iterative peephole optimization are much
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slower. This design permits efficient compilation (important for JIT
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environments) and aggressive optimization (used when generating code offline)
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by allowing components of varying levels of sophistication to be used for any
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step of compilation.</p>
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<p>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
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needed.</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). In order to allow the maximum amount of
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commonality to be factored out, the LLVM code generator uses
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the <a href="TableGenFundamentals.html">TableGen</a> tool to describe big
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chunks of the target machine, which allows the use of domain-specific and
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target-specific abstractions to reduce the amount of repetition.</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 the <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
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the surface area of the code generator that needs to be understood before
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someone can get something working. Second, it makes it easier to change
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things. In particular, if tables and other things are all emitted
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by <tt>tblgen</tt>, we only need a change in one place (<tt>tblgen</tt>) to
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update 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 (located in the
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<tt>include/llvm/Target</tt> directory) provide an abstract description of
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the 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
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pieces of code generation algorithms.</p>
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<p>All of the target description classes (except the
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<tt><a href="#targetdata">TargetData</a></tt> class) are designed to be
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subclassed by the concrete target implementation, and have virtual methods
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implemented. To get to these implementations, the
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<tt><a href="#targetmachine">TargetMachine</a></tt> class provides accessors
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that 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 a concrete target implementation
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(e.g., <tt>X86TargetMachine</tt>) which implements the various virtual
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methods. The only required target description class is
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the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the code
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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
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target lays out memory for structures, the alignment requirements for various
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data 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
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SelectionDAG operations. Among other things, this class indicates:</p>
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<ul>
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<li>an initial register class to use for various <tt>ValueType</tt>s,</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 <tt>setcc</tt> operations,</li>
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<li>the type to use for shift amounts, and</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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</ul>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a>
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</div>
|
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<div class="doc_text">
|
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<p>The <tt>TargetRegisterInfo</tt> class is used to describe the register file
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of the 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 integers. Physical registers (those that actually exist in the
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target description) are unique small numbers, and virtual registers are
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generally 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
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register (used for assembly output and debugging dumps) and a set of aliases
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(used to indicate whether one register overlaps with another).</p>
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<p>In addition to the per-register description, the <tt>TargetRegisterInfo</tt>
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class 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,
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it replaces virtual registers with a physical register in the set.</p>
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<p>The target-specific implementations of these classes is auto-generated from
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a <a href="TableGenFundamentals.html">TableGen</a> description of the
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register file.</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
|
|
and defs, whether the instruction has certain target-independent properties
|
|
(accesses memory, is commutable, etc), and holds any target-specific
|
|
flags.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
|
|
stack frame layout of the target. It holds the direction of stack growth, the
|
|
known stack alignment on entry to each function, and the offset to the local
|
|
area. The offset to the local area is the offset from the stack pointer on
|
|
function entry to the first location where function data (local variables,
|
|
spill locations) can be stored.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
|
|
specific chip set being targeted. A sub-target informs code generation of
|
|
which instructions are supported, instruction latencies and instruction
|
|
execution itinerary; i.e., which processing units are used, in what order,
|
|
and for how long.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>TargetJITInfo</tt> class exposes an abstract interface used by the
|
|
Just-In-Time code generator to perform target-specific activities, such as
|
|
emitting stubs. If a <tt>TargetMachine</tt> supports JIT code generation, it
|
|
should provide one of these objects through the <tt>getJITInfo</tt>
|
|
method.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="codegendesc">Machine code description classes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>At the high-level, LLVM code is translated to a machine specific
|
|
representation formed out of
|
|
<a href="#machinefunction"><tt>MachineFunction</tt></a>,
|
|
<a href="#machinebasicblock"><tt>MachineBasicBlock</tt></a>,
|
|
and <a href="#machineinstr"><tt>MachineInstr</tt></a> instances (defined
|
|
in <tt>include/llvm/CodeGen</tt>). This representation is completely target
|
|
agnostic, representing instructions in their most abstract form: an opcode
|
|
and a series of operands. This representation is designed to support both an
|
|
SSA representation for machine code, as well as a register allocated, non-SSA
|
|
form.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machineinstr">The <tt>MachineInstr</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Target machine instructions are represented as instances of the
|
|
<tt>MachineInstr</tt> class. This class is an extremely abstract way of
|
|
representing machine instructions. In particular, it only keeps track of an
|
|
opcode number and a set of operands.</p>
|
|
|
|
<p>The opcode number is a simple unsigned integer that only has meaning to a
|
|
specific backend. All of the instructions for a target should be defined in
|
|
the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values are
|
|
auto-generated from this description. The <tt>MachineInstr</tt> class does
|
|
not have any information about how to interpret the instruction (i.e., what
|
|
the semantics of the instruction are); for that you must refer to the
|
|
<tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
|
|
|
|
<p>The operands of a machine instruction can be of several different types: a
|
|
register reference, a constant integer, a basic block reference, etc. In
|
|
addition, a machine operand should be marked as a def or a use of the value
|
|
(though only registers are allowed to be defs).</p>
|
|
|
|
<p>By convention, the LLVM code generator orders instruction operands so that
|
|
all register definitions come before the register uses, even on architectures
|
|
that are normally printed in other orders. For example, the SPARC add
|
|
instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
|
|
and stores the result into the "%i3" register. In the LLVM code generator,
|
|
the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the
|
|
destination first.</p>
|
|
|
|
<p>Keeping destination (definition) operands at the beginning of the operand
|
|
list has several advantages. In particular, the debugging printer will print
|
|
the instruction like this:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
%r3 = add %i1, %i2
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Also if the first operand is a def, it is easier to <a href="#buildmi">create
|
|
instructions</a> whose only def is the first operand.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
|
|
located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
|
|
<tt>BuildMI</tt> functions make it easy to build arbitrary machine
|
|
instructions. Usage of the <tt>BuildMI</tt> functions look like this:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
|
|
// instruction. The '1' specifies how many operands will be added.
|
|
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
|
|
|
|
// Create the same instr, but insert it at the end of a basic block.
