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141 lines
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141 lines
7.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Global Reg Vars - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" title="Explicit Reg Vars">
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<link rel="next" href="Local-Reg-Vars.html#Local-Reg-Vars" title="Local Reg Vars">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Global-Reg-Vars"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Local-Reg-Vars.html#Local-Reg-Vars">Local Reg Vars</a>,
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Up: <a rel="up" accesskey="u" href="Explicit-Reg-Vars.html#Explicit-Reg-Vars">Explicit Reg Vars</a>
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<hr>
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</div>
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<h5 class="subsubsection">6.43.5.1 Defining Global Register Variables</h5>
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<p><a name="index-global-register-variables-3496"></a><a name="index-registers_002c-global-variables-in-3497"></a>
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You can define a global register variable in GNU C like this:
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<pre class="smallexample"> register int *foo asm ("a5");
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</pre>
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<p class="noindent">Here <code>a5</code> is the name of the register that should be used. Choose a
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register that is normally saved and restored by function calls on your
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machine, so that library routines will not clobber it.
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<p>Naturally the register name is CPU-dependent, so you need to
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conditionalize your program according to CPU type. The register
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<code>a5</code> is a good choice on a 68000 for a variable of pointer
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type. On machines with register windows, be sure to choose a “global”
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register that is not affected magically by the function call mechanism.
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<p>In addition, different operating systems on the same CPU may differ in how they
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name the registers; then you need additional conditionals. For
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example, some 68000 operating systems call this register <code>%a5</code>.
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<p>Eventually there may be a way of asking the compiler to choose a register
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automatically, but first we need to figure out how it should choose and
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how to enable you to guide the choice. No solution is evident.
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<p>Defining a global register variable in a certain register reserves that
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register entirely for this use, at least within the current compilation.
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The register is not allocated for any other purpose in the functions
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in the current compilation, and is not saved and restored by
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these functions. Stores into this register are never deleted even if they
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appear to be dead, but references may be deleted or moved or
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simplified.
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<p>It is not safe to access the global register variables from signal
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handlers, or from more than one thread of control, because the system
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library routines may temporarily use the register for other things (unless
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you recompile them specially for the task at hand).
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<p><a name="index-g_t_0040code_007bqsort_007d_002c-and-global-register-variables-3498"></a>It is not safe for one function that uses a global register variable to
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call another such function <code>foo</code> by way of a third function
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<code>lose</code> that is compiled without knowledge of this variable (i.e. in a
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different source file in which the variable isn't declared). This is
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because <code>lose</code> might save the register and put some other value there.
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For example, you can't expect a global register variable to be available in
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the comparison-function that you pass to <code>qsort</code>, since <code>qsort</code>
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might have put something else in that register. (If you are prepared to
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recompile <code>qsort</code> with the same global register variable, you can
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solve this problem.)
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<p>If you want to recompile <code>qsort</code> or other source files that do not
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actually use your global register variable, so that they do not use that
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register for any other purpose, then it suffices to specify the compiler
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option <samp><span class="option">-ffixed-</span><var>reg</var></samp>. You need not actually add a global
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register declaration to their source code.
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<p>A function that can alter the value of a global register variable cannot
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safely be called from a function compiled without this variable, because it
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could clobber the value the caller expects to find there on return.
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Therefore, the function that is the entry point into the part of the
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program that uses the global register variable must explicitly save and
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restore the value that belongs to its caller.
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<p><a name="index-register-variable-after-_0040code_007blongjmp_007d-3499"></a><a name="index-global-register-after-_0040code_007blongjmp_007d-3500"></a><a name="index-value-after-_0040code_007blongjmp_007d-3501"></a><a name="index-longjmp-3502"></a><a name="index-setjmp-3503"></a>On most machines, <code>longjmp</code> restores to each global register
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variable the value it had at the time of the <code>setjmp</code>. On some
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machines, however, <code>longjmp</code> does not change the value of global
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register variables. To be portable, the function that called <code>setjmp</code>
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should make other arrangements to save the values of the global register
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variables, and to restore them in a <code>longjmp</code>. This way, the same
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thing happens regardless of what <code>longjmp</code> does.
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<p>All global register variable declarations must precede all function
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definitions. If such a declaration could appear after function
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definitions, the declaration would be too late to prevent the register from
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being used for other purposes in the preceding functions.
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<p>Global register variables may not have initial values, because an
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executable file has no means to supply initial contents for a register.
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<p>On the SPARC, there are reports that g3 <small class="dots">...</small> g7 are suitable
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registers, but certain library functions, such as <code>getwd</code>, as well
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as the subroutines for division and remainder, modify g3 and g4. g1 and
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g2 are local temporaries.
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<p>On the 68000, a2 <small class="dots">...</small> a5 should be suitable, as should d2 <small class="dots">...</small> d7.
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Of course, it does not do to use more than a few of those.
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</body></html>
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