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diff --git a/share/doc/gccint/Liveness-information.html b/share/doc/gccint/Liveness-information.html new file mode 100644 index 0000000..4ffe083 --- /dev/null +++ b/share/doc/gccint/Liveness-information.html @@ -0,0 +1,128 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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). 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Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Liveness information</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Liveness information"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Liveness information"> +<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="previous" title="Maintaining the CFG"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +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.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Liveness-information"></a> +<div class="header"> +<p> +Previous: <a href="Maintaining-the-CFG.html#Maintaining-the-CFG" accesskey="p" rel="previous">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.cc</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="previous">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> |