|
|
MachineBasicBlock &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>
|
|
</div>
|
|
|
|
<p>The key thing to remember with the <tt>BuildMI</tt> functions is that you
|
|
have to specify the number of operands that the machine instruction will
|
|
take. This allows for efficient memory allocation. You also need to specify
|
|
if operands default to be uses of values, not definitions. If you need to
|
|
add a definition operand (other than the optional destination register), you
|
|
must explicitly mark it as such:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
MI.addReg(Reg, RegState::Define);
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="fixedregs">Fixed (preassigned) registers</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>One important issue that the code generator needs to be aware of is the
|
|
presence of fixed registers. In particular, there are often places in the
|
|
instruction stream where the register allocator <em>must</em> arrange for a
|
|
particular value to be in a particular register. This can occur due to
|
|
limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
|
|
with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like
|
|
calling conventions. In any case, the instruction selector should emit code
|
|
that copies a virtual register into or out of a physical register when
|
|
needed.</p>
|
|
|
|
<p>For example, consider this simple LLVM example:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
define i32 @test(i32 %X, i32 %Y) {
|
|
%Z = udiv i32 %X, %Y
|
|
ret i32 %Z
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>The X86 instruction selector produces this machine code for the <tt>div</tt>
|
|
and <tt>ret</tt> (use "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to
|
|
get this):</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
;; Start of div
|
|
%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
|
|
%reg1027 = sar %reg1024, 31
|
|
%EDX = mov %reg1027 ;; Sign extend X into EDX
|
|
idiv %reg1025 ;; Divide by Y (in reg1025)
|
|
%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
|
|
|
|
;; Start of ret
|
|
%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
|
|
ret
|
|
</pre>
|
|
</div>
|
|
|
|
<p>By the end of code generation, the register allocator has coalesced the
|
|
registers and deleted the resultant identity moves producing the following
|
|
code:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
;; X is in EAX, Y is in ECX
|
|
mov %EAX, %EDX
|
|
sar %EDX, 31
|
|
idiv %ECX
|
|
ret
|
|
</pre>
|
|
</div>
|
|
|
|
<p>This approach is extremely general (if it can handle the X86 architecture, it
|
|
can handle anything!) and allows all of the target specific knowledge about
|
|
the instruction stream to be isolated in the instruction selector. Note that
|
|
physical registers should have a short lifetime for good code generation, and
|
|
all physical registers are assumed dead on entry to and exit from basic
|
|
blocks (before register allocation). Thus, if you need a value to be live
|
|
across basic block boundaries, it <em>must</em> live in a virtual
|
|
register.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ssa">Machine code in SSA form</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and are
|
|
maintained in SSA-form until register allocation happens. For the most part,
|
|
this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes
|
|
become machine code PHI nodes, and virtual registers are only allowed to have
|
|
a single definition.</p>
|
|
|
|
<p>After register allocation, machine code is no longer in SSA-form because
|
|
there are no virtual registers left in the code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
|
|
(<tt><a href="#machineinstr">MachineInstr</a></tt> instances). It roughly
|
|
corresponds to the LLVM code input to the instruction selector, but there can
|
|
be a one-to-many mapping (i.e. one LLVM basic block can map to multiple
|
|
machine basic blocks). The <tt>MachineBasicBlock</tt> class has a
|
|
"<tt>getBasicBlock</tt>" method, which returns the LLVM basic block that it
|
|
comes from.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="machinefunction">The <tt>MachineFunction</tt> class</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
|
|
(<tt><a href="#machinebasicblock">MachineBasicBlock</a></tt> instances). It
|
|
corresponds one-to-one with the LLVM function input to the instruction
|
|
selector. In addition to a list of basic blocks,
|
|
the <tt>MachineFunction</tt> contains a a <tt>MachineConstantPool</tt>,
|
|
a <tt>MachineFrameInfo</tt>, a <tt>MachineFunctionInfo</tt>, and a
|
|
<tt>MachineRegisterInfo</tt>. See
|
|
<tt>include/llvm/CodeGen/MachineFunction.h</tt> for more information.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="codegenalgs">Target-independent code generation algorithms</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section documents the phases described in the
|
|
<a href="#high-level-design">high-level design of the code generator</a>.
|
|
It explains how they work and some of the rationale behind their design.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="instselect">Instruction Selection</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Instruction Selection is the process of translating LLVM code presented to
|
|
the code generator into target-specific machine instructions. There are
|
|
several well-known ways to do this in the literature. LLVM uses a
|
|
SelectionDAG based instruction selector.</p>
|
|
|
|
<p>Portions of the DAG instruction selector are generated from the target
|
|
description (<tt>*.td</tt>) files. Our goal is for the entire instruction
|
|
selector to be generated from these <tt>.td</tt> files, though currently
|
|
there are still things that require custom C++ code.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_intro">Introduction to SelectionDAGs</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The SelectionDAG provides an abstraction for code representation in a way
|
|
that is amenable to instruction selection using automatic techniques
|
|
(e.g. dynamic-programming based optimal pattern matching selectors). It is
|
|
also well-suited to other phases of code generation; in particular,
|
|
instruction scheduling (SelectionDAG's are very close to scheduling DAGs
|
|
post-selection). Additionally, the SelectionDAG provides a host
|
|
representation where a large variety of very-low-level (but
|
|
target-independent) <a href="#selectiondag_optimize">optimizations</a> may be
|
|
performed; ones which require extensive information about the instructions
|
|
efficiently supported by the target.</p>
|
|
|
|
<p>The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
|
|
<tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
|
|
operation code (Opcode) that indicates what operation the node performs and
|
|
the operands to the operation. The various operation node types are
|
|
described at the top of the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
|
|
file.</p>
|
|
|
|
<p>Although most operations define a single value, each node in the graph may
|
|
define multiple values. For example, a combined div/rem operation will
|
|
define both the dividend and the remainder. Many other situations require
|
|
multiple values as well. Each node also has some number of operands, which
|
|
are edges to the node defining the used value. Because nodes may define
|
|
multiple values, edges are represented by instances of the <tt>SDValue</tt>
|
|
class, which is a <tt><SDNode, unsigned></tt> pair, indicating the node
|
|
and result value being used, respectively. Each value produced by
|
|
an <tt>SDNode</tt> has an associated <tt>MVT</tt> (Machine Value Type)
|
|
indicating what the type of the value is.</p>
|
|
|
|
<p>SelectionDAGs contain two different kinds of values: those that represent
|
|
data flow and those that represent control flow dependencies. Data values
|
|
are simple edges with an integer or floating point value type. Control edges
|
|
are represented as "chain" edges which are of type <tt>MVT::Other</tt>.
|
|
These edges provide an ordering between nodes that have side effects (such as
|
|
loads, stores, calls, returns, etc). All nodes that have side effects should
|
|
take a token chain as input and produce a new one as output. By convention,
|
|
token chain inputs are always operand #0, and chain results are always the
|
|
last value produced by an operation.</p>
|
|
|
|
<p>A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
|
|
always a marker node with an Opcode of <tt>ISD::EntryToken</tt>. The Root
|
|
node is the final side-effecting node in the token chain. For example, in a
|
|
single basic block function it would be the return node.</p>
|
|
|
|
<p>One important concept for SelectionDAGs is the notion of a "legal" vs.
|
|
"illegal" DAG. A legal DAG for a target is one that only uses supported
|
|
operations and supported types. On a 32-bit PowerPC, for example, a DAG with
|
|
a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that
|
|
uses a SREM or UREM operation. The
|
|
<a href="#selectinodag_legalize_types">legalize types</a> and
|
|
<a href="#selectiondag_legalize">legalize operations</a> phases are
|
|
responsible for turning an illegal DAG into a legal DAG.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>SelectionDAG-based instruction selection consists of the following steps:</p>
|
|
|
|
<ol>
|
|
<li><a href="#selectiondag_build">Build initial DAG</a> — 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 <tt>div</tt>/<tt>rem</tt> pairs) for targets that support these meta
|
|
operations. This makes the resultant code more efficient and
|
|
the <a href="#selectiondag_select">select instructions from DAG</a> phase
|
|
(below) simpler.</li>
|
|
|
|
<li><a href="#selectiondag_legalize_types">Legalize SelectionDAG Types</a>
|
|
— This stage transforms SelectionDAG nodes to eliminate any types
|
|
that are unsupported on the target.</li>
|
|
|
|
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> — The
|
|
SelectionDAG optimizer is run to clean up redundancies exposed by type
|
|
legalization.</li>
|
|
|
|
<li><a href="#selectiondag_legalize">Legalize SelectionDAG Types</a> —
|
|
This stage transforms SelectionDAG nodes to eliminate any types that are
|
|
unsupported on the target.</li>
|
|
|
|
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> — The
|
|
SelectionDAG optimizer is run to eliminate inefficiencies introduced by
|
|
operation 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. The following options pop up a window
|
|
displaying the SelectionDAG at specific times (if you only get errors printed
|
|
to the console while using this, you probably
|
|
<a href="ProgrammersManual.html#ViewGraph">need to configure your system</a>
|
|
to add support for it).</p>
|
|
|
|
<ul>
|
|
<li><tt>-view-dag-combine1-dags</tt> displays the DAG after being built,
|
|
before the first optimization pass.</li>
|
|
|
|
<li><tt>-view-legalize-dags</tt> displays the DAG before Legalization.</li>
|
|
|
|
<li><tt>-view-dag-combine2-dags</tt> displays the DAG before the second
|
|
optimization pass.</li>
|
|
|
|
<li><tt>-view-isel-dags</tt> displays the DAG before the Select phase.</li>
|
|
|
|
<li><tt>-view-sched-dags</tt> displays the DAG before Scheduling.</li>
|
|
</ul>
|
|
|
|
<p>The <tt>-view-sunit-dags</tt> displays the Scheduler's dependency graph.
