1<html lang="en"> 2<head> 3<title>Hints implementation - 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="C-Implementation.html#C-Implementation" title="C Implementation"> 9<link rel="prev" href="Arrays-and-pointers-implementation.html#Arrays-and-pointers-implementation" title="Arrays and pointers implementation"> 10<link rel="next" href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" title="Structures unions enumerations and bit-fields implementation"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Hints-implementation"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation">Structures unions enumerations and bit-fields implementation</a>, 53Previous: <a rel="previous" accesskey="p" href="Arrays-and-pointers-implementation.html#Arrays-and-pointers-implementation">Arrays and pointers implementation</a>, 54Up: <a rel="up" accesskey="u" href="C-Implementation.html#C-Implementation">C Implementation</a> 55<hr> 56</div> 57 58<h3 class="section">4.8 Hints</h3> 59 60 <ul> 61<li><cite>The extent to which suggestions made by using the </cite><code>register</code><cite> 62storage-class specifier are effective (C90 6.5.1, C99 6.7.1).</cite> 63 64 <p>The <code>register</code> specifier affects code generation only in these ways: 65 66 <ul> 67<li>When used as part of the register variable extension, see 68<a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>. 69 70 <li>When <samp><span class="option">-O0</span></samp> is in use, the compiler allocates distinct stack 71memory for all variables that do not have the <code>register</code> 72storage-class specifier; if <code>register</code> is specified, the variable 73may have a shorter lifespan than the code would indicate and may never 74be placed in memory. 75 76 <li>On some rare x86 targets, <code>setjmp</code> doesn't save the registers in 77all circumstances. In those cases, GCC doesn't allocate any variables 78in registers unless they are marked <code>register</code>. 79 80 </ul> 81 82 <li><cite>The extent to which suggestions made by using the inline function 83specifier are effective (C99 6.7.4).</cite> 84 85 <p>GCC will not inline any functions if the <samp><span class="option">-fno-inline</span></samp> option is 86used or if <samp><span class="option">-O0</span></samp> is used. Otherwise, GCC may still be unable to 87inline a function for many reasons; the <samp><span class="option">-Winline</span></samp> option may be 88used to determine if a function has not been inlined and why not. 89 90</ul> 91 92 </body></html> 93 94