📄 explicit-reg-vars.html

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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>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 }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Explicit%20Reg%20Vars">Explicit Reg Vars</a>,Next:&nbsp;<a rel="next" accesskey="n" href="Alternate-Keywords.html#Alternate%20Keywords">Alternate Keywords</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Asm-Labels.html#Asm%20Labels">Asm Labels</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Variables in Specified Registers</h3><p>GNU C allows you to put a few global variables into specified hardwareregisters.  You can also specify the register in which an ordinaryregister variable should be allocated.     <ul><li>Global register variables reserve registers throughout the program. This may be useful in programs such as programming languageinterpreters which have a couple of global variables that are accessedvery often.     <li>Local register variables in specific registers do not reserve theregisters.  The compiler's data flow analysis is capable of determiningwhere the specified registers contain live values, and where they areavailable for other uses.  Stores into local register variables may be deletedwhen they appear to be dead according to dataflow analysis.  Referencesto local register variables may be deleted or moved or simplified.     <p>These local variables are sometimes convenient for use with the extended<code>asm</code> feature (see <a href="Extended-Asm.html#Extended%20Asm">Extended Asm</a>), if you want to write oneoutput of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraintsspecified for that operand in the <code>asm</code>.) </ul><ul class="menu"><li><a accesskey="1" href="Global-Reg-Vars.html#Global%20Reg%20Vars">Global Reg Vars</a>: <li><a accesskey="2" href="Local-Reg-Vars.html#Local%20Reg%20Vars">Local Reg Vars</a>: </ul>   </body></html>
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