|
|
This graph is based on the final SelectionDAG, with nodes that must be
|
|
scheduled together bundled into a single scheduling-unit node, and with
|
|
immediate operands and other nodes that aren't relevant for scheduling
|
|
omitted.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_build">Initial SelectionDAG Construction</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The initial SelectionDAG is naïvely peephole expanded from the LLVM
|
|
input by the <tt>SelectionDAGLowering</tt> class in the
|
|
<tt>lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp</tt> file. The intent of
|
|
this pass is to expose as much low-level, target-specific details to the
|
|
SelectionDAG as possible. This pass is mostly hard-coded (e.g. an
|
|
LLVM <tt>add</tt> turns into an <tt>SDNode add</tt> while a
|
|
<tt>getelementptr</tt> is expanded into the obvious arithmetic). This pass
|
|
requires target-specific hooks to lower calls, returns, varargs, etc. For
|
|
these features, the <tt><a href="#targetlowering">TargetLowering</a></tt>
|
|
interface is used.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Legalize phase is in charge of converting a DAG to only use the types
|
|
that are natively supported by the target.</p>
|
|
|
|
<p>There are two main ways of converting values of unsupported scalar types to
|
|
values of supported types: converting small types to larger types
|
|
("promoting"), and breaking up large integer types into smaller ones
|
|
("expanding"). For example, a target might require that all f32 values are
|
|
promoted to f64 and that all i1/i8/i16 values are promoted to i32. The same
|
|
target might require that all i64 values be expanded into pairs of i32
|
|
values. These changes can insert sign and zero extensions as needed to make
|
|
sure that the final code has the same behavior as the input.</p>
|
|
|
|
<p>There are two main ways of converting values of unsupported vector types to
|
|
value of supported types: splitting vector types, multiple times if
|
|
necessary, until a legal type is found, and extending vector types by adding
|
|
elements to the end to round them out to legal types ("widening"). If a
|
|
vector gets split all the way down to single-element parts with no supported
|
|
vector type being found, the elements are converted to scalars
|
|
("scalarizing").</p>
|
|
|
|
<p>A target implementation tells the legalizer which types are supported (and
|
|
which register class to use for them) by calling the
|
|
<tt>addRegisterClass</tt> method in its TargetLowering constructor.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Legalize phase is in charge of converting a DAG to only use the
|
|
operations that are natively supported by the target.</p>
|
|
|
|
<p>Targets often have weird constraints, such as not supporting every operation
|
|
on every supported datatype (e.g. X86 does not support byte conditional moves
|
|
and PowerPC does not support sign-extending loads from a 16-bit memory
|
|
location). Legalize takes care of this by open-coding another sequence of
|
|
operations to emulate the operation ("expansion"), by promoting one type to a
|
|
larger type that supports the operation ("promotion"), or by using a
|
|
target-specific hook to implement the legalization ("custom").</p>
|
|
|
|
<p>A target implementation tells the legalizer which operations are not
|
|
supported (and which of the above three actions to take) by calling the
|
|
<tt>setOperationAction</tt> method in its <tt>TargetLowering</tt>
|
|
constructor.</p>
|
|
|
|
<p>Prior to the existence of the Legalize passes, we required that every target
|
|
<a href="#selectiondag_optimize">selector</a> supported and handled every
|
|
operator and type even if they are not natively supported. The introduction
|
|
of the Legalize phases allows all of the canonicalization patterns to be
|
|
shared across targets, and makes it very easy to optimize the canonicalized
|
|
code because it is still in the form of a DAG.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_optimize">SelectionDAG Optimization Phase: the DAG
|
|
Combiner</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The SelectionDAG optimization phase is run multiple times for code
|
|
generation, immediately after the DAG is built and once after each
|
|
legalization. The first run of the pass allows the initial code to be
|
|
cleaned up (e.g. performing optimizations that depend on knowing that the
|
|
operators have restricted type inputs). Subsequent runs of the pass clean up
|
|
the messy code generated by the Legalize passes, which allows Legalize to be
|
|
very simple (it can focus on making code legal instead of focusing on
|
|
generating <em>good</em> and legal code).</p>
|
|
|
|
<p>One important class of optimizations performed is optimizing inserted sign
|
|
and zero extension instructions. We currently use ad-hoc techniques, but
|
|
could move to more rigorous techniques in the future. Here are some good
|
|
papers on the subject:</p>
|
|
|
|
<p>"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
|
|
integer arithmetic</a>"<br>
|
|
Kevin Redwine and Norman Ramsey<br>
|
|
International Conference on Compiler Construction (CC) 2004</p>
|
|
|
|
<p>"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
|
|
sign extension elimination</a>"<br>
|
|
Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
|
|
Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
|
|
and Implementation.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_select">SelectionDAG Select Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The Select phase is the bulk of the target-specific code for instruction
|
|
selection. This phase takes a legal SelectionDAG as input, pattern matches
|
|
the instructions supported by the target to this DAG, and produces a new DAG
|
|
of target code. For example, consider the following LLVM fragment:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
%t1 = fadd float %W, %X
|
|
%t2 = fmul float %t1, %Y
|
|
%t3 = fadd float %t2, %Z
|
|
</pre>
|
|
</div>
|
|
|
|
<p>This LLVM code corresponds to a SelectionDAG that looks basically like
|
|
this:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
|
|
</pre>
|
|
</div>
|
|
|
|
<p>If a target supports floating point multiply-and-add (FMA) operations, one of
|
|
the adds can be merged with the multiply. On the PowerPC, for example, the
|
|
output of the instruction selector might look like this DAG:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
(FMADDS (FADDS W, X), Y, Z)
|
|
</pre>
|
|
</div>
|
|
|
|
<p>The <tt>FMADDS</tt> instruction is a ternary instruction that multiplies its
|
|
first two operands and adds the third (as single-precision floating-point
|
|
numbers). The <tt>FADDS</tt> instruction is a simple binary single-precision
|
|
add instruction. To perform this pattern match, the PowerPC backend includes
|
|
the following instruction definitions:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
def FMADDS : AForm_1<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>
|
|
</div>
|
|
|
|
<p>The portion of the instruction definition in bold indicates the pattern used
|
|
to match the instruction. The DAG operators
|
|
(like <tt>fmul</tt>/<tt>fadd</tt>) are defined in
|
|
the <tt>include/llvm/Target/TargetSelectionDAG.td</tt> file. "
|
|
<tt>F4RC</tt>" is the register class of the input and result values.</p>
|
|
|
|
<p>The TableGen DAG instruction selector generator reads the instruction
|
|
patterns in the <tt>.td</tt> file and automatically builds parts of the
|
|
pattern matching code for your target. It has the following strengths:</p>
|
|
|
|
<ul>
|
|
<li>At compiler-compiler time, it analyzes your instruction patterns and tells
|
|
you if your patterns make sense or not.</li>
|
|
|
|
<li>It can handle arbitrary constraints on operands for the pattern match. In
|
|
particular, it is straight-forward to say things like "match any immediate
|
|
that is a 13-bit sign-extended value". For examples, see the
|
|
<tt>immSExt16</tt> and related <tt>tblgen</tt> classes in the PowerPC
|
|
backend.</li>
|
|
|
|
<li>It knows several important identities for the patterns defined. For
|
|
example, it knows that addition is commutative, so it allows the
|
|
<tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
|
|
well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
|
|
to specially handle this case.</li>
|
|
|
|
<li>It has a full-featured type-inferencing system. In particular, you should
|
|
rarely have to explicitly tell the system what type parts of your patterns
|
|
are. In the <tt>FMADDS</tt> case above, we didn't have to tell
|
|
<tt>tblgen</tt> that all of the nodes in the pattern are of type 'f32'.
