1<html lang="en"> 2<head> 3<title>Local Reg Vars - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" title="Explicit Reg Vars"> 9<link rel="prev" href="Global-Reg-Vars.html#Global-Reg-Vars" title="Global Reg Vars"> 10<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 11<!-- 12Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 131998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 142010 Free Software Foundation, Inc. 15 16Permission is granted to copy, distribute and/or modify this document 17under the terms of the GNU Free Documentation License, Version 1.3 or 18any later version published by the Free Software Foundation; with the 19Invariant Sections being ``Funding Free Software'', the Front-Cover 20Texts being (a) (see below), and with the Back-Cover Texts being (b) 21(see below). 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Note 65that this is the same syntax used for defining global register 66variables, but for a local variable it would appear within a function. 67 68 <p>Naturally the register name is cpu-dependent, but this is not a 69problem, since specific registers are most often useful with explicit 70assembler instructions (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>). Both of these things 71generally require that you conditionalize your program according to 72cpu type. 73 74 <p>In addition, operating systems on one type of cpu may differ in how they 75name the registers; then you would need additional conditionals. For 76example, some 68000 operating systems call this register <code>%a5</code>. 77 78 <p>Defining such a register variable does not reserve the register; it 79remains available for other uses in places where flow control determines 80the variable's value is not live. 81 82 <p>This option does not guarantee that GCC will generate code that has 83this variable in the register you specify at all times. You may not 84code an explicit reference to this register in the <em>assembler 85instruction template</em> part of an <code>asm</code> statement and assume it will 86always refer to this variable. However, using the variable as an 87<code>asm</code> <em>operand</em> guarantees that the specified register is used 88for the operand. 89 90 <p>Stores into local register variables may be deleted when they appear to be dead 91according to dataflow analysis. References to local register variables may 92be deleted or moved or simplified. 93 94 <p>As for global register variables, it's recommended that you choose a 95register which is normally saved and restored by function calls on 96your machine, so that library routines will not clobber it. A common 97pitfall is to initialize multiple call-clobbered registers with 98arbitrary expressions, where a function call or library call for an 99arithmetic operator will overwrite a register value from a previous 100assignment, for example <code>r0</code> below: 101<pre class="smallexample"> register int *p1 asm ("r0") = ...; 102 register int *p2 asm ("r1") = ...; 103</pre> 104 <p>In those cases, a solution is to use a temporary variable for 105each arbitrary expression. See <a href="Example-of-asm-with-clobbered-asm-reg.html#Example-of-asm-with-clobbered-asm-reg">Example of asm with clobbered asm reg</a>. 106 107 </body></html> 108 109