| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- 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. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Using the GNU Compiler Collection (GCC): Global Reg Vars</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Global Reg Vars"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Global Reg Vars"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" rel="up" title="Explicit Reg Vars"> |
| <link href="Local-Reg-Vars.html#Local-Reg-Vars" rel="next" title="Local Reg Vars"> |
| <link href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" rel="prev" title="Explicit Reg Vars"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Global-Reg-Vars"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Local-Reg-Vars.html#Local-Reg-Vars" accesskey="n" rel="next">Local Reg Vars</a>, Up: <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" accesskey="u" rel="up">Explicit Reg Vars</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Defining-Global-Register-Variables"></a> |
| <h4 class="subsection">6.44.1 Defining Global Register Variables</h4> |
| <a name="index-global-register-variables"></a> |
| <a name="index-registers_002c-global-variables-in"></a> |
| |
| <p>You can define a global register variable in GNU C like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">register int *foo asm ("a5"); |
| </pre></div> |
| |
| <p>Here <code>a5</code> is the name of the register that should be used. Choose a |
| register that is normally saved and restored by function calls on your |
| machine, so that library routines will not clobber it. |
| </p> |
| <p>Naturally the register name is cpu-dependent, so you need to |
| conditionalize your program according to cpu type. The register |
| <code>a5</code> is a good choice on a 68000 for a variable of pointer |
| type. On machines with register windows, be sure to choose a “global” |
| register that is not affected magically by the function call mechanism. |
| </p> |
| <p>In addition, different operating systems on the same CPU may differ in how they |
| name the registers; then you need additional conditionals. For |
| example, some 68000 operating systems call this register <code>%a5</code>. |
| </p> |
| <p>Eventually there may be a way of asking the compiler to choose a register |
| automatically, but first we need to figure out how it should choose and |
| how to enable you to guide the choice. No solution is evident. |
| </p> |
| <p>Defining a global register variable in a certain register reserves that |
| register entirely for this use, at least within the current compilation. |
| The register is not allocated for any other purpose in the functions |
| in the current compilation, and is not saved and restored by |
| these functions. Stores into this register are never deleted even if they |
| appear to be dead, but references may be deleted or moved or |
| simplified. |
| </p> |
| <p>It is not safe to access the global register variables from signal |
| handlers, or from more than one thread of control, because the system |
| library routines may temporarily use the register for other things (unless |
| you recompile them specially for the task at hand). |
| </p> |
| <a name="index-qsort_002c-and-global-register-variables"></a> |
| <p>It is not safe for one function that uses a global register variable to |
| call another such function <code>foo</code> by way of a third function |
| <code>lose</code> that is compiled without knowledge of this variable (i.e. in a |
| different source file in which the variable isn’t declared). This is |
| because <code>lose</code> might save the register and put some other value there. |
| For example, you can’t expect a global register variable to be available in |
| the comparison-function that you pass to <code>qsort</code>, since <code>qsort</code> |
| might have put something else in that register. (If you are prepared to |
| recompile <code>qsort</code> with the same global register variable, you can |
| solve this problem.) |
| </p> |
| <p>If you want to recompile <code>qsort</code> or other source files that do not |
| actually use your global register variable, so that they do not use that |
| register for any other purpose, then it suffices to specify the compiler |
| option <samp>-ffixed-<var>reg</var></samp>. You need not actually add a global |
| register declaration to their source code. |
| </p> |
| <p>A function that can alter the value of a global register variable cannot |
| safely be called from a function compiled without this variable, because it |
| could clobber the value the caller expects to find there on return. |
| Therefore, the function that is the entry point into the part of the |
| program that uses the global register variable must explicitly save and |
| restore the value that belongs to its caller. |
| </p> |
| <a name="index-register-variable-after-longjmp"></a> |
| <a name="index-global-register-after-longjmp"></a> |
| <a name="index-value-after-longjmp"></a> |
| <a name="index-longjmp"></a> |
| <a name="index-setjmp"></a> |
| <p>On most machines, <code>longjmp</code> restores to each global register |
| variable the value it had at the time of the <code>setjmp</code>. On some |
| machines, however, <code>longjmp</code> does not change the value of global |
| register variables. To be portable, the function that called <code>setjmp</code> |
| should make other arrangements to save the values of the global register |
| variables, and to restore them in a <code>longjmp</code>. This way, the same |
| thing happens regardless of what <code>longjmp</code> does. |
| </p> |
| <p>All global register variable declarations must precede all function |
| definitions. If such a declaration could appear after function |
| definitions, the declaration would be too late to prevent the register from |
| being used for other purposes in the preceding functions. |
| </p> |
| <p>Global register variables may not have initial values, because an |
| executable file has no means to supply initial contents for a register. |
| </p> |
| <p>On the SPARC, there are reports that g3 … g7 are suitable |
| registers, but certain library functions, such as <code>getwd</code>, as well |
| as the subroutines for division and remainder, modify g3 and g4. g1 and |
| g2 are local temporaries. |
| </p> |
| <p>On the 68000, a2 … a5 should be suitable, as should d2 … d7. |
| Of course, it does not do to use more than a few of those. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Local-Reg-Vars.html#Local-Reg-Vars" accesskey="n" rel="next">Local Reg Vars</a>, Up: <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" accesskey="u" rel="up">Explicit Reg Vars</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