|
|
It was able to infer and propagate this knowledge from the fact that
|
|
<tt>F4RC</tt> has type 'f32'.</li>
|
|
|
|
<li>Targets can define their own (and rely on built-in) "pattern fragments".
|
|
Pattern fragments are chunks of reusable patterns that get inlined into
|
|
your patterns during compiler-compiler time. For example, the integer
|
|
"<tt>(not x)</tt>" operation is actually defined as a pattern fragment
|
|
that expands as "<tt>(xor x, -1)</tt>", since the SelectionDAG does not
|
|
have a native '<tt>not</tt>' operation. Targets can define their own
|
|
short-hand fragments as they see fit. See the definition of
|
|
'<tt>not</tt>' and '<tt>ineg</tt>' for examples.</li>
|
|
|
|
<li>In addition to instructions, targets can specify arbitrary patterns that
|
|
map to one or more instructions using the 'Pat' class. For example, the
|
|
PowerPC has no way to load an arbitrary integer immediate into a register
|
|
in one instruction. To tell tblgen how to do this, it defines:
|
|
<br>
|
|
<br>
|
|
<div class="doc_code">
|
|
<pre>
|
|
// Arbitrary immediate support. Implement in terms of LIS/ORI.
|
|
def : Pat<(i32 imm:$imm),
|
|
(ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))>;
|
|
</pre>
|
|
</div>
|
|
<br>
|
|
If none of the single-instruction patterns for loading an immediate into a
|
|
register match, this will be used. This rule says "match an arbitrary i32
|
|
immediate, turning it into an <tt>ORI</tt> ('or a 16-bit immediate') and
|
|
an <tt>LIS</tt> ('load 16-bit immediate, where the immediate is shifted to
|
|
the left 16 bits') instruction". To make this work, the
|
|
<tt>LO16</tt>/<tt>HI16</tt> node transformations are used to manipulate
|
|
the input immediate (in this case, take the high or low 16-bits of the
|
|
immediate).</li>
|
|
|
|
<li>While the system does automate a lot, it still allows you to write custom
|
|
C++ code to match special cases if there is something that is hard to
|
|
express.</li>
|
|
</ul>
|
|
|
|
<p>While it has many strengths, the system currently has some limitations,
|
|
primarily because it is a work in progress and is not yet finished:</p>
|
|
|
|
<ul>
|
|
<li>Overall, there is no way to define or match SelectionDAG nodes that define
|
|
multiple values (e.g. <tt>SMUL_LOHI</tt>, <tt>LOAD</tt>, <tt>CALL</tt>,
|
|
etc). This is the biggest reason that you currently still <em>have
|
|
to</em> write custom C++ code for your instruction selector.</li>
|
|
|
|
<li>There is no great way to support matching complex addressing modes yet.
|
|
In the future, we will extend pattern fragments to allow them to define
|
|
multiple values (e.g. the four operands of the <a href="#x86_memory">X86
|
|
addressing mode</a>, which are currently matched with custom C++ code).
|
|
In addition, we'll extend fragments so that a fragment can match multiple
|
|
different patterns.</li>
|
|
|
|
<li>We don't automatically infer flags like isStore/isLoad yet.</li>
|
|
|
|
<li>We don't automatically generate the set of supported registers and
|
|
operations for the <a href="#selectiondag_legalize">Legalizer</a>
|
|
yet.</li>
|
|
|
|
<li>We don't have a way of tying in custom legalized nodes yet.</li>
|
|
</ul>
|
|
|
|
<p>Despite these limitations, the instruction selector generator is still quite
|
|
useful for most of the binary and logical operations in typical instruction
|
|
sets. If you run into any problems or can't figure out how to do something,
|
|
please let Chris know!</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The scheduling phase takes the DAG of target instructions from the selection
|
|
phase and assigns an order. The scheduler can pick an order depending on
|
|
various constraints of the machines (i.e. order for minimal register pressure
|
|
or try to cover instruction latencies). Once an order is established, the
|
|
DAG is converted to a list
|
|
of <tt><a href="#machineinstr">MachineInstr</a></tt>s and the SelectionDAG is
|
|
destroyed.</p>
|
|
|
|
<p>Note that this phase is logically separate from the instruction selection
|
|
phase, but is tied to it closely in the code because it operates on
|
|
SelectionDAGs.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="selectiondag_future">Future directions for the SelectionDAG</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<ol>
|
|
<li>Optional function-at-a-time selection.</li>
|
|
|
|
<li>Auto-generate entire selector from <tt>.td</tt> file.</li>
|
|
</ol>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ssamco">SSA-based Machine Code Optimizations</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="liveintervals">Live Intervals</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
|
|
They are used by some <a href="#regalloc">register allocator</a> passes to
|
|
determine if two or more virtual registers which require the same physical
|
|
register are live at the same point in the program (i.e., they conflict).
