<html lang="en"> <head> <title>Liveness information - GNU Compiler Collection (GCC) Internals</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="GNU Compiler Collection (GCC) Internals"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Control-Flow.html#Control-Flow" title="Control Flow"> <link rel="prev" href="Maintaining-the-CFG.html#Maintaining-the-CFG" title="Maintaining the CFG"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <!-- Copyright (C) 1988-2015 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.--> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Liveness-information"></a> <p> Previous: <a rel="previous" accesskey="p" href="Maintaining-the-CFG.html#Maintaining-the-CFG">Maintaining the CFG</a>, Up: <a rel="up" accesskey="u" href="Control-Flow.html#Control-Flow">Control Flow</a> <hr> </div> <h3 class="section">14.5 Liveness information</h3> <p><a name="index-Liveness-representation-3227"></a>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>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>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-3228"></a>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. <!-- Copyright (C) 2006-2015 Free Software Foundation, Inc. --> <!-- Free Software Foundation, Inc. --> <!-- This is part of the GCC manual. --> <!-- For copying conditions, see the file gcc.texi. --> <!-- --> <!-- Loop Representation --> <!-- --> </body></html>