<html lang="en"> <head> <title>Explicit Reg Vars - 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 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" title="Using Assembly Language with C"> <link rel="prev" href="Asm-Labels.html#Asm-Labels" title="Asm Labels"> <link rel="next" href="Size-of-an-asm.html#Size-of-an-asm" title="Size of an asm"> <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="Explicit-Reg-Vars"></a> <p> Next: <a rel="next" accesskey="n" href="Size-of-an-asm.html#Size-of-an-asm">Size of an asm</a>, Previous: <a rel="previous" accesskey="p" href="Asm-Labels.html#Asm-Labels">Asm Labels</a>, Up: <a rel="up" accesskey="u" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a> <hr> </div> <h4 class="subsection">6.43.5 Variables in Specified Registers</h4> <p><a name="index-explicit-register-variables-3492"></a><a name="index-variables-in-specified-registers-3493"></a><a name="index-specified-registers-3494"></a><a name="index-registers_002c-global-allocation-3495"></a> GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated. <ul> <li>Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters that have a couple of global variables that are accessed very often. <li>Local register variables in specific registers do not reserve the registers, except at the point where they are used as input or output operands in an <code>asm</code> statement and the <code>asm</code> statement itself is not deleted. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to 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-Asm">Extended Asm</a>), if you want to write one output of the assembler instruction directly into a particular register. (This works provided the register you specify fits the constraints specified for that operand in the <code>asm</code>.) </ul> <ul class="menu"> <li><a accesskey="1" href="Global-Reg-Vars.html#Global-Reg-Vars">Global Reg Vars</a> <li><a accesskey="2" href="Local-Reg-Vars.html#Local-Reg-Vars">Local Reg Vars</a> </ul> </body></html>