|
|
When this situation occurs, one virtual register must be <i>spilled</i>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="livevariable_analysis">Live Variable Analysis</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The first step in determining the live intervals of variables is to calculate
|
|
the set of registers that are immediately dead after the instruction (i.e.,
|
|
the instruction calculates the value, but it is never used) and the set of
|
|
registers that are used by the instruction, but are never used after the
|
|
instruction (i.e., they are killed). Live variable information is computed
|
|
for each <i>virtual</i> register and <i>register allocatable</i> physical
|
|
register in the function. This is done in a very efficient manner because it
|
|
uses SSA to sparsely compute lifetime information for virtual registers
|
|
(which are in SSA form) and only has to track physical registers within a
|
|
block. Before register allocation, LLVM can assume that physical registers
|
|
are only live within a single basic block. This allows it to do a single,
|
|
local analysis to resolve physical register lifetimes within each basic
|
|
block. If a physical register is not register allocatable (e.g., a stack
|
|
pointer or condition codes), it is not tracked.</p>
|
|
|
|
<p>Physical registers may be live in to or out of a function. Live in values are
|
|
typically arguments in registers. Live out values are typically return values
|
|
in registers. Live in values are marked as such, and are given a dummy
|
|
"defining" instruction during live intervals analysis. If the last basic
|
|
block of a function is a <tt>return</tt>, then it's marked as using all live
|
|
out values in the function.</p>
|
|
|
|
<p><tt>PHI</tt> nodes need to be handled specially, because the calculation of
|
|
the live variable information from a depth first traversal of the CFG of the
|
|
function won't guarantee that a virtual register used by the <tt>PHI</tt>
|
|
node is defined before it's used. When a <tt>PHI</tt> node is encountered,
|
|
only the definition is handled, because the uses will be handled in other
|
|
basic blocks.</p>
|
|
|
|
<p>For each <tt>PHI</tt> node of the current basic block, we simulate an
|
|
assignment at the end of the current basic block and traverse the successor
|
|
basic blocks. If a successor basic block has a <tt>PHI</tt> node and one of
|
|
the <tt>PHI</tt> node's operands is coming from the current basic block, then
|
|
the variable is marked as <i>alive</i> within the current basic block and all
|
|
of its predecessor basic blocks, until the basic block with the defining
|
|
instruction is encountered.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="liveintervals_analysis">Live Intervals Analysis</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>We now have the information available to perform the live intervals analysis
|
|
and build the live intervals themselves. We start off by numbering the basic
|
|
blocks and machine instructions. We then handle the "live-in" values. These
|
|
are in physical registers, so the physical register is assumed to be killed
|
|
by the end of the basic block. Live intervals for virtual registers are
|
|
computed for some ordering of the machine instructions <tt>[1, N]</tt>. A
|
|
live interval is an interval <tt>[i, j)</tt>, where <tt>1 <= i <= j
|
|
< N</tt>, for which a variable is live.</p>
|
|
|
|
<p><i><b>More to come...</b></i></p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="regalloc">Register Allocation</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <i>Register Allocation problem</i> consists in mapping a program
|
|
<i>P<sub>v</sub></i>, that can use an unbounded number of virtual registers,
|
|
to a program <i>P<sub>p</sub></i> that contains a finite (possibly small)
|
|
number of physical registers. Each target architecture has a different number
|
|
of physical registers. If the number of physical registers is not enough to
|
|
accommodate all the virtual registers, some of them will have to be mapped
|
|
into memory. These virtuals are called <i>spilled virtuals</i>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_represent">How registers are represented in LLVM</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>In LLVM, physical registers are denoted by integer numbers that normally
|
|
range from 1 to 1023. To see how this numbering is defined for a particular
|
|
architecture, you can read the <tt>GenRegisterNames.inc</tt> file for that
|
|
architecture. For instance, by
|
|
inspecting <tt>lib/Target/X86/X86GenRegisterNames.inc</tt> we see that the
|
|
32-bit register <tt>EAX</tt> is denoted by 15, and the MMX register
|
|
<tt>MM0</tt> is mapped to 48.</p>
|
|
|
|
<p>Some architectures contain registers that share the same physical location. A
|
|
notable example is the X86 platform. For instance, in the X86 architecture,
|
|
the registers <tt>EAX</tt>, <tt>AX</tt> and <tt>AL</tt> share the first eight
|
|
bits. These physical registers are marked as <i>aliased</i> in LLVM. Given a
|
|
particular architecture, you can check which registers are aliased by
|
|
inspecting its <tt>RegisterInfo.td</tt> file. Moreover, the method
|
|
<tt>TargetRegisterInfo::getAliasSet(p_reg)</tt> returns an array containing
|
|
all the physical registers aliased to the register <tt>p_reg</tt>.</p>
|
|
|
|
<p>Physical registers, in LLVM, are grouped in <i>Register Classes</i>.
|
|
Elements in the same register class are functionally equivalent, and can be
|
|
interchangeably used. Each virtual register can only be mapped to physical
|
|
registers of a particular class. For instance, in the X86 architecture, some
|
|
virtuals can only be allocated to 8 bit registers. A register class is
|
|
described by <tt>TargetRegisterClass</tt> objects. To discover if a virtual
|
|
register is compatible with a given physical, this code can be used:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
bool RegMapping_Fer::compatible_class(MachineFunction &mf,
|
|
unsigned v_reg,
|
|
unsigned p_reg) {
|
|
assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &&
|
|
"Target register must be physical");
|
|
const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
|
|
return trc->contains(p_reg);
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Sometimes, mostly for debugging purposes, it is useful to change the number
|
|
of physical registers available in the target architecture. This must be done
|
|
statically, inside the <tt>TargetRegsterInfo.td</tt> file. Just <tt>grep</tt>
|
|
for <tt>RegisterClass</tt>, the last parameter of which is a list of
|
|
registers. Just commenting some out is one simple way to avoid them being
|
|
used. A more polite way is to explicitly exclude some registers from
|
|
the <i>allocation order</i>. See the definition of the <tt>GR8</tt> register
|
|
class in <tt>lib/Target/X86/X86RegisterInfo.td</tt> for an example of this.
|
|
</p>
|
|
|
|
<p>Virtual registers are also denoted by integer numbers. Contrary to physical
|
|
registers, different virtual registers never share the same number. The
|
|
smallest virtual register is normally assigned the number 1024. This may
|
|
change, so, in order to know which is the first virtual register, you should
|
|
access <tt>TargetRegisterInfo::FirstVirtualRegister</tt>. Any register whose
|
|
number is greater than or equal
|
|
to <tt>TargetRegisterInfo::FirstVirtualRegister</tt> is considered a virtual
|
|
register. Whereas physical registers are statically defined in
|
|
a <tt>TargetRegisterInfo.td</tt> file and cannot be created by the
|
|
application developer, that is not the case with virtual registers. In order
|
|
to create new virtual registers, use the
|
|
method <tt>MachineRegisterInfo::createVirtualRegister()</tt>. This method
|
|
will return a virtual register with the highest code.</p>
|
|
|
|
<p>Before register allocation, the operands of an instruction are mostly virtual
|
|
registers, although physical registers may also be used. In order to check if
|
|
a given machine operand is a register, use the boolean
|
|
function <tt>MachineOperand::isRegister()</tt>. To obtain the integer code of
|
|
a register, use <tt>MachineOperand::getReg()</tt>. An instruction may define
|
|
or use a register. For instance, <tt>ADD reg:1026 := reg:1025 reg:1024</tt>
|
|
defines the registers 1024, and uses registers 1025 and 1026. Given a
|
|
register operand, the method <tt>MachineOperand::isUse()</tt> informs if that
|
|
register is being used by the instruction. The
|
|
method <tt>MachineOperand::isDef()</tt> informs if that registers is being
|
|
defined.</p>
|
|
|
|
<p>We will call physical registers present in the LLVM bitcode before register
|
|
allocation <i>pre-colored registers</i>. Pre-colored registers are used in
