You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
128 lines
6.1 KiB
HTML
128 lines
6.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
|
(see below). A copy of the license is included in the section entitled
|
|
"GNU Free Documentation License".
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Liveness information (GNU Compiler Collection (GCC) Internals)</title>
|
|
|
|
<meta name="description" content="Liveness information (GNU Compiler Collection (GCC) Internals)">
|
|
<meta name="keywords" content="Liveness information (GNU Compiler Collection (GCC) Internals)">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Control-Flow.html#Control-Flow" rel="up" title="Control Flow">
|
|
<link href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" rel="next" title="Loop Analysis and Representation">
|
|
<link href="Maintaining-the-CFG.html#Maintaining-the-CFG" rel="prev" title="Maintaining the CFG">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Liveness-information"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<a name="Liveness-information-1"></a>
|
|
<h3 class="section">15.5 Liveness information</h3>
|
|
<a name="index-Liveness-representation"></a>
|
|
<p>Liveness information is useful to determine whether some register is
|
|
“live” at given point of program, i.e. that it contains a value that
|
|
may be used at a later point in the program. This information is
|
|
used, for instance, during register allocation, as the pseudo
|
|
registers only need to be assigned to a unique hard register or to a
|
|
stack slot if they are live. The hard registers and stack slots may
|
|
be freely reused for other values when a register is dead.
|
|
</p>
|
|
<p>Liveness information is available in the back end starting with
|
|
<code>pass_df_initialize</code> and ending with <code>pass_df_finish</code>. Three
|
|
flavors of live analysis are available: With <code>LR</code>, it is possible
|
|
to determine at any point <code>P</code> in the function if the register may be
|
|
used on some path from <code>P</code> to the end of the function. With
|
|
<code>UR</code>, it is possible to determine if there is a path from the
|
|
beginning of the function to <code>P</code> that defines the variable.
|
|
<code>LIVE</code> is the intersection of the <code>LR</code> and <code>UR</code> and a
|
|
variable is live at <code>P</code> if there is both an assignment that reaches
|
|
it from the beginning of the function and a use that can be reached on
|
|
some path from <code>P</code> to the end of the function.
|
|
</p>
|
|
<p>In general <code>LIVE</code> is the most useful of the three. The macros
|
|
<code>DF_[LR,UR,LIVE]_[IN,OUT]</code> can be used to access this information.
|
|
The macros take a basic block number and return a bitmap that is indexed
|
|
by the register number. This information is only guaranteed to be up to
|
|
date after calls are made to <code>df_analyze</code>. See the file
|
|
<code>df-core.c</code> for details on using the dataflow.
|
|
</p>
|
|
|
|
<a name="index-REG_005fDEAD_002c-REG_005fUNUSED"></a>
|
|
<p>The liveness information is stored partly in the RTL instruction stream
|
|
and partly in the flow graph. Local information is stored in the
|
|
instruction stream: Each instruction may contain <code>REG_DEAD</code> notes
|
|
representing that the value of a given register is no longer needed, or
|
|
<code>REG_UNUSED</code> notes representing that the value computed by the
|
|
instruction is never used. The second is useful for instructions
|
|
computing multiple values at once.
|
|
</p>
|
|
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|