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128 lines
6.1 KiB
HTML
128 lines
6.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Liveness information (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Liveness information (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Liveness information (GNU Compiler Collection (GCC) Internals)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Control-Flow.html#Control-Flow" rel="up" title="Control Flow">
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<link href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" rel="next" title="Loop Analysis and Representation">
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<link href="Maintaining-the-CFG.html#Maintaining-the-CFG" rel="prev" title="Maintaining the CFG">
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</head>
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<body lang="en">
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<a name="Liveness-information"></a>
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<div class="header">
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<p>
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Previous: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="p" rel="prev">Maintaining the CFG</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Liveness-information-1"></a>
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<h3 class="section">15.5 Liveness information</h3>
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<a name="index-Liveness-representation"></a>
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<p>Liveness information is useful to determine whether some register is
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“live” at given point of program, i.e. that it contains a value that
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may be used at a later point in the program. This information is
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used, for instance, during register allocation, as the pseudo
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registers only need to be assigned to a unique hard register or to a
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stack slot if they are live. The hard registers and stack slots may
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be freely reused for other values when a register is dead.
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</p>
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<p>Liveness information is available in the back end starting with
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<code>pass_df_initialize</code> and ending with <code>pass_df_finish</code>. Three
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flavors of live analysis are available: With <code>LR</code>, it is possible
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to determine at any point <code>P</code> in the function if the register may be
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used on some path from <code>P</code> to the end of the function. With
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<code>UR</code>, it is possible to determine if there is a path from the
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beginning of the function to <code>P</code> that defines the variable.
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<code>LIVE</code> is the intersection of the <code>LR</code> and <code>UR</code> and a
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variable is live at <code>P</code> if there is both an assignment that reaches
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it from the beginning of the function and a use that can be reached on
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some path from <code>P</code> to the end of the function.
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</p>
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<p>In general <code>LIVE</code> is the most useful of the three. The macros
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<code>DF_[LR,UR,LIVE]_[IN,OUT]</code> can be used to access this information.
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The macros take a basic block number and return a bitmap that is indexed
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by the register number. This information is only guaranteed to be up to
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date after calls are made to <code>df_analyze</code>. See the file
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<code>df-core.c</code> for details on using the dataflow.
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</p>
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<a name="index-REG_005fDEAD_002c-REG_005fUNUSED"></a>
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<p>The liveness information is stored partly in the RTL instruction stream
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and partly in the flow graph. Local information is stored in the
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instruction stream: Each instruction may contain <code>REG_DEAD</code> notes
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representing that the value of a given register is no longer needed, or
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<code>REG_UNUSED</code> notes representing that the value computed by the
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instruction is never used. The second is useful for instructions
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computing multiple values at once.
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</p>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="p" rel="prev">Maintaining the CFG</a>, Up: <a href="Control-Flow.html#Control-Flow" accesskey="u" rel="up">Control Flow</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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