|
|
many different situations, for instance, to pass parameters of functions
|
|
calls, and to store results of particular instructions. There are two types
|
|
of pre-colored registers: the ones <i>implicitly</i> defined, and
|
|
those <i>explicitly</i> defined. Explicitly defined registers are normal
|
|
operands, and can be accessed
|
|
with <tt>MachineInstr::getOperand(int)::getReg()</tt>. In order to check
|
|
which registers are implicitly defined by an instruction, use
|
|
the <tt>TargetInstrInfo::get(opcode)::ImplicitDefs</tt>,
|
|
where <tt>opcode</tt> is the opcode of the target instruction. One important
|
|
difference between explicit and implicit physical registers is that the
|
|
latter are defined statically for each instruction, whereas the former may
|
|
vary depending on the program being compiled. For example, an instruction
|
|
that represents a function call will always implicitly define or use the same
|
|
set of physical registers. To read the registers implicitly used by an
|
|
instruction,
|
|
use <tt>TargetInstrInfo::get(opcode)::ImplicitUses</tt>. Pre-colored
|
|
registers impose constraints on any register allocation algorithm. The
|
|
register allocator must make sure that none of them are overwritten by
|
|
the values of virtual registers while still alive.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_howTo">Mapping virtual registers to physical registers</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>There are two ways to map virtual registers to physical registers (or to
|
|
memory slots). The first way, that we will call <i>direct mapping</i>, is
|
|
based on the use of methods of the classes <tt>TargetRegisterInfo</tt>,
|
|
and <tt>MachineOperand</tt>. The second way, that we will call <i>indirect
|
|
mapping</i>, relies on the <tt>VirtRegMap</tt> class in order to insert loads
|
|
and stores sending and getting values to and from memory.</p>
|
|
|
|
<p>The direct mapping provides more flexibility to the developer of the register
|
|
allocator; however, it is more error prone, and demands more implementation
|
|
work. Basically, the programmer will have to specify where load and store
|
|
instructions should be inserted in the target function being compiled in
|
|
order to get and store values in memory. To assign a physical register to a
|
|
virtual register present in a given operand,
|
|
use <tt>MachineOperand::setReg(p_reg)</tt>. To insert a store instruction,
|
|
use <tt>TargetRegisterInfo::storeRegToStackSlot(...)</tt>, and to insert a
|
|
load instruction, use <tt>TargetRegisterInfo::loadRegFromStackSlot</tt>.</p>
|
|
|
|
<p>The indirect mapping shields the application developer from the complexities
|
|
of inserting load and store instructions. In order to map a virtual register
|
|
to a physical one, use <tt>VirtRegMap::assignVirt2Phys(vreg, preg)</tt>. In
|
|
order to map a certain virtual register to memory,
|
|
use <tt>VirtRegMap::assignVirt2StackSlot(vreg)</tt>. This method will return
|
|
the stack slot where <tt>vreg</tt>'s value will be located. If it is
|
|
necessary to map another virtual register to the same stack slot,
|
|
use <tt>VirtRegMap::assignVirt2StackSlot(vreg, stack_location)</tt>. One
|
|
important point to consider when using the indirect mapping, is that even if
|
|
a virtual register is mapped to memory, it still needs to be mapped to a
|
|
physical register. This physical register is the location where the virtual
|
|
register is supposed to be found before being stored or after being
|
|
reloaded.</p>
|
|
|
|
<p>If the indirect strategy is used, after all the virtual registers have been
|
|
mapped to physical registers or stack slots, it is necessary to use a spiller
|
|
object to place load and store instructions in the code. Every virtual that
|
|
has been mapped to a stack slot will be stored to memory after been defined
|
|
and will be loaded before being used. The implementation of the spiller tries
|
|
to recycle load/store instructions, avoiding unnecessary instructions. For an
|
|
example of how to invoke the spiller,
|
|
see <tt>RegAllocLinearScan::runOnMachineFunction</tt>
|
|
in <tt>lib/CodeGen/RegAllocLinearScan.cpp</tt>.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_twoAddr">Handling two address instructions</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>With very rare exceptions (e.g., function calls), the LLVM machine code
|
|
instructions are three address instructions. That is, each instruction is
|
|
expected to define at most one register, and to use at most two registers.
|
|
However, some architectures use two address instructions. In this case, the
|
|
defined register is also one of the used register. For instance, an
|
|
instruction such as <tt>ADD %EAX, %EBX</tt>, in X86 is actually equivalent
|
|
to <tt>%EAX = %EAX + %EBX</tt>.</p>
|
|
|
|
<p>In order to produce correct code, LLVM must convert three address
|
|
instructions that represent two address instructions into true two address
|
|
instructions. LLVM provides the pass <tt>TwoAddressInstructionPass</tt> for
|
|
this specific purpose. It must be run before register allocation takes
|
|
place. After its execution, the resulting code may no longer be in SSA
|
|
form. This happens, for instance, in situations where an instruction such
|
|
as <tt>%a = ADD %b %c</tt> is converted to two instructions such as:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
%a = MOVE %b
|
|
%a = ADD %a %c
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Notice that, internally, the second instruction is represented as
|
|
<tt>ADD %a[def/use] %c</tt>. I.e., the register operand <tt>%a</tt> is both
|
|
used and defined by the instruction.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_ssaDecon">The SSA deconstruction phase</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>An important transformation that happens during register allocation is called
|
|
the <i>SSA Deconstruction Phase</i>. The SSA form simplifies many analyses
|
|
that are performed on the control flow graph of programs. However,
|
|
traditional instruction sets do not implement PHI instructions. Thus, in
|
|
order to generate executable code, compilers must replace PHI instructions
|
|
with other instructions that preserve their semantics.</p>
|
|
|
|
<p>There are many ways in which PHI instructions can safely be removed from the
|
|
target code. The most traditional PHI deconstruction algorithm replaces PHI
|
|
instructions with copy instructions. That is the strategy adopted by
|
|
LLVM. The SSA deconstruction algorithm is implemented
|
|
in <tt>lib/CodeGen/PHIElimination.cpp</tt>. In order to invoke this pass, the
|
|
identifier <tt>PHIEliminationID</tt> must be marked as required in the code
|
|
of the register allocator.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_fold">Instruction folding</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><i>Instruction folding</i> is an optimization performed during register
|
|
allocation that removes unnecessary copy instructions. For instance, a
|
|
sequence of instructions such as:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
%EBX = LOAD %mem_address
|
|
%EAX = COPY %EBX
|
|
</pre>
|
|
</div>
|
|
|
|
<p>can be safely substituted by the single instruction:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
%EAX = LOAD %mem_address
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Instructions can be folded with
|
|
the <tt>TargetRegisterInfo::foldMemoryOperand(...)</tt> method. Care must be
|
|
taken when folding instructions; a folded instruction can be quite different
|
|
from the original
|
|
instruction. See <tt>LiveIntervals::addIntervalsForSpills</tt>
|
|
in <tt>lib/CodeGen/LiveIntervalAnalysis.cpp</tt> for an example of its
|
|
use.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
|
|
<div class="doc_subsubsection">
|
|
<a name="regAlloc_builtIn">Built in register allocators</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The LLVM infrastructure provides the application developer with three
|
|
different register allocators:</p>
|
|
|
|
<ul>
|
|
<li><i>Simple</i> — This is a very simple implementation that does not
|
|
keep values in registers across instructions. This register allocator
|
|
immediately spills every value right after it is computed, and reloads all
|
|
used operands from memory to temporary registers before each
|
|
instruction.</li>
|
|
|
|
<li><i>Local</i> — This register allocator is an improvement on the
|
|
<i>Simple</i> implementation. It allocates registers on a basic block
|
|
level, attempting to keep values in registers and reusing registers as
|
|
appropriate.</li>
|
|
|
|
<li><i>Linear Scan</i> — <i>The default allocator</i>. This is the
|
|
well-know linear scan register allocator. Whereas the
|
|
<i>Simple</i> and <i>Local</i> algorithms use a direct mapping
|
|
implementation technique, the <i>Linear Scan</i> implementation
|
|
uses a spiller in order to place load and stores.</li>
|
|
</ul>
|
|
|
|
<p>The type of register allocator used in <tt>llc</tt> can be chosen with the
|
|
command line option <tt>-regalloc=...</tt>:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
$ llc -regalloc=simple file.bc -o sp.s;
|
|
$ llc -regalloc=local file.bc -o lc.s;
|
|
$ llc -regalloc=linearscan file.bc -o ln.s;
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="proepicode">Prolog/Epilog Code Insertion</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="latemco">Late Machine Code Optimizations</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="codeemit">Code Emission</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="codeemit_asm">Generating Assembly Code</a>
|
|
</div>
|
|
<div class="doc_text"><p>To Be Written</p></div>
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="codeemit_bin">Generating Binary Machine Code</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>For the JIT or <tt>.o</tt> file writer</p>
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="targetimpls">Target-specific Implementation Notes</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section of the document explains features or design decisions that are
|
|
specific to the code generator for a particular target.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="tailcallopt">Tail call optimization</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Tail call optimization, callee reusing the stack of the caller, is currently
|
|
supported on x86/x86-64 and PowerPC. It is performed if:</p>
|
|
|
|
<ul>
|
|
<li>Caller and callee have the calling convention <tt>fastcc</tt> or
|
|
<tt>cc 10</tt> (GHC call convention).</li>
|
|
|
|
<li>The call is a tail call - in tail position (ret immediately follows call
|
|
and ret uses value of call or is void).</li>
|
|
|
|
<li>Option <tt>-tailcallopt</tt> is enabled.</li>
|
|
|
|
<li>Platform specific constraints are met.</li>
|
|
</ul>
|
|
|
|
<p>x86/x86-64 constraints:</p>
|
|
|
|
<ul>
|
|
<li>No variable argument lists are used.</li>
|
|
|
|
<li>On x86-64 when generating GOT/PIC code only module-local calls (visibility
|
|
= hidden or protected) are supported.</li>
|
|
</ul>
|
|
|
|
<p>PowerPC constraints:</p>
|
|
|
|
<ul>
|
|
<li>No variable argument lists are used.</li>
|
|
|
|
<li>No byval parameters are used.</li>
|
|
|
|
<li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
|
|
</ul>
|
|
|
|
<p>Example:</p>
|
|
|
|
<p>Call as <tt>llc -tailcallopt test.ll</tt>.</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
|
|
|
|
define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
|
|
%l1 = add i32 %in1, %in2
|
|
%tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
|
|
ret i32 %tmp
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Implications of <tt>-tailcallopt</tt>:</p>
|
|
|
|
<p>To support tail call optimization in situations where the callee has more
|
|
arguments than the caller a 'callee pops arguments' convention is used. This
|
|
currently causes each <tt>fastcc</tt> call that is not tail call optimized
|
|
(because one or more of above constraints are not met) to be followed by a
|
|
readjustment of the stack. So performance might be worse in such cases.</p>
|
|
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="sibcallopt">Sibling call optimization</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Sibling call optimization is a restricted form of tail call optimization.
|
|
Unlike tail call optimization described in the previous section, it can be
|
|
performed automatically on any tail calls when <tt>-tailcallopt</tt> option
|
|
is not specified.</p>
|
|
|
|
<p>Sibling call optimization is currently performed on x86/x86-64 when the
|
|
following constraints are met:</p>
|
|
|
|
<ul>
|
|
<li>Caller and callee have the same calling convention. It can be either
|
|
<tt>c</tt> or <tt>fastcc</tt>.
|
|
|
|
<li>The call is a tail call - in tail position (ret immediately follows call
|
|
and ret uses value of call or is void).</li>
|
|
|
|
<li>Caller and callee have matching return type or the callee result is not
|
|
used.
|
|
|
|
<li>If any of the callee arguments are being passed in stack, they must be
|
|
available in caller's own incoming argument stack and the frame offsets
|
|
must be the same.
|
|
</ul>
|
|
|
|
<p>Example:</p>
|
|
<div class="doc_code">
|
|
<pre>
|
|
declare i32 @bar(i32, i32)
|
|
|
|
define i32 @foo(i32 %a, i32 %b, i32 %c) {
|
|
entry:
|
|
%0 = tail call i32 @bar(i32 %a, i32 %b)
|
|
ret i32 %0
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="x86">The X86 backend</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
|
|
code generator is capable of targeting a variety of x86-32 and x86-64
|
|
processors, and includes support for ISA extensions such as MMX and SSE.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_tt">X86 Target Triples supported</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The following are the known target triples that are supported by the X86
|
|
backend. This is not an exhaustive list, and it would be useful to add those
|
|
that people test.</p>
|
|
|
|
<ul>
|
|
<li><b>i686-pc-linux-gnu</b> — 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>i386-pc-mingw32msvc</b> — MingW crosscompiler on Linux</li>
|
|
|
|
<li><b>i686-apple-darwin*</b> — Apple Darwin on X86</li>
|
|
|
|
<li><b>x86_64-unknown-linux-gnu</b> — Linux</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_cc">X86 Calling Conventions supported</a>
|
|
</div>
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The following target-specific calling conventions are known to backend:</p>
|
|
|
|
<ul>
|
|
<li><b>x86_StdCall</b> — stdcall calling convention seen on Microsoft
|
|
Windows platform (CC ID = 64).</li>
|
|
|
|
<li><b>x86_FastCall</b> — fastcall calling convention seen on Microsoft
|
|
Windows platform (CC ID = 65).</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The x86 has a very flexible way of accessing memory. It is capable of
|
|
forming memory addresses of the following expression directly in integer
|
|
instructions (which use ModR/M addressing):</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32
|
|
</pre>
|
|
</div>
|
|
|
|
<p>In order to represent this, LLVM tracks no less than 5 operands for each
|
|
memory operand of this form. This means that the "load" form of
|
|
'<tt>mov</tt>' has the following <tt>MachineOperand</tt>s in this order:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
Index: 0 | 1 2 3 4 5
|
|
Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement Segment
|
|
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Stores, and all other instructions, treat the four memory operands in the
|
|
same way and in the same order. If the segment register is unspecified
|
|
(regno = 0), then no segment override is generated. "Lea" operations do not
|
|
have a segment register specified, so they only have 4 operands for their
|
|
memory reference.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_memory">X86 address spaces supported</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>x86 has an experimental feature which provides
|
|
the ability to perform loads and stores to different address spaces
|
|
via the x86 segment registers. A segment override prefix byte on an
|
|
instruction causes the instruction's memory access to go to the specified
|
|
segment. LLVM address space 0 is the default address space, which includes
|
|
the stack, and any unqualified memory accesses in a program. Address spaces
|
|
1-255 are currently reserved for user-defined code. The GS-segment is
|
|
represented by address space 256, while the FS-segment is represented by
|
|
address space 257. Other x86 segments have yet to be allocated address space
|
|
numbers.</p>
|
|
|
|
<p>While these address spaces may seem similar to TLS via the
|
|
<tt>thread_local</tt> keyword, and often use the same underlying hardware,
|
|
there are some fundamental differences.</p>
|
|
|
|
<p>The <tt>thread_local</tt> keyword applies to global variables and
|
|
specifies that they are to be allocated in thread-local memory. There are
|
|
no type qualifiers involved, and these variables can be pointed to with
|
|
normal pointers and accessed with normal loads and stores.
|
|
The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
|
|
level (though LLVM doesn't yet have implementations of it for some
|
|
configurations).<p>
|
|
|
|
<p>Special address spaces, in contrast, apply to static types. Every
|
|
load and store has a particular address space in its address operand type,
|
|
and this is what determines which address space is accessed.
|
|
LLVM ignores these special address space qualifiers on global variables,
|
|
and does not provide a way to directly allocate storage in them.
|
|
At the LLVM IR level, the behavior of these special address spaces depends
|
|
in part on the underlying OS or runtime environment, and they are specific
|
|
to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>
|
|
|
|
<p>Some operating systems and runtime environments use (or may in the future
|
|
use) the FS/GS-segment registers for various low-level purposes, so care
|
|
should be taken when considering them.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="x86_names">Instruction naming</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>An instruction name consists of the base name, a default operand size, and a
|
|
a character per operand with an optional special size. For example:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
ADD8rr -> add, 8-bit register, 8-bit register
|
|
IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
|
|
IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
|
|
MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ppc">The PowerPC backend</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The PowerPC code generator lives in the lib/Target/PowerPC directory. The
|
|
code generation is retargetable to several variations or <i>subtargets</i> of
|
|
the PowerPC ISA; including ppc32, ppc64 and altivec.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ppc_abi">LLVM PowerPC ABI</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
|
|
relative (PIC) or static addressing for accessing global values, so no TOC
|
|
(r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth
|
|
of a stack frame. LLVM takes advantage of having no TOC to provide space to
|
|
save the frame pointer in the PowerPC linkage area of the caller frame.
|
|
Other details of PowerPC ABI can be found at <a href=
|
|
"http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html"
|
|
>PowerPC ABI.</a> Note: This link describes the 32 bit ABI. The 64 bit ABI
|
|
is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved
|
|
for system use.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ppc_frame">Frame Layout</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The size of a PowerPC frame is usually fixed for the duration of a
|
|
function's invocation. Since the frame is fixed size, all references
|
|
into the frame can be accessed via fixed offsets from the stack pointer. The
|
|
exception to this is when dynamic alloca or variable sized arrays are
|
|
present, then a base pointer (r31) is used as a proxy for the stack pointer
|
|
and stack pointer is free to grow or shrink. A base pointer is also used if
|
|
llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is
|
|
always aligned to 16 bytes, so that space allocated for altivec vectors will
|
|
be properly aligned.</p>
|
|
|
|
<p>An invocation frame is laid out as follows (low memory at top);</p>
|
|
|
|
<table class="layout">
|
|
<tr>
|
|
<td>Linkage<br><br></td>
|
|
</tr>
|
|
<tr>
|
|
<td>Parameter area<br><br></td>
|
|
</tr>
|
|
<tr>
|
|
<td>Dynamic area<br><br></td>
|
|
</tr>
|
|
<tr>
|
|
<td>Locals area<br><br></td>
|
|
</tr>
|
|
<tr>
|
|
<td>Saved registers area<br><br></td>
|
|
</tr>
|
|
<tr style="border-style: none hidden none hidden;">
|
|
<td><br></td>
|
|
</tr>
|
|
<tr>
|
|
<td>Previous Frame<br><br></td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>The <i>linkage</i> area is used by a callee to save special registers prior
|
|
to allocating its own frame. Only three entries are relevant to LLVM. The
|
|
first entry is the previous stack pointer (sp), aka link. This allows
|
|
probing tools like gdb or exception handlers to quickly scan the frames in
|
|
the stack. A function epilog can also use the link to pop the frame from the
|
|
stack. The third entry in the linkage area is used to save the return
|
|
address from the lr register. Finally, as mentioned above, the last entry is
|
|
used to save the previous frame pointer (r31.) The entries in the linkage
|
|
area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit
|
|
mode and 48 bytes in 64 bit mode.</p>
|
|
|
|
<p>32 bit linkage area</p>
|
|
|
|
<table class="layout">
|
|
<tr>
|
|
<td>0</td>
|
|
<td>Saved SP (r1)</td>
|
|
</tr>
|
|
<tr>
|
|
<td>4</td>
|
|
<td>Saved CR</td>
|
|
</tr>
|
|
<tr>
|
|
<td>8</td>
|
|
<td>Saved LR</td>
|
|
</tr>
|
|
<tr>
|
|
<td>12</td>
|
|
<td>Reserved</td>
|
|
</tr>
|
|
<tr>
|
|
<td>16</td>
|
|
<td>Reserved</td>
|
|
</tr>
|
|
<tr>
|
|
<td>20</td>
|
|
<td>Saved FP (r31)</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>64 bit linkage area</p>
|
|
|
|
<table class="layout">
|
|
<tr>
|
|
<td>0</td>
|
|
<td>Saved SP (r1)</td>
|
|
</tr>
|
|
<tr>
|
|
<td>8</td>
|
|
<td>Saved CR</td>
|
|
</tr>
|
|
<tr>
|
|
<td>16</td>
|
|
<td>Saved LR</td>
|
|
</tr>
|
|
<tr>
|
|
<td>24</td>
|
|
<td>Reserved</td>
|
|
</tr>
|
|
<tr>
|
|
<td>32</td>
|
|
<td>Reserved</td>
|
|
</tr>
|
|
<tr>
|
|
<td>40</td>
|
|
<td>Saved FP (r31)</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>The <i>parameter area</i> is used to store arguments being passed to a callee
|
|
function. Following the PowerPC ABI, the first few arguments are actually
|
|
passed in registers, with the space in the parameter area unused. However,
|
|
if there are not enough registers or the callee is a thunk or vararg
|
|
function, these register arguments can be spilled into the parameter area.
|
|
Thus, the parameter area must be large enough to store all the parameters for
|
|
the largest call sequence made by the caller. The size must also be
|
|
minimally large enough to spill registers r3-r10. This allows callees blind
|
|
to the call signature, such as thunks and vararg functions, enough space to
|
|
cache the argument registers. Therefore, the parameter area is minimally 32
|
|
bytes (64 bytes in 64 bit mode.) Also note that since the parameter area is
|
|
a fixed offset from the top of the frame, that a callee can access its spilt
|
|
arguments using fixed offsets from the stack pointer (or base pointer.)</p>
|
|
|
|
<p>Combining the information about the linkage, parameter areas and alignment. A
|
|
stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
|
|
mode.</p>
|
|
|
|
<p>The <i>dynamic area</i> starts out as size zero. If a function uses dynamic
|
|
alloca then space is added to the stack, the linkage and parameter areas are
|
|
shifted to top of stack, and the new space is available immediately below the
|
|
linkage and parameter areas. The cost of shifting the linkage and parameter
|
|
areas is minor since only the link value needs to be copied. The link value
|
|
can be easily fetched by adding the original frame size to the base pointer.
|
|
Note that allocations in the dynamic space need to observe 16 byte
|
|
alignment.</p>
|
|
|
|
<p>The <i>locals area</i> is where the llvm compiler reserves space for local
|
|
variables.</p>
|
|
|
|
<p>The <i>saved registers area</i> is where the llvm compiler spills callee
|
|
saved registers on entry to the callee.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ppc_prolog">Prolog/Epilog</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The llvm prolog and epilog are the same as described in the PowerPC ABI, with
|
|
the following exceptions. Callee saved registers are spilled after the frame
|
|
is created. This allows the llvm epilog/prolog support to be common with
|
|
other targets. The base pointer callee saved register r31 is saved in the
|
|
TOC slot of linkage area. This simplifies allocation of space for the base
|
|
pointer and makes it convenient to locate programatically and during
|
|
debugging.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ppc_dynamic">Dynamic Allocation</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><i>TODO - More to come.</i></p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
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src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
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|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
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</address>
|
|
|
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